Re: aim7 -30% regression in 2.6.24-rc1

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

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!

-yanmin