Luca's meaningless thoughts  

CDGC experimental branch in Druntime

by Leandro Lucarella on 2010- 11- 09 15:51 (updated on 2010- 11- 09 15:51)
tagged cdgc, d, dgc, druntime, en, sean kelly - with 0 comment(s)

Sean Kelly just created a new experimental branch in Druntime with CDGC as the GC for D2. The new branch is completely untested though, so only people wanting to help testing should try it out (which will be very appreciated).

DMD beta

by Leandro Lucarella on 2010- 01- 27 21:01 (updated on 2010- 01- 27 21:01)
tagged beta, compiler, d, development model, dmd, druntime, en, phobos, software - with 2 comment(s)

After some discussion [*] in the D newsgroup about the value of having release candidates for DMD (due to the high number of regressions introduced in new versions mostly), Walter agreed to make public what he called beta versions of the compiler, which he sent privately to people who asked for them (like some Tango developers).

The new DMD betas are announced in a special mailing list (available through Gmane too). It seems like Walter want to keep the beta releases with some kind of secrecy, or only for people really interested on them (the zip files are even password protected! But the password is announced in a public mailing list, that doesn't make much sense =/). I think he should encourage people to try them as much as possible instead, but one step at the time, at least now people have a way to test the compiler before it's released.

I can say without fear that the experience has been very successful already, even when there is no DMD release yet that came from a beta pre-release, you can see in the beta mailing list that multiple regressions have been discovered and fixed because this new beta releases. I think the reliability of the compiler has been increased already. Is really interesting to see how the quality of a product increases proportionally to the level of openness and the numbers of eyes doing peer review.

The new DMD release should be published very soon, as all the regressions seems to be fixed now and big projects like Tango, GtkD and QTD compiles (a lot of focus on fixing bugs that prevented the later to compile has been put into this release, specially from Rainer Schuetze, who submitted a lot of patches).

So kudos for a new era in D, I think this is another big milestone for having a reliable compiler.

[*]I'm sure there was previos requests for having release candidates, I know I asked for it, but I can't find the threads in the archives =)

D and open development model

by Leandro Lucarella on 2009- 10- 15 17:09 (updated on 2009- 10- 15 17:09)
tagged compiler, d, development model, dmd, druntime, en, phobos, software - with 6 comment(s)


Long post ahead =)

I'm very glad that yesterday DMD had the first releases (DMD 1.050 and DMD 2.035) with a decent revision history. It took some time to Walter Bright to understand how the open source development model works, and I think he still has a lot more to learn, but I have some hope now about the future of D.

Not much time ago, neither Phobos, DMD nor Druntime had revision control. Druntime didn't even exist, making D 1 split in two because of the Phobos vs Tango dichotomy. DMD back-end sources were not available either, and Walter Bright was the only person writing stuff (sometimes not because people didn't want to, but because he was too anal retentive to let them ;). It was almost impossible to make patches back then (your only chance was hacking GDC, which is pretty hard).

Now I can say that DMD, Phobos and Druntime have full source availability (DMD back-end is not free/libre though), almost all the parts of DMD have the sources published under a source control system. The core team has been expanded and even when Walter Bright is still in charge, at least 3 developers are now very committed to D: Andrei Alexandrescu (in charge of Phobos), Sean Kelly (in charge of Druntime) and Don Clugston (squashing DMD bugs at full speed, specially in the back-end). Other people are contributing patches in a regular basis. There were about 72 patches submitted to bugzilla before DMD was distributed with full source (72 patches in ~10 years) , since then, 206 patches were submitted (that is, 206 patches in less than 8 months).

But even with this great improvement, there is much left to do yet (and I'm talking only about the development model). This is a small list of what I think it's necessary to keep moving to a more open development model:


The release process should be improved. Me and other people are suggesting release candidates. This will allow people to test the new releases to find any regressions. As things are now, releases are not much different from a nightly build, except that you don't have one available every night :). People get very frustrated when downloading a new version of the compiler and things stop working, and this holds back front-end updates in other compilers, like LDC (which is frozen at 1.045 because of the regressions found in the next 5 versions).

I think Walter Bright is suffering from premature releasing too. Releases comes from nowhere, when nobody expects them. Nobody knows when a new compiler version will be released. I think that hurts the language reliability.

I think the releases should be more predictable. A release schedule (even when not very accurate, like in many other open source projects) gives you some peace of mind.

Peer review

Even when commits are fairly small now in DMD, I think they are far from ideal. Is very common to see unrelated changes in a commit (the classic example is the compiler version number being bumped in an bug fix). See revision 214 for example: the compiler version is bumped and there are some changes to the new JSON output, totally unrelated to bug 3401, which is supposed to fix; or revision 213, which announces the release of DMD 1.050 and DMD 2.035, introducing a bunch of changes that who knows what are supposed to do (well, they look like the introduction of the new type T[new], but that's not even documented in the release changelog :S). This is bad for several reasons:

  • Reviewing a patch with unrelated changes is hard.
  • If you want to fold in a individual patch (let's say, LDC guys want to fold a bug fix), you have a lot of junk to take care of.
  • If you want to do some sort of bisection to find a regression, you still have to figure out which is the group of related changes that introduced the regression.

I'm sure there are more...

Commit messages lacks a good description of the problem and the solution. Most commit messages in DMD are "bugzilla N". You have to go to the bugzilla bug to know what's all about. For example, Don's patches usually comes with very good and juicy information about the bug causes and why the patch fixes it (see an example). That is a good commit message. You can learn a lot about the code by reading well commented patches, which can lead to more contributions in the future.

