Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to3.6-rc5 on AMD chipsets - bisected

[Mike Galbraith]

On Sat, 2012-09-15 at 12:44 +0200, Borislav Petkov wrote: 
> 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).

Increasing /proc/sys/kernel/sched_min_granularity_ns to roughly half of
sched_latency_ns should also help.  That will allow LAST_BUDDY to do
it's job, try to hand the CPU back to a preempted task if possible.  The
change that increased sched_nr_latency to 8 should have injured
postgress as well.  ATM, it's disabled unless you're massively loaded.

I _think_ it's about preemption, but it doesn't matter, patch is toast.

-Mike

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