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Date: Sat, 15 Sep 2012 12:44:51 +0200
From: Borislav Petkov <bp@alien8.de>
To: Mike Galbraith <efault@gmx.de>
Cc: Nikolay Ulyanitsky <lystor@gmail.com>, linux-kernel@vger.kernel.org,
        Andreas Herrmann <andreas.herrmann3@amd.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
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On Sat, Sep 15, 2012 at 06:11:02AM +0200, Mike Galbraith wrote:
> My wild (and only) theory is that this is userspace spinlock related.
> If so, starting the server and benchmark SCHED_BATCH should not only
> kill the regression, but likely improve throughput as well.

FWIW,

I went and tried it. Here are the exact steps:

$ ps ax | grep postgres
 2066 ?        S      0:01 /usr/lib/postgresql/9.1/bin/postgres -D /var/lib/postgresql/9.1/main -c config_file=/etc/postgresql/9.1/main/postgresql.conf
 2070 ?        Ss     0:07 postgres: writer process
 2071 ?        Ss     0:05 postgres: wal writer process
 2072 ?        Ss     0:01 postgres: autovacuum launcher process
 2073 ?        Ss     0:01 postgres: stats collector process
 5788 pts/0    S+     0:00 grep postgres

# set to SCHED_BATCH
$ schedtool -B 2066 2070 2071 2072 2073

# verify:
$ schedtool 2066 2070 2071 2072 2073
PID  2066: PRIO   0, POLICY B: SCHED_BATCH   , NICE   0, AFFINITY 0x3f
PID  2070: PRIO   0, POLICY B: SCHED_BATCH   , NICE   0, AFFINITY 0x3f
PID  2071: PRIO   0, POLICY B: SCHED_BATCH   , NICE   0, AFFINITY 0x3f
PID  2072: PRIO   0, POLICY B: SCHED_BATCH   , NICE   0, AFFINITY 0x3f
PID  2073: PRIO   0, POLICY B: SCHED_BATCH   , NICE   0, AFFINITY 0x3f

$ su - postgres
postgres@hhost:~$ export PATH=$PATH:/usr/lib/postgresql/9.1/bin/
postgres@hhost:~$ schedtool -B -e pgbench -i pgbench && pgbench -c 10 -t 10000 pgbench

...

tps = 4388.118940 (including connections establishing)
tps = 4391.771875 (excluding connections establishing)

=> even better than the results with 3.5 (had something around 3900ish
on that particular configuration).

HTH.

-- 
Regards/Gruss,
    Boris.