Commits in Phobos can be even worse. The commits with a message "bugzilla N" are usually the good ones. There are 56 commits that have "minor" as the commit message. Yes, just "minor". That's pretty useless, it's very hard to review a patch when you don't know what is supposed to do. Commit messages are the base of peer reviewing, and peer reviewing is the base for high quality code.

So I think that D developers should focus a lot more in commit message. I know it can sound silly at first, but I think I would be a huge gain with too little effort.

Besides this, commits should be mailed to a newsgroup or mailing list to easy peer review. Now it's a little hard to make comments about a commit, you have to post the comment in the D newsgroup or make the comment by personal e-mail to the author. The former is not that bad but it's not easy to include context and people reading the comment will probably have to open a browser and search for the commented commit. This clearly make peer reviewing more difficult when the ideal would be to encourage it. The private mail is simply wrong because other people can't see the comments.

Source control and versioning

This one is tightly related to the previous two topics. Using a good DVCS can make help a lot too. Subversion has a lot of problems with branching, which makes releases harder too (as having a branch for each release is very painful). Is bad for commit messages too, because there is no real difference in branches and directories, so know every commit is duplicated (both changes for DMD 1 and 2 are included). It's not easy to cherry-pick single commits either, and you can't fix you commits if you messed up, which leads to a lot of commits of the style "Woops! Fix the typo in the previous commit.".

I'm sure both the release process and peer reviewing can be greatly improved by using a better DVCS.

Easy branching can also lead to a more fast evolving and reliable language. Yes, both are possible with branches. Now there are 2 branches: stable (D1) and experimental (D2). D1 is almost frozen and people is seeing less and less interest on it as it goes old, and D2 is too unstable for real use. Having some intermediate can be really helpful. For example, it has been announced that the concurrency model proposed by Bartosz Milewski will be not part of D2 because there is not enough time to implement it, since D2 should be release fairly soon as Andrei Alexandrescu is writing a book that has a deadline and the language has to be finalized by the time the book is published.

So concurrency (as AST macros) are delayed to D3. D2 is more than 2 years old, so one should expect that D3 will be not available in less than 5 years from now (assuming D2 would take 2.5 years and D3 would take the same). This might be too much time.

I think the language should adopt a model closer to Python, where a minor language version (with backward compatible improvements) is release every 1 ~ 1.5 years. Last mayor version took about 8 years, but considering how many new features Python included in minor versions that's not a big issue. The last mayor version was mostly a clean up of old stuff/nasty stuff, not huge changes to the language.


I think the DMD back-end should have a better license. Personal use is simply not enough for a reference implementation of a language that wants to hit mainstream. If you plan to do business with it, not being able to patch the compiler if you need to and distribute it is not an option.

This is for the sake of DMD only, because other compilers (like LDC and GDC) are fully free/libre.


Some of the things I mention are really hard to change, as they modify how people work and imply learning new tools. But other are fairly easy, and can be done progressively (like providing release candidates and improving commits and commit messages).

I hope Walter Bright & Co. keep walking the openness road =)

Understanding the current GC, conclusion

by Leandro Lucarella on 2009- 04- 11 16:36 (updated on 2009- 04- 11 16:36)
tagged book, conclusion, d, dgc, druntime, en, gc, mark-sweep, understanding the current gc - with 0 comment(s)

Now that I know fairly deeply the implementation details about the current GC, I can compare it to the techniques exposed in the GC Book.

Tri-colour abstraction

Since most literature speaks in terms of the tri-colour abstraction, now it's a good time to translate how this is mapped to the D GC implementation.

As we all remember, each cell (bin) in D has several bits associated to them. Only 3 are interesting in this case:

  • mark
  • scan
  • free (freebits)

So, how we can translate this bits into the tri-colour abstraction?


Cells that were marked and scanned (there are no pointer to follow) are coloured black. In D this cells has the bits:

mark = 1
scan = 0
free = 0

Cells that has been marked, but they have pointers to follow in them are coloured grey. In D this cells has the bits:

mark = 1
scan = 1
free = 0

Cells that has not been visited at all are coloured white (all cells should be colored white before the marking starts). In D this cells has the bits:

mark = 0
scan = X


free = 1

The scan bit is not important in this case (but in D it should be 0 because scan bits are cleared before the mark phase starts). The free bit is used for the cells in the free list. They are marked before other cells get marked with bits mark=1 and free=1. This way the cells in the free list don't get scanned (mark=1, scan=0) and are not confused with black cells (free=1), so they can be kept in the free list after the mark phase is done. I think this is only necessary because the free list is regenerated.


Here is a summary of improvements proposed by the GC Book, how the current GC is implemented in regards to this improvements and what optimization opportunities can be considered.

Mark stack

The simplest version of the marking algorithm is recursive:

    if not cell.marked
        cell.marked = true
        for child in cell.children

The problem here is, of course, stack overflow for very deep heap graphs (and the space cost).

The book proposes using a marking stack instead, and several ways to handle stack overflow, but all these are only useful for relieving the symptom, they are not a cure.

As a real cure, pointer reversal is proposed. The idea is to use the very same pointers to store the mark stack. This is constant in space, and needs only one pass through the help, so it's a very tempting approach. The bad side is increased complexity and probably worse cache behavior (writes to the heap dirties the entire heap, and this can kill the cache).

Current implementation

The D GC implementation does none of this. Instead it completes the mark phase by traversing the heap (well, not really the heap, only the bit sets) in several passes, until no more data to scan can be found (all cells are painted black or white). While the original algorithm only needs one pass through the heap, this one need several. This trades space (and the complexity of stack overflow handling) for time.

