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<h3>
Ben Gamari pushed to branch wip/gc/optimize
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/395ab8d5e6a0cc467a7b280142fe8e567cc5a70d">395ab8d5</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:23:46Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">rts: Implement concurrent collection in the nonmoving collector
This extends the non-moving collector to allow concurrent collection.
The full design of the collector implemented here is described in detail
in a technical note
B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
Compiler" (2018)
This extension involves the introduction of a capability-local
remembered set, known as the /update remembered set/, which tracks
objects which may no longer be visible to the collector due to mutation.
To maintain this remembered set we introduce a write barrier on
mutations which is enabled while a concurrent mark is underway.
The update remembered set representation is similar to that of the
nonmoving mark queue, being a chunked array of `MarkEntry`s. Each
`Capability` maintains a single accumulator chunk, which it flushed
when it (a) is filled, or (b) when the nonmoving collector enters its
post-mark synchronization phase.
While the write barrier touches a significant amount of code it is
conceptually straightforward: the mutator must ensure that the referee
of any pointer it overwrites is added to the update remembered set.
However, there are a few details:
* In the case of objects with a dirty flag (e.g. `MVar`s) we can
exploit the fact that only the *first* mutation requires a write
barrier.
* Weak references, as usual, complicate things. In particular, we must
ensure that the referee of a weak object is marked if dereferenced by
the mutator. For this we (unfortunately) must introduce a read
barrier, as described in Note [Concurrent read barrier on deRefWeak#]
(in `NonMovingMark.c`).
* Stable names are also a bit tricky as described in Note [Sweeping
stable names in the concurrent collector] (`NonMovingSweep.c`).
We take quite some pains to ensure that the high thread count often seen
in parallel Haskell applications doesn't affect pause times. To this end
we allow thread stacks to be marked either by the thread itself (when it
is executed or stack-underflows) or the concurrent mark thread (if the
thread owning the stack is never scheduled). There is a non-trivial
handshake to ensure that this happens without racing which is described
in Note [StgStack dirtiness flags and concurrent marking].
Co-Authored-by: Ömer Sinan Ağacan <omer@well-typed.com>
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/59894c9036959ca72d9f7102799a10f6fffe39f6">59894c90</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:23:46Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Nonmoving: Disable memory inventory with concurrent collection
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/f9a609b8fcca842f61ae7702a1cafe916d590e24">f9a609b8</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:16Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Nonmoving: Allow aging and refactor static objects logic
This commit does two things:
* Allow aging of objects during the preparatory minor GC
* Refactor handling of static objects to avoid the use of a hashtable
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/2fadc5bbc50ea52a6f21ab7635700d6412ef513d">2fadc5bb</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:16Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Disable aging when doing deadlock detection GC
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/bbb0532f078f081846f5fe655e314d1ac846e3d3">bbb0532f</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:16Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">More comments for aging
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/91b7397a88a8171b752ab3f4a79c10858e07b98a">91b7397a</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Eliminate integer division in nonmovingBlockCount
Perf showed that the this single div was capturing up to 10% of samples
in nonmovingMark. However, the overwhelming majority of cases is looking
at small block sizes. These cases we can easily compute explicitly,
allowing the compiler to turn the division into a significantly more
efficient division-by-constant.
While the increase in source code looks scary, this all optimises down
to very nice looking assembler. At this point the only remaining
hotspots in nonmovingBlockCount are due to memory access.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/69255fd34e8a3f4b878f6fc05e62f5c1006af831">69255fd3</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Allocate mark queues in larger block groups
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/0fa4699e9b23319c34aa7b520236b49906329fad">0fa4699e</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMovingMark: Optimize representation of mark queue
This shortens MarkQueueEntry by 30% (one word)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/ce01e27cb344fa63eb260f2f64a9ed5d4c64178c">ce01e27c</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Optimize bitmap search during allocation
Use memchr instead of a open-coded loop. This is nearly twice as fast in
a synthetic benchmark.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/0f1b25458dc457d1c0a7feaf785879ff177cdc79">0f1b2545</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">rts: Add prefetch macros
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/068c0031a97688e1cfb60994cec77e60590736d3">068c0031</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Prefetch when clearing bitmaps
Ensure that the bitmap of the segmentt that we will clear next is in
cache by the time we reach it.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/6d0a1ba189fcc77e18681bb8e91e33bd339f37a7">6d0a1ba1</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Inline nonmovingClearAllBitmaps
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/681ee157d98905d7d49e44c82aea69bcb6c9d027">681ee157</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Fuse sweep preparation into mark prep
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/0a2681ce8d969055c9ef7e72e8bcfd9c5a799fcf">0a2681ce</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Pre-fetch during mark
This improved overall runtime on nofib's constraints test by nearly 10%.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/3e43b8efd884447b9b3bebff5a8a623d94670888">3e43b8ef</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Prefetch segment header
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/6721548598a1e2d34d4f440a961c4c796716f7a7">67215485</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Optimise allocator cache behavior
Previously we would look at the segment header to determine the block
size despite the fact that we already had the block size at hand.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/a1a4c13a2d41d09a642a747d5cd0f2143f1b21f0">a1a4c13a</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:43Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMovingMark: Eliminate redundant check_in_nonmoving_heaps
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/c7ecb8a101b7c5288d769eb64784a4c7aaff4e88">c7ecb8a1</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:44Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">NonMoving: Don't do major GC if one is already running
Previously we would perform a preparatory moving collection, resulting
in many things being added to the mark queue. When we finished with this
we would realize in nonmovingCollect that there was already a collection
running, in which case we would simply not run the nonmoving collector.
However, it was very easy to end up in a "treadmilling" situation: all
subsequent GC following the first failed major GC would be scheduled as
major GCs. Consequently we would continuously feed the concurrent
collector with more mark queue entries and it would never finish.
This patch aborts the major collection far earlier, meaning that we
avoid adding nonmoving objects to the mark queue and allowing the
concurrent collector to finish.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/b1c0e77701efa7507dd5f4179cfa9d1edc78c71a">b1c0e777</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-19T15:24:44Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Nonmoving: Ensure write barrier vanishes in non-threaded RTS
</pre>
</li>
</ul>
<h4>30 changed files:</h4>
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<a href="#db6936871693fa68c3fdcbd692eb7dcffc3a9372">
compiler/cmm/CLabel.hs
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compiler/codeGen/StgCmmBind.hs
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compiler/codeGen/StgCmmPrim.hs
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compiler/codeGen/StgCmmUtils.hs
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includes/Cmm.h
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includes/Rts.h
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includes/rts/storage/ClosureMacros.h
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includes/rts/storage/GC.h
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includes/rts/storage/TSO.h
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includes/stg/MiscClosures.h
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rts/Capability.c
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rts/Capability.h
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rts/Exception.cmm
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rts/Messages.c
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rts/PrimOps.cmm
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rts/RaiseAsync.c
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rts/RtsStartup.c
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rts/RtsSymbols.c
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rts/STM.c
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rts/Schedule.c
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rts/StableName.c
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rts/ThreadPaused.c
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rts/Threads.c
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rts/Updates.h
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rts/sm/Evac.c
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rts/sm/GC.c
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rts/sm/GC.h
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rts/sm/GCAux.c
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<h5>The diff was not included because it is too large.</h5>
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