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Subject: Re: [PATCH] percpu_counter: increase batch count
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <0bf90e07-8758-b238-b3f3-a330725a1134@kernel.dk>
 <20210218151644.df430e4f77f763b7db2a004f@linux-foundation.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <919b3aa5-7332-0817-3b70-4f20ae6410a8@kernel.dk>
Date:   Thu, 18 Feb 2021 16:23:47 -0700
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On 2/18/21 4:16 PM, Andrew Morton wrote:
> On Thu, 18 Feb 2021 14:36:31 -0700 Jens Axboe <axboe@kernel.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