Optimization opportunities

This seems like a fair trade-off, but alternatives can be explored.

Bitmap marking

The simplest mark-sweep algorithm suggests to store marking bits in the very own cells. This can be very bad for the cache because a full traversal should be done across the entire heap. As an optimization, a bitmap can be used, because they are much small and much more likely to fit in the cache, marking can be greatly improved using them.

Current implementation

Current implementation uses bitmaps for mark, scan, free and other bits. The bitmap implementation is GCBits and is a general approach.

The bitmap stores a bit for each 16 bytes chunks, no matter what cell size (Bins, or bin size) is used. This means that 4096/16 = 256 bits (32 bytes) are used for each bitmap for every page in the GC heap. Being 5 bitmaps (mark, scan, freebits, finals and noscan), the total spaces per page is 160 bytes. This is a 4% space overhead in bits only.

This wastes some space for larger cells.

Optimization opportunities

The space overhead of bitmaps seems to be fairly small, but each byte counts for the mark phase because of the cache. A heap with 64 MiB uses 2.5 MiB in bitmaps. Modern processors come with about that much cache, and a program using 64 MiB doesn't seems very rare. So we are pushing the limits here if we want our bitmaps to fit in the cache to speed up the marking phase.

I think there is a little room for improvement here. A big object, lets say it's 8 MiB long, uses 640 KiB of memory for bitmaps it doesn't need. I think some specialized bitmaps can be used for large object, for instance, to minimize the bitmaps space overhead.

There are some overlapping bits too. mark=0 and scan=1 can never happen for instance. I think it should be possible to use that combination for freebits, and get rid of an entire bitmap.

Lazy sweep

The sweep phase is done generally right after the mark phase. Since normally the collection is triggered by an allocation, this can be a little disrupting for the thread that made that allocation, that has to absorb all the sweeping itself.

Another alternative is to do the sweeping incrementally, by doing it lazy. Instead of finding all the white cells and linking them to the free list immediately, this is done on each allocation. If there is no free cells in the free list, a little sweeping is done until new space can be found.

This can help minimize pauses for the allocating thread.

Current implementation

The current implementation does an eager sweeping.

Optimization opportunities

The sweeping phase can be made lazy. The only disadvantage I see is (well, besides extra complexity) that could make the heap more likely to be fragmented, because consecutive requests are not necessarily made on the same page (a free() call can add new cells from another page to the free list), making the heap more sparse, (which can be bad for the cache too). But I think this is only possible if free() is called explicitly, and this should be fairly rare in a garbage collected system, so I guess this could worth trying.

Lazy sweeping helps the cache too, because in the sweep phase, you might trigger cache misses when linking to the free list. When sweeping lazily, the cache miss is delayed until it's really necessary (the cache miss will happen anyway when you are allocating the free cell).


Even when the current GC is fairly optimized, there is plenty of room for improvements, even preserving the original global design.

Understanding the current GC, the end

by Leandro Lucarella on 2009- 04- 11 01:46 (updated on 2009- 04- 15 01:10)
tagged d, dgc, druntime, en, gc, mark, mark-sweep, sweep, understanding the current gc - with 0 comment(s)

In this post I will take a closer look at the Gcx.mark() and Gcx.fullcollect() functions.

This is a simplified version of the mark algorithm:

mark(from, to)
    changes = 0
    while from < to
        pool = findPool(from)
        offset = from - pool.baseAddr
        page_index = offset / PAGESIZE
        bin_size = pool.pagetable[page_index]
        bit_index = find_bit_index(bin_size, pool, offset)
        if not pool.mark.test(bit_index)
            if not pool.noscan.test(bit_index)
                changes = true
        anychanges |= changes // anychanges is global

In the original version, there are some optimizations and the find_bit_index() function doesn't exist (it does some bit masking to find the right bit index for the bit set). But everything else is pretty much the same.

So far, is evident that the algorithm don't mark the whole heap in one step, because it doesn't follow pointers. It just marks a consecutive chunk of memory, assuming that pointers can be at any place in that memory, as long as they are aligned (from increments in word-sized steps).

fullcollect() is the one in charge of following pointers, and marking chunks of memory. It does it in an iterative way (that's why mark() informs about anychanges (when new pointer should be followed to mark them, or, speaking in the tri-colour abstraction, when grey cells are found).

fullcollect() is huge, so I'll split it up in smaller pieces for the sake of clarity. Let's see what are the basic blocks (see the second part of this series):

    thread_scanAll(mark, stackTop)

Generaly speaking, all the functions that have some CamelCasing are real functions and the ones that are all_lowercase and made up by me.

Let's see each function.

This is part of the threads runtime (found in src/common/core/thread.d). A simple peak at it shows it uses SIGUSR1 to stop the thread. When the signal is caught it pushes all the registers into the stack to be sure any pointers there are scanned in the future. The threads waits for SIGUSR2 to resume.
foreach pool in pooltable
foreach n in B_16 .. B_PAGE
    foreach node in bucket
        pool = findPool(node)
        pool.freebits.set(find_bit_index(pool, node))
        pool.mark.set(find_bit_index(pool, node))
This function, as the name suggests, uses the provided mark function callback to scan the program's static data.
thread_scanAll(mark, stackTop)
This is another threads runtime function, used to mark the suspended threads stacks. I does some calculation about the stack bottom and top, and calls mark(bottom, top), so at this point we have marked all reachable memory from the stack(s).
mark(roots, roots + nroots)
foreach range in ranges
    mark(range.pbot, range.ptop)

This is where most of the marking work is done. The code is really ugly, very hard to read (mainly because of bad variable names) but what it does it's relatively simple, here is the simplified algorithm:

