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Make group commit more effective.

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When a backend needs to flush the WAL, and someone else is already flushing
the WAL, wait until it releases the WALInsertLock and check if we still need
to do the flush or if the other backend already did the work for us, before
acquiring WALInsertLock. This helps group commit, because when the WAL flush
finishes, all the backends that were waiting for it can be woken up in one
go, and the can all concurrently observe that they're done, rather than
waking them up one by one in a cascading fashion.

This is based on a new LWLock function, LWLockWaitUntilFree(), which has
peculiar semantics. If the lock is immediately free, it grabs the lock and
returns true. If it's not free, it waits until it is released, but then
returns false without grabbing the lock. This is used in XLogFlush(), so
that when the lock is acquired, the backend flushes the WAL, but if it's
not, the backend first checks the current flush location before retrying.

Original patch and benchmarking by Peter Geoghegan and Simon Riggs, although
this patch as committed ended up being very different from that.
This commit is contained in:
Heikki Linnakangas
2012-01-30 16:40:58 +02:00
parent ba1868ba31
commit 9b38d46d9f
7 changed files with 200 additions and 18 deletions
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2
src/backend/utils/probes.d
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@@ -35,6 +35,8 @@ provider postgresql {
probe lwlock__wait__done(LWLockId, LWLockMode);
probe lwlock__condacquire(LWLockId, LWLockMode);
probe lwlock__condacquire__fail(LWLockId, LWLockMode);
probe lwlock__wait__until__free(LWLockId, LWLockMode);
probe lwlock__wait__until__free__fail(LWLockId, LWLockMode);
probe lock__wait__start(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE);
probe lock__wait__done(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE);