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Re: pgbench cpu overhead (was Re: lazy vxid locks, v1)

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>
Cc: pgsql-hackers(at)postgresql(dot)org, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: pgbench cpu overhead (was Re: lazy vxid locks, v1)
Date: 2011-07-24 01:50:34
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Lists: pgsql-hackers
On Mon, Jun 13, 2011 at 7:03 AM, Stefan Kaltenbrunner
<stefan(at)kaltenbrunner(dot)cc> wrote:
> On 06/13/2011 01:55 PM, Stefan Kaltenbrunner wrote:
> [...]
>> all those tests are done with pgbench running on the same box - which
>> has a noticable impact on the results because pgbench is using ~1 core
>> per 8 cores of the backend tested in cpu resoures - though I don't think
>> it causes any changes in the results that would show the performance
>> behaviour in a different light.
> actuall testing against sysbench with the very same workload shows the
> following performance behaviour:
> with 40 threads(aka the peak performance point):
> pgbench:        223308 tps
> sysbench:       311584 tps
> with 160 threads (backend contention dominated):
> pgbench:        57075
> sysbench:       43437
> so it seems that sysbench is actually significantly less overhead than
> pgbench and the lower throughput at the higher conncurency seems to be
> cause by sysbench being able to stress the backend even more than
> pgbench can.
> for those curious - the profile for pgbench looks like:
> samples  %        symbol name
> 29378    41.9087  doCustom
> 17502    24.9672  threadRun
> 7629     10.8830  pg_strcasecmp
> 5871      8.3752  compareVariables
> 2568      3.6633  getVariable
> 2167      3.0913  putVariable
> 2065      2.9458  replaceVariable
> 1971      2.8117  parseVariable
> 534       0.7618  xstrdup
> 278       0.3966  xrealloc
> 137       0.1954  xmalloc

Hi Stefan,

How was this profile generated?  I get a similar profile using
--enable-profiling and gprof, but I find it not believable.  The
complete absence of any calls to libpq is not credible.  I don't know
about your profiler, but with gprof they should be listed in the call
graph even if they take a negligible amount of time.  So I think
pgbench is linking to libpq libraries that do not themselves support
profiling (I have no idea how that could happen though).  If the calls
graphs are not getting recorded correctly, surely the timing can't be
reliable either.

(I also tried profiling pgbench with "perf", but in that case I get
nothing other than kernel and libc calls showing up.  I don't know
what that means)

To support this, I've dummied up doCustom so that does all the work of
deciding what needs to happen, executing the metacommands,
interpolating the variables into the SQL string, but then simply
refrains from calling the PQ functions to send and receive the query
and response.  (I had to make a few changes around the select loop in
threadRun to support this).

The result is that the dummy pgbench is charged with only 57% more CPU
time than the stock one, but it gets over 10 times as many
"transactions" done.  I think this supports the notion that the CPU
bottleneck is not in pgbench.c, but somewhere in the libpq or the



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