// anychanges is global and was set by the mark()ing of the
// stacks and root set
while anychanges
    anychanges = 0
    foreach pool in pooltable
        foreach bit_pos in pool.scan
            if not pool.scan.test(bit_pos)
            pool.scan.clear(bit_pos) // mark as already scanned
            bin_size = find_bin_for_bit(pool, bit_pos)
            bin_base_addr = find_base_addr_for_bit(pool, bit_pos)
            if bin_size < B_PAGE // small object
                bin_top_addr = bin_base_addr + bin_size
            else if bin_size in [B_PAGE, B_PAGEPLUS] // big object
                page_num = (bin_base_addr - pool.baseAddr) / PAGESIZE
                if bin == B_PAGEPLUS // search for the base page
                    while pool.pagetable[page_num - 1] != B_PAGE
                n_pages = 1
                while page_num + n_pages < pool.ncommitted
                        and pool.pagetable[page_num + n_pages] == B_PAGEPLUS
                bin_top_addr = bin_base_addr + n_pages * PAGESIZE
            mark(bin_base_addr, bin_top_addr)

The original algorithm has some optimizations for proccessing bits in clusters (skips groups of bins without the scan bit) and some kind-of bugs too.

Again, the functions in all_lower_case don't really exist, some pointer arithmetics are done in place for finding those values.

Note that the pools are iterated over and over again until there are no unvisited bins. I guess this is a fair price to pay for not having a mark stack (but I'm not really sure =).

This is, again, part of the threads runtime and resume all the paused threads by signaling a SIGUSR2 to them.

This (invented) function looks for unmarked bins and set the freebits bit on them if they are small objects (bin size smaller than B_PAGE) or mark the entire page as free (B_FREE) in case of large objects.

This step is in charge of executing destructors too (through rt_finalize() the runtime function).


This (also invented) function first clear the free list (bucket) and then rebuild it using the information collected in the previous step.

As usual, only bins with size smaller than B_PAGE are linked to the free list, except if the pages they belong to have all the bins freed, in which case the page is marked with the special B_FREE bin size. The same goes for big objects freed in the previous step.

I think rebuilding the whole free list is not necessary, the new free bins could be just linked to the existing free list. I guess this step exists to help reducing fragmentation, since the rebuilt free list group bins belonging to the same page together.

Understanding the current GC, part IV

by Leandro Lucarella on 2009- 04- 10 18:33 (updated on 2009- 04- 10 18:33)
tagged d, dgc, druntime, en, freeing, gc, mark-sweep, reallocation, understanding the current gc - with 0 comment(s)

What about freeing? Well, is much simpler than allocation =) is a thread-safe wrapper for GC.freeNoSync(ptr).

GC.freeNoSync(ptr) gets the Pool that ptr belongs to and clear its bits. Then, if ptr points to a small object (bin size smaller than B_PAGE), it simply link that bin to the free list (Gcx.bucket). If ptr is a large object, the number of pages used by the object is calculated then all the pages marked as B_FREE (done by Pool.freePages(start, n_pages)).

Then, there is reallocation, which is a little more twisted than free, but doesn't add much value to the analysis. It does what you think it should (maybe except for a possible bug) using functions already seen in this post or in the previous ones.

Understanding the current GC, part III

by Leandro Lucarella on 2009- 04- 10 02:28 (updated on 2009- 04- 10 02:28)
tagged allocation, d, dgc, druntime, en, gc, mark-sweep, understanding the current gc - with 0 comment(s)

In the previous post we focused on the Gcx object, the core of the GC in druntime (and Phobos and Tango, they are all are based on the same implementation). In this post we will focus on allocation, which a little more complex than it should be in my opinion.

It was not an easy task to follow how allocation works. A GC.malloc() call spawns into this function calls:

GC.malloc(size, bits)
 '---> GC.mallocNoSync(size, bits)
        |---> Gcx.allocPage(bin_size)
        |      |
        |      '---> Pool.allocPages(n_pages)
        |             |
        |             '---> Pool.extendPages(n_pages)
        |                    |
        |                    '---> os_mem_commit(addr, offset, size)
        |---> Gcx.fullcollectshell()
        |---> Gcx.newPool(n_pages)
        |      |
        |      '---> Pool.initialize(n_pages)
        |             |
        |             |---> os_mem_map(mem_size)
        |             |
        |             '---> GCBits.alloc(bits_size)
        '---> Gcx.bigAlloc(size)
               |---> Pool.allocPages(n_pages)
               |      '---> (...)
               |---> Gcx.fullcollectshell()
               |---> Gcx.minimize()
               |      |
               |      '---> Pool.Dtor()
               |             |
               |             |---> os_mem_decommit(addr, offset, size)
               |             |
               |             |---> os_mem_map(addr, size)
               |             |
               |             '---> GCBits.Dtor()
               '---> Gcx.newPool(n_pages)
                      '---> (...)

Doesn't look so simple, ugh?

The map/commit differentiation of Windows doesn't exactly help simplicity. Note that Pool.initialize() maps the memory (reserve the address space) while Pool.allocPages() (through Pool.extendPages()) commit the new memory (ask the OS to actually reserve the virtual memory). I don't know how good is this for Windows (or put in another way, how bad could it be for Windows if all mapped memory gets immediately committed), but it adds a new layer of complexity (that's not even needed in Posix OSs). The whole branch starting at Gcx.allocPage(bin_size) would be gone if this distinction it's not made. Besides this, it worsen Posix OSs performance, because there are some non-trivial lookups to handle this non-existing non-committed pages, even when the os_mem_commit() and os_mem_decommit() functions are NOP and can be optimized out, the lookups are there.

