Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Wednesday 12 March 2008 18:58, Peter Zijlstra wrote:
> On Wed, 2008-03-12 at 12:21 +1100, Nick Piggin wrote:
> > (Back onto lkml)
> >
> > On Tuesday 11 March 2008 23:02, Ingo Molnar wrote:
> > > another thing to try would be to increase:
> > >
> > >   /proc/sys/kernel/sched_migration_cost
> > >
> > > from its 500 usecs default to a few msecs ?
> >
> > This doesn't really help either (at 10ms).
> >
> > (For the record, I've tried turning SD_WAKE_IDLE, SD_WAKE_AFFINE
> > on and off for each domain and that hasn't helped either).
> >
> > I've also tried increasing sched_latency_ns as far as it can go.
> > BTW. this is a pretty nasty behaviour if you ask my opinion. It
> > starts *increasing* the number of involuntary context switches
> > as resources get oversubscribed. That's completely unintuitive as
> > far as I can see -- when we get overloaded, the obvious thing to
> > do is try to increase efficiency, or at least try as hard as
> > possible not to lose it. So context switches should be steady or
> > decreasing as I add more processes to a runqueue.
> >
> > It seems to max out at nearly 100 context switches per second,
> > and this has actually shown to be too frequent for modern CPUs
> > and big caches.
> >
> > Increasing the tunable didn't help for this workload, but it really
> > needs to be fixed so it doesn't decrease timeslices as the number
> > of processes increases.
>
> /proc/sys/kernel/sched_min_granularity_ns
> /proc/sys/kernel/sched_latency_ns
>
> period := max(latency, nr_running * min_granularity)
> slice := period * w_{i} / W
> W := \Sum_{i} w_{i}
>
> So if you want to increase the slice length for loaded systems, up
> min_granularity.

OK, but the very concept of reducing efficiency when load increases
is nasty, and leads to nasty feedback loops. It's just a very bad
behaviour to have out of the box, and as a general observation, 10ms
is too short a default timeslice IMO.

I don't see how it is really helpful for interactive processes either.
By definition, if they are not CPU bound, then they should be run
quite soon after waking up; if they are CPU bound, then reducing
efficiency by increasing context switches is effectively going to
increase their latency anyway.

Can this be changed by default, please?