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Date:   Mon, 22 Feb 2021 14:14:07 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Jens Axboe <axboe@kernel.dk>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] percpu_counter: increase batch count
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On Mon, 22 Feb 2021, Jens Axboe wrote:
> On 2/22/21 2:31 PM, Hugh Dickins wrote:
> > 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@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.
> > 
> > I want to quibble with the word "capping" here, it's misleading -
> > but I'm sorry I cannot think of the right word.
> 
> Agree, it's not the best wording. And if you can't think of a better
> one, then I'm at a loss too :-)
> 
> > 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?
> 
> Right, that actually had me confused. The box in question has 64 threads,
> so my effective count was 128, or 256 with the patch.

Ah, yes, so there I *was* confused in saying "1 to 15",
the improvement was for "1 to 127" of course - thanks.

> 
> > Though, io_uring could as well use percpu_counter_add_batch() instead?
> 
> That might be a simpler/better choice!
> 
> > (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.)
> 
> I don't think you're confused, and honestly I think using the batch
> version instead would likely improve our situation without potentially
> changing behavior for everyone else. So it's likely the right way to go.

You're too polite! But yes, if percpu_counter_add_batch() suits, great.

> 
> Thanks Hugh!
> 
> -- 
> Jens Axboe