Re: profiling pgbench

From: Andres Freund <andres(at)anarazel(dot)de>
To: pgsql-hackers(at)postgresql(dot)org
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: profiling pgbench
Date: 2010-11-24 21:19:09
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On Wednesday 24 November 2010 22:14:04 Andres Freund wrote:
> On Wednesday 24 November 2010 21:24:43 Robert Haas wrote:
> > I'd like to get access to a box with (a lot) more cores, to see
> > whether the lock stuff moves up in the profile. A big chunk of that
> > hash_search_with_hash_value overhead is coming from
> > LockAcquireExtended. The __strcmp_sse2 is almost entirely parsing
> > overhead. In general, I'm not sure there's much hope for reducing the
> > parsing overhead, although ScanKeywordLookup() can certainly be done
> > better. XLogInsert() is spending a lot of time doing CRC's.
> > LWLockAcquire() is dropping cycles in many different places.
> I can get you profiles of machines with up two 24 real cores, unfortunately
> I can't give access away.
> Regarding CRCs:
> I spent some time optimizing these, as you might remember. The wall I hit
> optimizing it benefit-wise is that the single CRC calls (4 for a
> non-indexed single-row insert on a table with 1 column inside a
> transaction) are just too damn small to get more efficient. Its causing
> pipeline stalls all over... (21, 5, 1, 28 bytes).
> I have a very preliminary patch calculating the CRC over the whole thing in
> one go if it can do so (no switch, no xl buffers wraparound), but its
> highly ugly as it needs to read from the xl insert buffers and then
> reinsert the crc at the correct position.
> While it shows a noticable improvement, that doesn't seem to be a good way
> to go. It could be made to work properly though.
> I played around with some ideas to do that more nicely, but none were
> gratifying.
> Recarding LWLockAcquire costs:
> Yes, its pretty noticeable - on loads of different usages. On a bunch of
> production machines its the second (begind XLogInsert) on some the most
> expensive function. Most of the time
AllocSetAlloc is the third, battling with hash_search_with_hash value. To
complete that sentence...


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