Anyone interested in improving postgresql scaling?

[Kris Kennaway]

On Tue, Apr 10, 2007 at 12:04:32AM -0400, Kevin Way wrote:
> Kris Kennaway wrote:
> >If so, then your task is the following:
> >
> >Make SYSV semaphores less dumb about process wakeups.  Currently
> >whenever the semaphore state changes, all processes sleeping on the
> >semaphore are woken, even if we only have released enough resources
> >for one waiting process to claim.  i.e. there is a thundering herd
> >wakeup situation which destroys performance at high loads.  Fixing
> >this will involve replacing the wakeup() calls with appropriate
> >amounts of wakeup_one().
> Could this cause problem cause a situation where an 8-Core system was 
> 50-75% slower than an otherwise equivalent 2-Core system?
> 
> I have a graph of my sysbench/pgsql results here:
> 
>    http://blog.insidesystems.net/files/sysctl-pgsq-amd64-wtf.png
> 
> As the graph shows, the 8-core system is about half the speed of the 
> 2-core system at 2 simultaneous threads, and it decays down to 
> approximately 1/4 the speed of the 2-core system as the # of threads hits 5.
> 
> All other (non-pgsql, non-sysv) tests came back approximately as 
> expected, but I'm left wondering if I did something wrong, or if 8 cpus 
> are slower than 2, when it comes to Postgres on currently available FreeBSD.

I wouldn't expect 6.2 to have good scaling on 8 cpus for this kind of
benchmark.  Fixing that was what we have been working on for 7.0 (and
basically succeeded).  The most important fixes are already in 7.0
CVS, but a possible merge to 6.x will only happen after a suitably
conservative delay.

Kris
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