Re: hackbench [pthread mode] regression with 2.6.29-rc3

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2009-02-02 at 09:12 +0800, Zhang, Yanmin wrote:

> process timer (by setitimer) isn't good. Is per-cpu itimer to improve it?

> So the per-cpu itimer could improve this situation when thread number is far bigger than
> cpu number. I didn't retry specweb2005 with 2.6.28.

1) process wide itimers are rubbish, 2) per-cpu itimers are rubbish too,
for the very simple reason they waste gobs of memory for sane programs.

I would rather go back to the old model where we iterate all threads,
and find a way to not make programs with too many threads for their own
good lock up the kernel, but instead get poor service.

Now the problems appears to be that I overlooked that we keep the itimer
clock running at all times, not only when we have a pending timer, and I
suppose that the standard mandates that behaviour :/

Anyway, I will try to sort something out, at worst we'll have to revert
to the .28 state and live with the per-cpu crap for another release.