linux-kernel - Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu counter

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Date:   Wed, 07 Sep 2022 09:01:53 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Jiebin Sun <jiebin.sun@...el.com>, akpm@...ux-foundation.org,
        vasily.averin@...ux.dev, shakeelb@...gle.com, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, ebiederm@...ssion.com,
        legion@...nel.org, manfred@...orfullife.com,
        alexander.mikhalitsyn@...tuozzo.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     tim.c.chen@...el.com, feng.tang@...el.com, ying.huang@...el.com,
        tianyou.li@...el.com, wangyang.guo@...el.com
Subject: Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu
 counter

On Thu, 2022-09-08 at 01:25 +0800, Jiebin Sun wrote:
> The msg_bytes and msg_hdrs atomic counters are frequently
> updated when IPC msg queue is in heavy use, causing heavy
> cache bounce and overhead. Change them to percpu_counter
> greatly improve the performance. Since there is one percpu
> struct per namespace, additional memory cost is minimal.
> Reading of the count done in msgctl call, which is infrequent.
> So the need to sum up the counts in each CPU is infrequent.
> 
> 
> Apply the patch and test the pts/stress-ng-1.4.0
> -- system v message passing (160 threads).
> 
> Score gain: 3.17x
> 
> 
...
>  
> +/* large batch size could reduce the times to sum up percpu counter */
> +#define MSG_PERCPU_COUNTER_BATCH 1024
> +

Jiebin, 

1024 is a small size (1/4 page). 
The local per cpu counter could overflow to the gloabal count quickly
if it is limited to this size, since our count tracks msg size.
  
I'll suggest something larger, say 8*1024*1024, about
8MB to accommodate about 2 large page worth of data.  Maybe that
will further improve throughput on stress-ng by reducing contention
on adding to the global count.

Tim