On Mon, Sep 18, 2017 at 06:33:20PM +0300, Tariq Toukan wrote:
> 
> 
> On 18/09/2017 10:44 AM, Aaron Lu wrote:
> > On Mon, Sep 18, 2017 at 03:34:47PM +0800, Aaron Lu wrote:
> > > On Sun, Sep 17, 2017 at 07:16:15PM +0300, Tariq Toukan wrote:
> > > > 
> > > > It's nice to have the option to dynamically play with the parameter.
> > > > But maybe we should also think of changing the default fraction guaranteed
> > > > to the PCP, so that unaware admins of networking servers would also benefit.
> > > 
> > > I collected some performance data with will-it-scale/page_fault1 process
> > > mode on different machines with different pcp->batch sizes, starting
> > > from the default 31(calculated by zone_batchsize(), 31 is the standard
> > > value for any zone that has more than 1/2MiB memory), then incremented
> > > by 31 upwards till 527. PCP's upper limit is 6*batch.
> > > 
> > > An image is plotted and attached: batch_full.png(full here means the
> > > number of process started equals to CPU number).
> > 
> > To be clear: X-axis is the value of batch size(31, 62, 93, ..., 527),
> > Y-axis is the value of per_process_ops, generated by will-it-scale,

One correction here, Y-axis isn't per_process_ops but per_process_ops *
nr_processes. Still, higher is better.

> > higher is better.
> > 
> > > 
> > >  From the image:
> > > - For EX machines, they all see throughput increase with increased batch
> > >    size and peaked at around batch_size=310, then fall;
> > > - For EP machines, Haswell-EP and Broadwell-EP also see throughput
> > >    increase with increased batch size and peaked at batch_size=279, then
> > >    fall, batch_size=310 also delivers pretty good result. Skylake-EP is
> > >    quite different in that it doesn't see any obvious throughput increase
> > >    after batch_size=93, though the trend is still increasing, but in a very
> > >    small way and finally peaked at batch_size=403, then fall.
> > >    Ivybridge EP behaves much like desktop ones.
> > > - For Desktop machines, they do not see any obvious changes with
> > >    increased batch_size.
> > > 
> > > So the default batch size(31) doesn't deliver good enough result, we
> > > probbaly should change the default value.
> 
> Thanks Aaron for sharing your experiment results.
> That's a good analysis of the effect of the batch value.
> I agree with your conclusion.
> 
> From networking perspective, we should reconsider the defaults to be able to
> reach the increasing NICs linerates.
> Not only for pcp->batch, but also for pcp->high.

I guess I didn't make it clear in my last email: when pcp->batch is
changed, pcp->high is also changed. Their relationship is:
pcp->high = pcp->batch * 6.

Manipulating percpu_pagelist_fraction could increase pcp->high, but not
pcp->batch(it has an upper limit as 96 currently).

My test shows even when pcp->high being the same, changing pcp->batch
could further improve will-it-scale's performance. e.g. in the below two
cases, pcp->high are both set to 1860 but with different pcp->batch:

                 will-it-scale    native_queued_spin_lock_slowpath(perf)
pcp->batch=96    15762348         79.95%
pcp->batch=310   19291492 +22.3%  74.87% -5.1%

Granted, this is the case for will-it-scale and may not apply to your
case. I have a small patch that adds a batch interface for debug
purpose, echo a value could set batch and high will be batch * 6. You
are welcome to give it a try if you think it's worth(attached).

Regards,
Aaron