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

Stephen Hemminger
2008-03-11T23:48:01Z

On Tue, 11 Mar 2008 16:12:12 -0700
Nicholas Miell <nmiell@comcast.net> wrote:

>
> On Tue, 2008-03-11 at 22:34 +0100, Cyrus Massoumi wrote:
> > Nicholas Miell wrote:
> >
> > > (Also, ignoring MySQL because it's a terrible piece of software at least
> > > when regarding it's scalability is a bad idea. It's the M in LAMP, it
> > > has a huge user base, and FreeBSD manages to outperform Linux with the
> > > same unscalable piece of software.)
> >
> > Did you actually see this?
> > http://www.kernel.org/pub/linux/kernel/people/npiggin/sysbench/
> >
> > FreeBSD does not outperform Linux, it's actually a bit faster according
> > to Nick's tests.
>
> I am aware of those results, but in the mail I was responding to, Nick
> Piggin said the following:
>
> > The problem with MySQL contention means that if the scheduler
> > unluckily chooses to deschedule a lock holder, then you can get
> > idle time building up on other cores and you can get context switch
> > cascades as things all pile up onto this heavily contended lock. As
> > such, it means MySQL is not an ideal candidate for looking at
> > performance behaviour. I discounted the relatively worse scaling of
> > MySQL with 2.6.25-rc (than 2.6.22) as such an effect.
>
> which I interpreted to mean that MySQL performs worse on 2.6.23+ than on
> 2.6.22 but for some reason this doesn't matter.
>

How many of these problems are due to poorly implemented userlevel
spinlocks? Do the database spinlocks map to futexes?