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And ensure accesses to n_capabilities are atomic (although with relaxed
ordering). This is necessary as RTS API callers may concurrently call
into the RTS without holding a capability.
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There was a lock-order reversal between lockTSO() and the TVar lock,
see #15136 for the details.
It turns out we can fix this pretty easily by just deleting all the
locking code(!). The principle for unblocking a `BlockedOnSTM` thread
then becomes the same as for other kinds of blocking: if the TSO
belongs to this capability then we do it directly, otherwise we send a
message to the capability that owns the TSO. That is, a thread blocked
on STM is owned by its capability, as it should be.
The possible downside of this is that we might send multiple messages
to wake up a thread when the thread is on another capability. This is
safe, it's just not very efficient. I'll try to do some experiments
to see if this is a problem.
Test Plan: Test case from #15136 doesn't deadlock any more.
Reviewers: bgamari, osa1, erikd
Reviewed By: osa1
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: #15136
Differential Revision: https://phabricator.haskell.org/D4956
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The existing internal counters:
* gc_alloc_block_sync
* whitehole_spin
* gen[g].sync
* gen[1].sync
are now not shown in the -s report unless --internal-counters is also passed.
If --internal-counters is passed we now show the counters above, reformatted, as
well as several other counters. In particular, we now count the yieldThread()
calls that SpinLocks do as well as their spins.
The added counters are:
* gc_spin (spin and yield)
* mut_spin (spin and yield)
* whitehole_threadPaused (spin only)
* whitehole_executeMessage (spin only)
* whitehole_lockClosure (spin only)
* waitForGcThreadsd (spin and yield)
As well as the following, which are not SpinLock-like things:
* any_work
* do_work
* scav_find_work
See the Note for descriptions of what these counters are.
We add busy_wait_nops in these loops along with the counter increment where it
was absent.
Old internal counters output:
```
gc_alloc_block_sync: 0
whitehole_gc_spin: 0
gen[0].sync: 0
gen[1].sync: 0
```
New internal counters output:
```
Internal Counters:
Spins Yields
gc_alloc_block_sync 323 0
gc_spin 9016713 752
mut_spin 57360944 47716
whitehole_gc 0 n/a
whitehole_threadPaused 0 n/a
whitehole_executeMessage 0 n/a
whitehole_lockClosure 0 0
waitForGcThreads 2 415
gen[0].sync 6 0
gen[1].sync 1 0
any_work 2017
no_work 2014
scav_find_work 1004
```
Test Plan:
./validate
Check it builds with #define PROF_SPIN removed from includes/rts/Config.h
Reviewers: bgamari, erikd, simonmar, hvr
Reviewed By: simonmar
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: #3553, #9221
Differential Revision: https://phabricator.haskell.org/D4302
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Our new CPP linter enforces this.
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This both says what we mean and silences a bunch of spurious CPP linting
warnings. This pragma is supported by all CPP implementations which we
support.
Reviewers: austin, erikd, simonmar, hvr
Reviewed By: simonmar
Subscribers: rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3482
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Summary:
The aim here is to reduce the number of remote memory accesses on
systems with a NUMA memory architecture, typically multi-socket servers.
Linux provides a NUMA API for doing two things:
* Allocating memory local to a particular node
* Binding a thread to a particular node
When given the +RTS --numa flag, the runtime will
* Determine the number of NUMA nodes (N) by querying the OS
* Assign capabilities to nodes, so cap C is on node C%N
* Bind worker threads on a capability to the correct node
* Keep a separate free lists in the block layer for each node
* Allocate the nursery for a capability from node-local memory
* Allocate blocks in the GC from node-local memory
For example, using nofib/parallel/queens on a 24-core 2-socket machine:
```
$ ./Main 15 +RTS -N24 -s -A64m
Total time 173.960s ( 7.467s elapsed)
$ ./Main 15 +RTS -N24 -s -A64m --numa
Total time 150.836s ( 6.423s elapsed)
```
The biggest win here is expected to be allocating from node-local
memory, so that means programs using a large -A value (as here).
According to perf, on this program the number of remote memory accesses
were reduced by more than 50% by using `--numa`.
Test Plan:
* validate
* There's a new flag --debug-numa=<n> that pretends to do NUMA without
actually making the OS calls, which is useful for testing the code
on non-NUMA systems.
* TODO: I need to add some unit tests
Reviewers: erikd, austin, rwbarton, ezyang, bgamari, hvr, niteria
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D2199
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