Cc: Matt Mackall <mpm@...>, Ingo Molnar <mingo@...>, Roman Zippel <zippel@...>, Mike Galbraith <efault@...>, Linus Torvalds <torvalds@...>, Andrew Morton <akpm@...>, <linux-kernel@...>
On Thu, Aug 02, 2007 at 09:31:19PM -0700, Arjan van de Ven wrote:
But if we rely on the same sampling method, at least we will report
something consistent with what happens. And sampling is often the
correct method to get finer resolution on a macroscopic scale.
I mean, we're telling users that we include the "completely fair scheduler"
in 2.6.23, a scheduler which will ensure that all tasks get a fair share of
CPU time. A user starts top and sees 33%+32%+32+3% for 4 tasks while he
would have expected to see 25%+25%+25%+25%. You can try to explain users
that it's the fairest distribution, but they will have a hard time believing
it, especially when they measure the time spent on CPU with the "time"
command. OK this is all sampling, but we should try to avoid relying on
different sources of data for computation and reporting. Time and Top
should report something close to 4*25% for comparable tasks. And if not,
because of some sampling problem, maybe the scheduler cannot be that fair
in some situations, but either it should make use of the sampling time
and top use, or top and time should rely on the view of the scheduler.
I'll try to quickly hack up a program which makes use of rdtsc from
userspace to precisely measure user-space time, and disable TSC use
from the kernel to see how the values diverge.
Regards,
Willy
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