Re: Lock partitions

"Strong, David" <david(dot)strong(at)unisys(dot)com> writes:
> When using 16 buffer and 16 lock partitions, we see that BufMapping
> takes 809 seconds to acquire locks and 174 seconds to release locks. The
> LockMgr takes 362 seconds to acquire locks and 26 seconds to release
> locks.

> When using 128 buffer and 128 lock partitions, we see that BufMapping
> takes 277 seconds (532 seconds improvement) to acquire locks and 78
> seconds (96 seconds improvement) to release locks. The LockMgr takes 235
> seconds (127 seconds improvement) to acquire locks and 22 seconds (4
> seconds improvement) to release locks.

While I don't see any particular penalty to increasing
NUM_BUFFER_PARTITIONS, increasing NUM_LOCK_PARTITIONS carries a very
significant penalty (increasing PGPROC size as well as the work needed
during LockReleaseAll, which is executed at every transaction end).
I think 128 lock partitions is probably verging on the ridiculous
... particularly if your benchmark only involves touching half a dozen
tables. I'd be more interested in comparisons between 4 and 16 lock
partitions. Also, please vary the two settings independently rather
than confusing the issue by changing them both at once.

> With the improvements in the various locking times, one might expect an
> improvement in the overall benchmark result. However, a 16 partition run
> produces a result of 198.74 TPS and a 128 partition run produces a
> result of 203.24 TPS.

> Part of the time saved from BufMapping and LockMgr partitions is
> absorbed into the WALInsertLock lock. For a 16 partition run, the total
> time to lock/release the WALInsertLock lock is 5845 seconds. For 128
> partitions, the WALInsertLock lock takes 6172 seconds, an increase of
> 327 seconds. Perhaps we have our WAL configured incorrectly?

I fear this throws your entire measurement procedure into question. For
a fixed workload the number of acquisitions of WALInsertLock ought to be
fixed, so you shouldn't see any more contention for WALInsertLock if the
transaction rate didn't change materially.

regards, tom lane