Messages in this thread Patch in this message | | | Date | Mon, 13 Sep 2010 13:45:33 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH] check_preempt_tick should not compare vruntime with wall time |
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* Linus Torvalds (torvalds@linux-foundation.org) wrote: > On Mon, Sep 13, 2010 at 9:16 AM, Mathieu Desnoyers > <mathieu.desnoyers@efficios.com> wrote: > > > > OK, the long IRC discussions we just had convinced me that the current scheme > > takes things into account by adapting the granularity dynamically, but also got > > me to notice that check_preempt seems to compare vruntime with wall time, which > > is utterly incorrect. So maybe all my patch was doing was to expose this bug: > > Do you have latency numbers for this patch?
Sure, see below,
In addition to this patch, I also used Peter's approach of reducing the minimum granularity (given that now I am confident that the value of sysctl_sched_min_granularity won't affect the preemption granularity directly, but rather indirectly through the period length). The result, succinctly, is that just the "check_preempt" fix does not seem to do much difference in terms of fork wakeup latency (the results below give an order of magnitude, but should be taken with a grain of salt due to the noise). I ran 3 the tests 3 times each, and took the average, but there is clearly some noise.
FWIW, Xorg and firefox feel _much more_ responsive with the fix I propose when running with a make -j10. The system is even usable with a make -j20 on my UP machine, even though I can start feeling a some lag. This is probably a more important, yet less scientific, result.
It turns out that the min_granularity value (and hence the associated nr_latency values) have been brought from 20 down to 5 (commit 722aab0c3bbd7648d66790515c14d95d10a15bf3), and then down to 3 lately (commit 21406928afe43f1db6acab4931bb8c886f4d04ce). I would not be surprised that this last change might have been trying to hide the vruntime vs wall time comparison bug, which has been introduced in commit f685ceacab07d3f6c236f04803e2f2f0dbcc5afb.
2.0GHz Pentium M
* wakeup-latency.c (SIGEV_THREAD) with make -j10
- Mainline 2.6.35.2 kernel
maximum latency: 45762.1 µs average latency: 7348.6 µs missed timer events: 0
- check_preempt fix
maximum latency: 39858.9 µs average latency: 7728.1 µs missed timer events: 0
- With only Peter's smaller min_gran (shown below):
maximum latency: 29100.6 µs average latency: 6684.1 µs missed timer events: 0
- check_preempt fix + Peter's smaller min_gran: maximum latency: 20433.0 µs average latency: 5112.5 µs missed timer events: 0
Thanks,
Mathieu
Diminish min granularity patch:
--- kernel/sched_fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
Index: linux-2.6-lttng.git/kernel/sched_fair.c =================================================================== --- linux-2.6-lttng.git.orig/kernel/sched_fair.c +++ linux-2.6-lttng.git/kernel/sched_fair.c @@ -54,13 +54,13 @@ enum sched_tunable_scaling sysctl_sched_ * Minimal preemption granularity for CPU-bound tasks: * (default: 2 msec * (1 + ilog(ncpus)), units: nanoseconds) */ -unsigned int sysctl_sched_min_granularity = 2000000ULL; -unsigned int normalized_sysctl_sched_min_granularity = 2000000ULL; +unsigned int sysctl_sched_min_granularity = 750000ULL; +unsigned int normalized_sysctl_sched_min_granularity = 750000ULL; /* * is kept at sysctl_sched_latency / sysctl_sched_min_granularity */ -static unsigned int sched_nr_latency = 3; +static unsigned int sched_nr_latency = 8; /* * After fork, child runs first. If set to 0 (default) then -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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