linux-kernel - Re: [PATCH] percpu_counter: increase batch count

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Date:   Thu, 18 Feb 2021 16:23:47 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_counter: increase batch count

On 2/18/21 4:16 PM, Andrew Morton wrote:
> On Thu, 18 Feb 2021 14:36:31 -0700 Jens Axboe <axboe@...nel.dk> wrote:
> 
>> Currently we cap the batch count at max(32, 2*nr_online_cpus), which these
>> days is kind of silly as systems have gotten much bigger than in 2009 when
>> this heuristic was introduced.
>>
>> Bump it to capping it at 256 instead. This has a noticeable improvement
>> for certain io_uring workloads, as io_uring tracks per-task inflight count
>> using percpu counters.
>>
> 
> It will also make percpu_counter_read() and
> percpu_counter_read_positive() more inaccurate than at present.  Any
> effects from this will take a while to discover.

It will, but the value of 32 is very low, especially when you are potentially
doing millions of these per second. So I do think it should track the times
a bit.

> But yes, worth trying - I'll add it to the post-rc1 pile.

Thanks!

-- 
Jens Axboe