Re: aim7 -30% regression in 2.6.24-rc1

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Thu, 2007-11-01 at 11:02 +0100, Cyrus Massoumi wrote:
> Zhang, Yanmin wrote:
> > On Wed, 2007-10-31 at 17:57 +0800, Zhang, Yanmin wrote:
> >> On Tue, 2007-10-30 at 16:36 +0800, Zhang, Yanmin wrote:
> >>> On Tue, 2007-10-30 at 08:26 +0100, Ingo Molnar wrote:
> >>>> * Zhang, Yanmin <yanmin_zhang@linux.intel.com> wrote:
> >>>>
> >>>>> sub-bisecting captured patch 
> >>>>> 38ad464d410dadceda1563f36bdb0be7fe4c8938(sched: uniform tunings) 
> >>>>> caused 20% regression of aim7.
> >>>>>
> >>>>> The last 10% should be also related to sched parameters, such like 
> >>>>> sysctl_sched_min_granularity.
> >>>> ah, interesting. Since you have CONFIG_SCHED_DEBUG enabled, could you 
> >>>> please try to figure out what the best value for 
> >>>> /proc/sys/kernel_sched_latency, /proc/sys/kernel_sched_nr_latency and 
> >>>> /proc/sys/kernel_sched_min_granularity is?
> >>>>
> >>>> there's a tuning constraint for kernel_sched_nr_latency: 
> >>>>
> >>>> - kernel_sched_nr_latency should always be set to 
> >>>>   kernel_sched_latency/kernel_sched_min_granularity. (it's not a free 
> >>>>   tunable)
> >>>>
> >>>> i suspect a good approach would be to double the value of 
> >>>> kernel_sched_latency and kernel_sched_nr_latency in each tuning 
> >>>> iteration, while keeping kernel_sched_min_granularity unchanged. That 
> >>>> will excercise the tuning values of the 2.6.23 kernel as well.
> >>> I followed your idea to test 2.6.24-rc1. The improvement is slow.
> >>> When sched_nr_latency=2560 and sched_latency_ns=640000000, the performance
> >>> is still about 15% less than 2.6.23.
> >> I got the aim7 30% regression on my new upgraded stoakley machine. I found
> >> this mahcine is slower than the old one. Maybe BIOS has issues, or memeory(Might not
> >> be dual-channel?) is slow. So I retested it on the old machine and found on the old
> >> stoakley machine, the regression is about 6%, quite similiar to the regression on tigerton
> >> machine.
> >>
> >> By sched_nr_latency=640 and sched_latency_ns=640000000 on the old stoakley machine,
> >> the regression becomes about 2%. Other latency has more regression.
> >>
> >> On my tulsa machine, by sched_nr_latency=640 and sched_latency_ns=640000000,
> >> the regression becomes less than 1% (The original regression is about 20%).
> > I rerun SPECjbb by ched_nr_latency=640 and sched_latency_ns=640000000. On tigerton,
> > the regression is still more than 40%. On stoakley machine, it becomes worse (26%,
> > original is 9%). I will do more investigation to make sure SPECjbb regression is
> > also casued by the bad default values.
> > 
> > We need a smarter method to calculate the best default values for the key tuning
> > parameters.
> > 
> > One interesting is sysbench+mysql(readonly) got the same result like 2.6.22 (no
> > regression). Good job!
> 
> Do you mean you couldn't reproduce the regression which was reported 
> with 2.6.23 (http://lkml.org/lkml/2007/10/30/53) with 2.6.24-rc1?
It looks like you missed my emails.

Firstly, I reproduced (or just find the same myself :) ) the issue with kernel 2.6.22,
2.6.23-rc and 2.6.23.

Ingo wrote a big patch to fix it and the new patch is in 2.6.24-rc1 now.

Then I retested it with 2.6.24-rc1 on a couple of x86_64 machines. The issue
disappeared. You could test it with 2.6.24-rc1.

>  It 
> would be nice if you could provide some numbers for 2.6.22, 2.6.23 and 
> 2.6.24-rc1.
Sorry. Intel policy doesn't allow me to publish the numbers because only
specific departments in Intel could do that. But I could talk the regression
percentage.

-yanmin