Potential improvement in compacting GC

Thu Sep 2 15:47:08 UTC 2021

Here's another improvement that fixes a very old issue in GHC's compacting GC
[1].

To summarize the problem: when untreading an object we update references to the
object that we've seen so far to the object's new location. But to get the
object's new location we need to know the object's size, because depending on
the size we may need to move the object to a new block (when the current block
does not have enough space for the object).

For this we currently walk the thread twice, once to get the info table (used
to get the size), then again to update references to the object. Ideally we
want to do just one pass when unthreading.

The solution is explained in The GC Handbook, section 3.4. Instead of using one
bit to mark an object, we use two bits: one for the first word of the object,
one for the last word. Using two bits is not a problem in GHC because heap
objects are at least 2 words. For example, an object with two words is marked
with `11`, 3 words is marked with `101` and so on.

Now we can scan the bitmap to find object size, and unthread it without having
to find the info table first.

Ömer

[1]: https://github.com/ghc/ghc/blob/922c6bc8dd8d089cfe4b90ec2120cb48959ba2b5/rts/sm/Compact.c#L844-L848

Ömer Sinan Ağacan <omeragacan at gmail.com>, 14 Tem 2021 Çar, 09:27
tarihinde şunu yazdı:
>
> Two other ideas that should improve GHC's compacting GC much more
> significantly. I've implemented both of these in another project and the
> results were great. In a GC benchmark (mutator does almost no work other than
> allocating data using a bump allocator), first one reduced Wasm instructions
> executed by 14%, second one 19.8%.
>
> Both of these ideas require pushing object headers to the mark stack with the
> objects, which means larger mark stacks. This is the only downside of these
> algorithms.
>
> - Instead of marking and then threading in the next pass, mark phase threads
>   all fields when pushing the fields to the mark stack. We still need two other
>   passes: one to unthread headers, another to move the objects. (we can't do
>   both in one pass, let me know if you're curious and I can elaborate)
>
>   This has the same number of passes as the current implementation, but it only
>   visits object fields once. Currently, we visit fields once when marking, to
>   mark fields, then again in `update_fwd`. This implementation does both in one
>   pass over the fields. `update_fwd` does not visit fields.
>
>   This reduced Wasm instructions executed by 14% in my benchmark.
>
> - Marking phase threads backwards pointers (ignores forwards pointers). Then we
>   do one pass instead of two: for a marked object, unthread it (update
>   forwards pointers to the object's new location), move it to its new location,
>   then thread its forwards pointers. This completely eliminates one of the 3
>   passes, but fields need to be visited twice as before (unlike the algorithm
>   above).
>
>   I think this one is originally described in "An Efficient Garbage Compaction
>   Algorithm", but I found that paper to be difficult to follow. A short
>   description of the same algorithm exists in "High-Performance Garbage
>   Collection for Memory-Constrained Environments" in section 5.1.2.
>
>   This reduced Wasm instructions executed by 19.8% in my benchmark.
>
>   In this algorithm, fields that won't be moved can be threaded any time before
>   the second pass (pointed objects need to be marked and pushed to the mark
>   stack with headers before threading a field). For example, in GHC, mut list
>   entries can be threaded before or after marking (but before the second pass)
>   as IIRC mut lists are not moved. Same for fields of large objects.
>
> As far as I can see, mark-compact GC is still the default when max heap size is
> specified and the oldest generation size is (by default) more than 30% of the
> max heap size. I'm not sure if max heap size is specified often (it's off by
> default), so not sure what would be the impact of these improvements be, but if
> anyone would be interested in funding me to implement these ideas (second
> algorithm above, and the bitmap iteration in the previous email) I could try to
> allocate one or two days a week to finish in a few months.
>
> Normally these are simple changes, but it's difficult to test and debug GHC's
> RTS as we don't have a test suite readily available and the code is not easily
> testable. In my previous implementations of these algorithms I had unit tests
> for the GC where I could easily generate arbitrary graphs (with cycles,
> backwards ptrs, forwards ptrs, ptrs from/to roots etc.) and test GC in
> isolation. Implementation of (2) took less than a day, and I didn't have to
> debug it more once the tests passed. It's really unfortunate that GHC's RTS
> makes this kind of thing difficult..
>
> Ömer
>
> Ömer Sinan Ağacan <omeragacan at gmail.com>, 7 Oca 2021 Per, 20:42
> tarihinde şunu yazdı:
> >
> > Hello,
> >
> > I recently implemented the algorithm used by GHC's compacting GC in another
> > project. The algorithm (after marking) makes two passes over the heap
> > /generation. In GHC, these passes are implemented in [1] and in the next
> > function.
> >
> > In my implementation I tried 3 ways of implementing these passes, one of which
> > is the same as GHC's code, and benchmarked each version. I found that the
> > fastest implementation is not what's used in GHC, but it could easily be used.
> >
> > I should mention that my code targets Wasm, and I benchmarked Wasm instructions
> > executed. Not CPU cycles, CPU instructions, or anything like that. It's very
> > likely that the results will be different when benchmarking code running on
> > actual hardware.
> >
> > Secondly, in my case the heap is mostly dead (residency is low). In GHC,
> > compaction for the oldest generation is enabled when residency passes a
> > threshold, so the assumption is the heap is mostly live. I'm guessing this
> > should also make some difference.
> >
> > Anyway, the first implementation I tried was similar to GHC's scan, but I did
> >
> >     scan += object_size(scan);
> >
> > instead of bumping scan by one, as GHC does in [2]. This was the slowest
> > version.
> >
> > Second implementation did the same as GHC (bumped scan by one). This was
> > faster, but still slower than the next version.
> >
> > What I found to be the best is scanning the bitmap, not the heap. The bitmap
> > iterator reads one word at a time. In each iteration it checks if the bitmap
> > word is 0. In GHC, in the best case this can skip 16 words on heap on 32-bit
> > systems, and 32 words on 64-bit. Reminder: we need two bits per object in the
> > bitmap, see [3]. (this is not the case in my implementation so the payoff is
> > better)
> >
> > When the bitmap word is not 0 I use "count trailing zeros" (or "count leading
> > zeros" depending on the bitmap implementation) to get the number of words to
> > skip. This is a single instruction on Wasm and x86 (TZCNT or LZCNT, available
> > via __builtin_ctzl and __builtin_clzl in gcc).
> >
> > So instead of skipping one word at a time, this can potentially skip 16 words
> > (or 32 on 64-bit architectures). When that's not possible, it can still skip
> > multiple words by using ctz/clz.
> >
> > Ömer
> >
> > [1]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rts/sm/Compact.c#L824-L879
> > [2]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rts/sm/Compact.c#L838
> > [3]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rts/sm/Compact.h#L18-L55