netdev - Re: Page allocator bottleneck

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Date:   Mon, 18 Sep 2017 15:44:04 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Tariq Toukan <tariqt@...lanox.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-mm <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: Page allocator bottleneck

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,
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.