Re: Patch: fix lock contention for HASHHDR.mutex

On Tue, Dec 15, 2015 at 7:25 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2015-12-11 17:00:01 +0300, Aleksander Alekseev wrote:
>> The problem is that code between LWLockAcquire (lock.c:881) and
>> LWLockRelease (lock.c:1020) can _sometimes_ run up to 3-5 ms. Using
>> old-good gettimeofday and logging method I managed to find a bottleneck:
>>
>> -- proclock = SetupLockInTable [lock.c:892]
>>  `-- proclock = (PROCLOCK *) hash_search_with_hash_value [lock.c:1105]
>>    `-- currBucket = get_hash_entry(hashp) [dynahash.c:985]
>>      `-- SpinLockAcquire(&hctl->mutex) [dynahash.c:1187]
>>
>> If my measurements are not wrong (I didn't place gettimeofday between
>> SpinLockAquire/SpinLockRelease, etc) we sometimes spend about 3 ms here
>> waiting for a spinlock, which doesn't seems right.
>
> Well, it's quite possible that your process was scheduled out, while
> waiting for that spinlock. Which'd make 3ms pretty normal.
>
> I'd consider using a LWLock instead of a spinlock here. I've seen this
> contended in a bunch of situations, and the queued behaviour, combined
> with directed wakeups on the OS level, ought to improve the worst case
> behaviour measurably.

Amit had the idea a while back of trying to replace the HASHHDR mutex
with something based on atomic ops.  It seems hard to avoid the
attendant A-B-A problems but maybe there's a way.

-- 
Robert Haas
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