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

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Date:   Mon, 22 Feb 2021 13:31:18 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Jens Axboe <axboe@...nel.dk>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu_counter: increase batch count

On Thu, 18 Feb 2021, Jens Axboe wrote:
> 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.

I want to quibble with the word "capping" here, it's misleading -
but I'm sorry I cannot think of the right word.

The macro is max() not min(): you're making an improvement for
certain io_uring workloads on machines with 1 to 15 cpus, right?
Does "bigger than in 2009" apply to those?

Though, io_uring could as well use percpu_counter_add_batch() instead?

(Yeah, this has nothing to do with me really, but I was looking at
percpu_counter_compare() just now, for tmpfs reasons, so took more
interest.  Not objecting to a change, but the wording leaves me
wondering if the patch does what you think - or, not for the
first time, I'm confused.)

Hugh

> >>
> > 
> > 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