Re: Misaligned BufferDescriptors causing major performance problems on AMD

On Wed, Dec 24, 2014 at 11:20 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> I just verified that I can still reproduce the problem:
>
> # aligned case (max_connections=401)
> afreund(at)axle:~$ pgbench -P 1 -h /tmp/ -p5440 postgres -n -M prepared -c 96 -j 96 -T 100 -S
> progress: 1.0 s, 405170.2 tps, lat 0.195 ms stddev 0.928
> progress: 2.0 s, 467011.1 tps, lat 0.204 ms stddev 0.140
> progress: 3.0 s, 462832.1 tps, lat 0.205 ms stddev 0.154
> progress: 4.0 s, 471035.5 tps, lat 0.202 ms stddev 0.154
> progress: 5.0 s, 500329.0 tps, lat 0.190 ms stddev 0.132
>
> BufferDescriptors is at 0x7f63610a6960 (which is 32byte aligned)
>
> # unaligned case (max_connections=400)
> afreund(at)axle:~$ pgbench -P 1 -h /tmp/ -p5440 postgres -n -M prepared -c 96 -j 96 -T 100 -S
> progress: 1.0 s, 202271.1 tps, lat 0.448 ms stddev 1.232
> progress: 2.0 s, 223823.4 tps, lat 0.427 ms stddev 3.007
> progress: 3.0 s, 227584.5 tps, lat 0.414 ms stddev 4.760
> progress: 4.0 s, 221095.6 tps, lat 0.410 ms stddev 4.390
> progress: 5.0 s, 217430.6 tps, lat 0.454 ms stddev 7.913
> progress: 6.0 s, 210275.9 tps, lat 0.411 ms stddev 0.606
> BufferDescriptors is at 0x7f1718aeb980 (which is 64byte aligned)

So, should we increase ALIGNOF_BUFFER from 32 to 64? Seems like
that's what these results are telling us.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company