Re: Processor speed relative to postgres transactions per second

Recently I ran a set of tests on two systems: a 4-core server with 5
disks (OS + WAL + 3 for DB) on a battery backed disk controller, and a
newer Hyper-threaded design with 4 physical cores turning into 8 virtual
ones--but only a single disk and no RAID controller, so I had to turn
off its write cache to get reliable database operation. (See
http://www.postgresql.org/docs/current/interactive/wal-reliability.html )

When running pgbench with its simple built-in SELECT-only test, on a
tiny data set that fits in RAM, I went from a peak of 28336 TPS on the
4-core system to a peak of 58164 TPS on the 8-core one.

On the default write-heavy test, the 4-core server peaked at 4047 TPS.
The 8-core one peaked at 94 TPS because that's as fast as its single
disk could commit data.

The moral is that a faster processor or more cores only buys you
additional speed if enough of your data fits in RAM that the processor
speed is the bottleneck. If you're waiting on disks, a faster processor
will just spin without any work to do. You can't answer "will I get
more transactions per second?" without specifying what your transaction
is, and knowing what the current limiter is.

--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us