linux-kernel - Re: [RFC PATCH v2 0/4] Eliminate zone->lock contention for will-it-scale/page

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Date:   Thu, 22 Mar 2018 09:30:49 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH v2 0/4] Eliminate zone->lock contention for
 will-it-scale/page_fault1 and parallel free

On Wed, Mar 21, 2018 at 01:44:25PM -0400, Daniel Jordan wrote:
> On 03/20/2018 04:54 AM, Aaron Lu wrote:
> ...snip...
> > reduced zone->lock contention on free path from 35% to 1.1%. Also, it
> > shows good result on parallel free(*) workload by reducing zone->lock
> > contention from 90% to almost zero(lru lock increased from almost 0 to
> > 90% though).
> 
> Hi Aaron, I'm looking through your series now.  Just wanted to mention that I'm seeing the same interaction between zone->lock and lru_lock in my own testing.  IOW, it's not enough to fix just one or the other: both need attention to get good performance on a big system, at least in this microbenchmark we've both been using.

Agree.

> 
> There's anti-scaling at high core counts where overall system page faults per second actually decrease with more CPUs added to the test.  This happens when either zone->lock or lru_lock contention are completely removed, but the anti-scaling goes away when both locks are fixed.
> 
> Anyway, I'll post some actual data on this stuff soon.

Looking forward to that, thanks.

In the meantime, I'll also try your lru_lock optimization work on top of
this patchset to see if the lock contention shifts back to zone->lock.