Mental Note

See if getting rid of the commit()/decommit() stuff improves Linux performance.

But well, let's forget about this issue for now and live with it. Here is a summary of what all this functions do.


I recommend to give another read to the (updated) previous posts of this series, specially if you are not familiar with the Pool concept and implementation.

GC.malloc(size, bits)
This is just a wrapper for multi-threaded code, it takes the GCLock if necessary and calls GC.mallocNoSync(size, bits).
GC.mallocNoSync(size, bits)

This function has 2 different algorithms for small objects (less than a page of 4KiB) and another for big objects.

It does some common work for both cases, like logging and adding a sentinel for debugging purposes (if those feature are enabled), finding the bin size (bin_size) that better fits size (and cache the result as an optimization for consecutive calls to malloc with the same size) and setting the bits (NO_SCAN, NO_MOVE, FINALIZE) to the allocated bin.

Small objects (bin_size < B_PAGE)
Looks at the free list (Gcx.bucket) trying to find a page with the minimum bin size that's equals or bigger than size. If it can't succeed, it calls Gcx.allocPage(bin_size) to find room in uncommitted pages. If there still no room for the requested amount of memory, it triggers a collection (Gcx.fullcollectshell()). If there is still no luck, Gcx.newPage(1) is called to ask the OS for more memory. Then it calls again Gcx.allocPage(bin_size) (remember the new memory is just mmap'ped but not commit'ed) and if there is no room in the free list still, an out of memory error is issued.
Big objects (B_PAGE and B_PAGEPLUS)
It simply calls Gcx.bigAlloc(size) and issue an out of memory error if that call fails to get the requested memory.
This function linearly search the pooltable for a Pool with an allocable page (i.e. a page already mapped by not yet committed). This is done through a call to Pool.allocPages(1). If a page is found, its bin size is set to bin_size via the Pool's pagetable, and all the bins of that page are linked to the free list (Gcx.bucket).
Search for n_pages consecutive free pages (B_FREE) in the committed pages (pages in the pagetable with index up to ncommited). If they're not found, Pool.extendPages(n_pages) is called to commit some more mapped pages to fulfill the request.
Commit n_pages already mapped pages (calling os_mem_commit()), setting them as free (B_FREE) and updating the ncommited attribute. If there are not that many uncommitted pages, it returns an error.
This function adds a new Pool to the pooltable. It first adjusts the n_pages variable using various rules (for example, it duplicates the current allocated memory until 8MiB are allocated and then allocates 8MiB pools always, unless more memory is requested in the first place, of course). Then a new Pool is created with the adjusted n_pages value and it's initialized calling to Pool.initialize(n_pages), the pooltable is resized to fit the new number of pools (npools) and sorted using Pool.opCmp() (which uses the baseAddr to compare). Finally the minAddr and maxAddr attributes are updated.
Initializes all the Pool attributes, mapping the requested number of pages (n_pages) using os_mem_map(). All the bit sets (mark, scan, freebits, noscan) are allocated (using GCBits.alloc()) to n_pages * PAGESIZE / 16 bits and the pagetable too, setting all bins to B_UNCOMMITTED and ncommitted to 0.

This is the weirdest function by far. There are very strange things, but I'll try to explain what I understand from it (what I think it's trying to do).

It first make a simple lookup in the pooltable for n_pages consecutive pages in any existing Pool (calling Pool.allocPages(n_pages) as in Gcx.allocPage()). If this fails, it runs a fullcollectshell() (if not disabled) then calls to minimize() (to prevent bloat) and then create a new pool (calling newPool() followed by Pool.allocPages()). If all that fails, it returns an error. If something succeed, the bin size for the first page is set to B_PAGE and the remaining pages are set to B_PAGEPLUS (if any). If there is any unused memory at the end, it's initialized to 0 (to prevent false positives when scanning I guess).

The weird thing about this, is that a lot of lookups into the pooltable are done in certain condition, but I think they are not needed because there are no changes that can make new room.

I don't know if this is legacy code that never got updated and have a lot of useless lookups or if I'm getting something wrong. Help is welcome!

There is not much to say about os_mem_xxx(), Gcx.minimize() and Gcx.fullcollectshell() functions, they were briefly described in the previous posts of this series. Pool.Dtor() just undo what was done in Pool.initialize().

A final word about the free list (Gcx.bucket). It's just a simple linked list. It uses the first size_t bytes of the free bin to point to the next free bin (there's always room for a pointer in a bin because their minimum size is 16 bytes). A simple structure is used to easy this:

struct List {
    List *next;

Then, the memory cell is casted to this structure to use the next pointer, like this:

p = gcx.bucket[bin]
gcx.bucket[bin] = (cast(List*) p).next

I really have my doubts if this is even a little less cryptic than:

p = gcx.bucket[bin]
gcx.bucket[bin] = *(cast(void**) p)

But what the hell, this is no really important =)

Understanding the current GC, part II

by Leandro Lucarella on 2009- 04- 05 21:00 (updated on 2009- 04- 15 01:10)
tagged d, dgc, druntime, en, gc, gcx, mark-sweep, understanding the current gc - with 0 comment(s)

Back to the analysis of the current GC implementation, in this post I will focus on the Gcx object structure and methods.

