Re: [tbench regression fixes]: digging out smelly deadmen.

Ingo Molnar
2008-10-27T09:31:41Z

* David Miller <davem@davemloft.net> wrote:

> From: Evgeniy Polyakov <zbr@ioremap.net>
> Date: Sun, 26 Oct 2008 13:05:55 +0300
>
> > I'm not surprised there were no changes when I reported hrtimers to be
> > the main guilty factor in my setup for dbench tests, and only when David
> > showed that they also killed his sparks via wake_up(), something was
> > done. Now this regression even dissapeared from the list.
> > Good direction, we should always follow this.
>
> Yes, this situation was in my opinion a complete fucking joke.
> Someone like me shouldn't have to do all of the hard work for the
> scheduler folks in order for a bug like this to get seriously looked
> at.

yeah, that overhead was bad, and once it became clear that you had
high-resolution timers enabled for your benchmaking runs (which is
default-off and which is still rare for benchmarking runs - despite
being a popular end-user feature) we immediately disabled the hrtick via
this upstream commit:

0c4b83d: sched: disable the hrtick for now

that commit is included in v2.6.28-rc1 so this particular issue should
be resolved.

high-resolution timers are still default-disabled in the upstream
kernel, so this never affected usual configs that folks keep
benchmarking - it only affected those who decided they want higher
resolution timers and more precise scheduling.

Anyway, the sched-hrtick is off now, and we wont turn it back on without
making sure that it's really low cost in the hotpath.

Regarding tbench, a workload that context-switches in excess of 100,000
per second is inevitably going to show scheduler overhead - so you'll
get the best numbers if you eliminate all/most scheduler code from the
hotpath. We are working on various patches to mitigate the cost some
more - and your patches and feedback is welcome as well.

But it's a difficult call with no silver bullets. On one hand we have
folks putting more and more stuff into the context-switching hotpath on
the (mostly valid) point that the scheduler is a slowpath compared to
most other things. On the other hand we've got folks doing
high-context-switch ratio benchmarks and complaining about the overhead
whenever something goes in that improves the quality of scheduling of a
workload that does not context-switch as massively as tbench. It's a
difficult balance and we cannot satisfy both camps.

Nevertheless, this is not a valid argument in favor of the hrtick
overhead: that was clearly excessive overhead and we zapped it.

Ingo