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Re: PostgreSQL 8.4 performance tuning questions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Scott Carey <scott(at)richrelevance(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Matthew Wakeling <matthew(at)flymine(dot)org>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: PostgreSQL 8.4 performance tuning questions
Date: 2009-07-31 17:04:52
Message-ID: (view raw or whole thread)
Lists: pgsql-performance
Greg Stark <gsstark(at)mit(dot)edu> writes:
> On Thu, Jul 30, 2009 at 11:30 PM, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I did some tracing and verified that pg_dump passes data to deflate()
>> one table row at a time. I'm not sure about the performance
>> implications of that, but it does seem like it might be something to
>> look into.

> I suspect if this was a problem the zlib people would have added
> internal buffering ages ago. I find it hard to believe we're not the
> first application to use it this way.

I dug into this a bit more.  zlib *does* have internal buffering --- it
has to, because it needs a minimum lookahead of several hundred bytes
to ensure that compression works properly.  The per-call overhead of
deflate() looks a bit higher than one could wish when submitting short
chunks, but oprofile shows that "pg_dump -Fc" breaks down about like

samples  %        image name               symbol name
1103922  74.7760            longest_match
215433   14.5927            deflate_slow
55368     3.7504            compress_block
41715     2.8256            fill_window
17535     1.1878              memcpy
13663     0.9255            adler32
4613      0.3125              _int_malloc
2942      0.1993              free
2552      0.1729              malloc
2155      0.1460            pqdownheap
2128      0.1441              _int_free
1702      0.1153            deflate
1648      0.1116              mempcpy

longest_match is the core lookahead routine and is not going to be
affected by submission sizes, because it isn't called unless adequate
data (ie, the longest possible match length) is available in zlib's
internal buffer.  It's possible that doing more buffering on our end
would reduce the deflate_slow component somewhat, but it looks like
the most we could hope to get that way is in the range of 10% speedup.
So I'm wondering if anyone can provide concrete evidence of large
wins from buffering zlib's input.

			regards, tom lane

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