Gcx attributes

Root set
roots (nroots, rootdim)
An array of root pointers.
ranges (nranges, rangedim)
An array of root ranges (a range of memory that should be scanned for root pointers).
Beginning of the stack (stackBottom)
A pointer to the stack bottom (assuming it grows up).
Pool table (pooltable, npools)
An array of pointers to Pool objects (the heap itself).
Free list (bucket)
A free list for each Bins size.
Internal state
Set if the marking of a range has actually marked anything (and then using in the full collection.
Set if the GC has been initialized.
Behaviour changing attributes
Don't scan the stack if activated.
Turn on logging if activated.
Don't run the collector if activated.
Cache (for optimizations and such)
p_cache, size_cache
Querying the size of a heap object is an expensive task. This caches the last query as an optimization.
minAddr, maxAddr
All the heap is in this range. It's used as an optimization when looking if a pointer can be pointing into the heap (if the pointer is not in this range it can be safely discarded, but if it's in the range, a full search in the pooltable should be done).

Gcx main methods

Initialization, set the Gcx object attributes to 0, except for the stackBottom (which is set to the address of a dummy local variable, this works because this function is one of the first functions called by the runtime) and the inited flag, that is set to 1. The log is initialized too.
Destruction, free all the memory.
Root set manipulation
addRoot(p), removeRoot(p), rootIter(dg)
Add, remove and iterate over single root pointers.
addRange(pbot, ptop), remove range(pbot), rangeIter(dg)
Add, remove and iterate over root pointer ranges. This methods are almost the same as the previous ones, so the code duplication here can be improved here.
Flags manipulation

Each Bin has some flags associated (as explained before). With this functions the user can manipulate some of them:

  • FINALIZE: this pool has destructors to be called (final flag)
  • NO_SCAN: this pool should not be scanned for pointers (noscan flag)
  • NO_MOVE: this pool shouldn't be moved (not implemented)
getBits(pool, biti)
Get which of the flags specified by biti are set for the pool Pool.
setBits(pool, mask)
Set the flags specified by mask for the pool Pool.
clrBits(pool, mask)
Clear the flags specified by mask for the pool Pool.
Find the Pool object that pointer p is in.
Find the base address of block containing pointer p.
Find the size of the block pointed by p.
Get information on the pointer p. The information is composed of: base (the base address of the block), size (the size of the block) and attr (the flags associated to the block, as shown in Flag manipulation). This information is returned as a structure called the BlkInfo.
Compute Bins (bin size) for an object of size size.
Heap (pagetable) manipulation

The pooltable is kept sorted always.

Allocate a new Pool of at least size bytes.
Minimizes physical memory usage by returning free pools to the OS.
Allocate a chunk of memory that is larger than a page.
Allocate a new Pool with at least npages pages in it.
Allocate a page of bin size.
mark(pbot, ptop)

This is the mark phase. It search a range of memory values and mark any pointers into the GC heap. The mark bit is set, and if the noscan bit is unset, the scan bit is activated (indicating that the block should be scanned for pointers, equivalent to coloring the cell grey in the tri-colour abstraction).

The mark phase is not recursive (nor a mark stack is used). Only the passed range is marked, pointers are not followed here.

That's why the anychanges flag is used, if anything has got marked, anychanges is set to true. The marking phase is done iteratively until no more blocks are marked, in which case we can safely assume that we marked all the live blocks.

The purpose of the shell is to ensure all the registers get put on the stack so they'll be scanned.

Collect memory that is not referenced by the program. The algorithm is something like this:

  1. Stop the world (all other threads)
  2. Clear all the mark/scan bits in the pools
  3. Manually mark each free list entry (bucket), so it doesn't get scanned
  4. mark() the static data
  5. mark() stacks and registers for each paused thread
  6. mark() the root set (both roots and ranges)
  7. mark() the heap iteratively until no more changes are detected (anychanges is false)
  8. Start the world (all other threads)
  9. Sweep (free up everything not marked)
  10. Free complete pages, rebuild free list


This is a very summarized version of the algorithm, what I could understand from a quick look into the code, which is pretty much undocumented. A deeper analysis should be done in a following post.

Understanding the current GC

by Leandro Lucarella on 2009- 01- 04 18:37 (updated on 2009- 04- 09 19:53)
tagged bin, d, dgc, druntime, en, gc, intro, mark-sweep, pool, understanding the current gc - with 1 comment(s)

Oh, yeah! A new year, a new air, and the same thesis =)

After a little break, I'm finally starting to analyze the current D (druntime) GC (basic) implementation in depth.

First I want to say I found the code really, but really, hard to read and follow. Things are split in several parts without apparent reason, which make it really hard to understand and it's pretty much undocumented.

I hope I can fully understand it in some time to be able to make a full rewrite of it (in a first pass, conserving the main design).


I'll start with a big picture overview, and then I'll try to describe each component with more detail.

The implementation in split in several files:

I didn't took a look at this one yet, but I guess it's about stats =).
A custom bitset implementation for collector bit/flags (mark, scan, etc.).
A wrapper for memory allocation with several versions (malloc, win32, mmap and valloc). 4 functions are provided: map, unmap, commit and decommit. The (de)commit stuff if because (in Sean Kelly's words) Windows has a 2-phase allocation process. You can reserve the address space via map and unmap, but the virtual memory isn't actually created until you call commit. So decommit gets rid of the virtual memory but retains ownership of the address space.
The real GC implementation, split in 2 main classes/structs: GC and Gcx. GC seems to be a thin wrapper over Gcx that only provides the allocation logic (alloc/realloc/free) and Gcx seems to be the responsible for the real GC work (and holding the memory).
This is just a thin wrapper over gcx.d to adapt it to the druntime GC interface.

The Gcx struct is where most magic happens. It holds the GC memory organized in pools. It holds the information about roots, the stack and free list, but in this post I'll focus in the memory pools:

Pool Concept

A pool is a group of pages, each page has a bin size (Bins) and host a fixed number of bins (PAGESIZE / Bins, for example, if Bins == 1024 and PAGESIZE == 4096, the page holds 4 bins).

