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To: "Mkrtchyan, Tigran" <tigran.mkrtchyan@desy.de>
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 postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: postgres 9.3 vs. 9.4
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On 25/09/14 01:03, Mkrtchyan, Tigran wrote:
>
> With pg_test_timing I can see, that overhead is 48 nsec on my server and 32 nsec on the laptop.
> what makes this difference and have it any influence on the overall performance?
>

Hmm - 22 nsec for my workstation, so while it could be a factor, your 
laptop and my workstation performed the pgbench about the same, so I'd 
look elsewhere - in particlular sync IO performance:


$ cd <where ssd mounted>
$ pg_test_fsync
5 seconds per test
O_DIRECT supported on this platform for open_datasync and open_sync.

Compare file sync methods using one 8kB write:
(in wal_sync_method preference order, except fdatasync
is Linux's default)
         open_datasync                       140.231 ops/sec    7131 
usecs/op
         fdatasync                           138.159 ops/sec    7238 
usecs/op
         fsync                               137.680 ops/sec    7263 
usecs/op
         fsync_writethrough                              n/a
         open_sync                           137.202 ops/sec    7289 
usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync
is Linux's default)
         open_datasync                        68.832 ops/sec   14528 
usecs/op
         fdatasync                           135.994 ops/sec    7353 
usecs/op
         fsync                               137.454 ops/sec    7275 
usecs/op
         fsync_writethrough                              n/a
         open_sync                            69.092 ops/sec   14473 
usecs/op

Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB
in different write open_sync sizes.)
          1 * 16kB open_sync write           136.904 ops/sec    7304 
usecs/op
          2 *  8kB open_sync writes           68.857 ops/sec   14523 
usecs/op
          4 *  4kB open_sync writes           34.744 ops/sec   28782 
usecs/op
          8 *  2kB open_sync writes      write failed: Invalid argument



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