Re: [PATCH v3 3/4] block: drop shared-tag fairness throttling

[Sumit Saxena <sumit.saxena@xxxxxxxxxxxx>]

On Wed, Jun 10, 2026 at 10:06 PM Keith Busch <kbusch@xxxxxxxxxx> wrote:
>
> On Wed, Jun 10, 2026 at 09:16:11PM +0530, Sumit Saxena wrote:
> > The motivation for this change stems from performance issue we
> > encountered due to false sharing of the 'nr_active_requests_shared_tags'
> > counter
> > on certain CPU architectures. I initially submitted a patch to move that
> > counter to
> > its own cache line to avoid conflicts with 'nr_requests' and other hot
> > fields
> > (see:
> > https://patchwork.kernel.org/project/linux-scsi/patch/20260402074637.92417-3-sumit.saxena@xxxxxxxxxxxx/
> > ).
> >
> > During the review, Bart shared his work, which eliminates the
> > counter entirely by removing the fairness throttling. My testing confirmed
> > that
> > this approach resolved the performance issues and improved IOPS.
> > This patch is part of a larger set, and I have reported the cumulative
> > performance
> > improvements in the cover letter.
>
> So the problem is just the atomic operation accounting overhead? I
> previously thought the device just really needed to consume all the tags
> to hit performance.

That's correct, it's the atomic operation accounting overhead.

Thanks,

Sumit

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