Each bin has some bits of information:

Setted when the Bin is visited by the mark phase.
Setted when the Bin is has been visited by the mark phase (the mark bit is set) but it has pointers yet to be scanned.
Setted when the Bin is free (linked to a free list).
The object stored in this bin has a destructor that must be called when freed.
This bin should be not scanned by the collector (it has no pointers).
| Page 0 (bin size: Bins)                | ... | Page (npages-1) |
|                                        |     |                 |
| +--------+-----+---------------------+ |     |                 |
| | Bin 0  | ... | Bin (PAGESIZE/Bins) | |     |                 |
| +--------+-----+---------------------+ |     |                 |
| | mark   | ... |                     | |     |                 |
| | scan   | ... |                     | |     |       ...       |
| | free   | ... |         ...         | |     |                 |
| | final  | ... |                     | |     |                 |
| | noscan | ... |                     | |     |                 |
| +--------+-----+---------------------+ |     |                 |

Pool Implementation

A single chunk of memory is allocated for the whole pool, the baseAddr points to the start of the chunk, the topAddr, to the end. A pagetable holds the bin size (Bins) of each page

.          ,-- baseAddr                                   topAddr --,
           |                   ncommitted = i                       |
           |                                                        |
           |--- committed pages ---,------ uncommitted pages -------|
           V                       |                                V
    memory | Page 0 | Page 1 | ... | Page i | ... | Page (npages-1) |
               /\       /\      /\     /\      /\          /\
               ||       ||      ||     ||      ||          ||
 pagetable | Bins 0 | Bins 1 | ... | Bins i | ... | Bins (npages-1) |
(bin size) +--------+--------+-----+--------+-----+-----------------+

The bin size can be one of:

The XXX is a power of 2 from 16 to 4096. The special name B_PAGE is used for the size 4096.
The whole page is a continuation of a large object (the first page of a large object has size B_PAGE).
The page is completely free.
The page is not committed yet.
Not really a value, used for iteration or allocation. Pages can't have this value.

The information bits are stored in a custom bit set (GCBits). npages * PAGESIZE / 16 bits are allocated (since the smallest bin is 16 bytes long) and each bit is addressed using this formula:

bit(pointer) = (pointer - baseAddr) / 16

This means that a bit is reserved each 16 bytes. For large bin sizes, a lot of bits are wasted.

The minimum pool size is 256 pages. With 4096 bytes pages, that is 1 MiB.

The GCBits implementation deserves another post, it's a little complex and I still don't understand why.

druntime developers FAQ

by Leandro Lucarella on 2008- 12- 05 09:38 (updated on 2008- 12- 05 09:38)
tagged d, dgc, druntime, en, faq - with 0 comment(s)

I've compiled some of the questions I asked about druntime to Sean Kelly and added them to a (really) small FAQ page in the wiki.

Improved druntime getting started documentation

by Leandro Lucarella on 2008- 12- 02 21:07 (updated on 2008- 12- 02 21:07)
tagged d, dgc, druntime, en, howto - with 0 comment(s)

I've expanded the druntime Getting Started documentation. I basically added all the information I've posted in this blog so far: how to change the GC implementation and rebuild phobos.

Testing druntime modifications

by Leandro Lucarella on 2008- 11- 30 01:51 (updated on 2008- 11- 30 01:51)
tagged d, dgc, druntime, en, howto, phobos - with 0 comment(s)

Now that we can compile druntime, we should be able to compile some programs that use our fresh, modified, druntime library.

Since DMD 2.021, druntime is built into phobos, so if we want to test some code we need to rebuild phobos too, to include our new druntime.

Since I'm particularly interested in the GC, let's say we want to use the GC stub implementation (instead of the basic default).

We can add a simple "init" message to see that something is actually happening. For example, open src/gc/stub/gc.d and add this import:

private import core.sys.posix.unistd: write;

Then, in the gc_init() function add this line:

write(1, "init\n".ptr, 5);

Now, we must tell druntime we want to use the stub GC implementation. Edit dmd-posix.mak, search for DIR_GC variable and change it from basic to stub:


Great, now recompile druntime.

Finally, go to your DMD installation, and edit src/phobos/linux.mak. Search for the DRUNTIME variable and set it to the path to your newly generated libdruntime.a (look in the druntime lib directory). For me it's something like:


Now recompile phobos. I have to do this, because my DMD compiler is named dmd2:

make -f linux.mak DMD=dmd2

Now you can compile some trivial D program (compile it in the src druntime directory so its dmd.conf is used to search for the libraries and imports) and see how "init" get printed when the program starts. For example:

druntime/src$ cat hello.d

import core.sys.posix.unistd: write;

void main()
    write(1, "hello!\n".ptr, 7);

druntime/src$ dmd2 -L-L/home/luca/tesis/dmd2/lib hello.d
druntime/src$ ./hello

Note that I passed my DMD compiler's lib path so it can properly find the newly created libphobos2.a.

druntime build system

by Leandro Lucarella on 2008- 11- 26 00:28 (updated on 2008- 11- 26 00:28)
tagged build system, d, dgc, druntime, en, patch - with 0 comment(s)

I have to be honest on this one. I'm not crazy about the druntime build system. I know there are a lot of other more important thing to work on, but I can't help myself, and if I'm not comfortable with the build system, I get too much distracted, so I have no choice but to try to improve it a little =)

First, I don't like the HOME environment variable override hack in (I won't talk about the Windows build because I don't have Windows, so I can't test it).

So I've made a simple patch to tackle this. It just adds a dmd.conf configuration file in each directory owning a makefile. I think it's a fair price to pay adding this extra files to be hable to just use make and get rid of the script.

I've added a ticket on this and another related ticket with a patch too.

Getting started with druntime

by Leandro Lucarella on 2008- 11- 25 00:27 (updated on 2008- 11- 25 00:27)
tagged d, dgc, druntime, en, howto - with 0 comment(s)

I've added a brief draft about how to get started in the druntime wiki, which I plan to expand a little in the future.

I hope somebody find it useful.

BTW, the -version=Posix fix is now included in the main repo.

My druntime repository

by Leandro Lucarella on 2008- 11- 24 00:17 (updated on 2008- 11- 24 00:17)
tagged d, dgc, druntime, en, git, repository - with 0 comment(s)

I've finally published my own git druntime repository. It has both branches, the one for D2 (the svn trunk, called master in my repo) and the one for D1 (D1.0 in svn, d1 in my repo).

For now, there are only changes in the master branch.

Hacking druntime

by Leandro Lucarella on 2008- 11- 22 14:38 (updated on 2009- 03- 28 17:17)
tagged d, dgc, druntime, en, howto, patch - with 0 comment(s)

I've been reading the source code of the druntime, and it's time to get my hands dirty and do some real work.

First I have to do to start hacking it is build it and start trying things out. There is no documentation at all yet, so I finally bothered Sean Kelly and asked him how to get started.

Here is what I had to do to get druntime compiled:

First of all, I'll introduce my environment and tools. I'll use DMD because there's no other option for now (druntime doesn't have support for GDC, but Sean says it's coming soon, and LDC will not be included until the support it's added to Tango runtime).

The trunk in the druntime repository is for D2, but there is a branch for D1 too.

I use Debian (so you'll see some apt stuff here) and I love git, and there's is no way I will go back to subversion. Fortunately there is git-svn, so that's what I'm gonna use =)

Now, what I did step by step.

  1. Get the git goodies:

    sudo aptitude install git-core git-svn
  2. Make a directory where to put all the D-related stuff:

    mkdir ~/d
    cd ~/d
  3. Get D2 (bleeding edge version) and unpack it:

    unzip # "install" the D2 compiler
    rm -fr dm dmd/linux/bin/{sc.ini,readme.txt,*.{exe,dll,hlp}} # cut the fat
    chmod a+x dmd/linux/bin/{dmd,rdmd,dumpobj,obj2asm} # make binaries executable
    mv dmd dmd2 # reserve the dmd directory for D1 compiler
  4. Make it accessible, for example:

    echo '#!/bin/sh' > ~/bin/dmd2 # reserve dmd name for the D1 compiler
    echo 'exec ~/d/dmd2/linux/bin/dmd "$@"' >> ~/bin/dmd2
    chmod a+x ~/bin/dmd2
  5. Get D1 and install it too:

    rm -fr dm dmd/linux/bin/{sc.ini,readme.txt,*.{exe,dll,hlp}}
    chmod a+x dmd/linux/bin/{dmd,rdmd,dumpobj,obj2asm}
    echo '#!/bin/sh' > ~/bin/dmd
    echo 'exec ~/d/dmd/linux/bin/dmd "$@"' >> ~/bin/dmd
    chmod a+x ~/bin/dmd
  6. Get druntime for D1 and D2 as separated repositories (you can get all in one git repository using git branches but since I'll work on both at the same time I prefer to use two separated repositories):

    git svn clone \
    git svn clone druntime2
  7. Build druntime for D1:

    cd druntime
    cd -
  8. Build druntime for D2.

    This one is a little trickier. The trunk version have some changes for a feature that is not yet released (this being changed from a pointer to a reference for structs). Fortunately this is well isolated in a single commit, so reverting this change is really easy, first, get the abbreviated hash for the commit 44:

    cd druntime2
    git log --grep='trunk@44' --pretty=format:%h

    This should give you a small string (mine is cae2326). Now, revert that change:

    git revert cae2326

    Done! You now have that change reverted, we can remove this new commit later when the new version of DMD that implements the this change appear.

    But this is not all. Then I find a problem about redefining the Posix version:

    Error: version identifier 'Posix' is reserved and cannot be set

    To fix this you just have to remove the -version=Posix from

    But there is still one more problem, but this is because I have renamed the bianries to have both dmd and dmd2. The compiler we have to use to build things is called dmd2 for me, but don't override properly the DC environment variable when calling make, so dmd is used instead.

    This is a simple and quick fix:

    diff --git a/src/ b/src/
    old mode 100644
    new mode 100755
    index d6be599..8f3b163
    --- a/src/
    +++ b/src/
    @@ -11,9 +11,10 @@ goerror(){
         exit 1
    -make clean -fdmd-posix.mak           || goerror
    -make lib doc install -fdmd-posix.mak || goerror
    -make clean -fdmd-posix.mak           || goerror
    +test -z "$DC" && DC=dmd
    +make DC=$DC clean -fdmd-posix.mak           || goerror
    +make DC=$DC lib doc install -fdmd-posix.mak || goerror
    +make DC=$DC clean -fdmd-posix.mak           || goerror
     chmod 644 ../import/*.di             || goerror
     export HOME=$OLDHOME

    (to apply the patch just copy&paste it to fix.patch and then do git apply fix.patch; that should do the trick)

    Now you can do something like this to build druntime for D2:

    export DC=dmd2

That's it for now. I'll be publishing my druntime (git) repository soon with this changes (and probably submitting some patches to upstream) so stay tuned ;)