From: Keith Busch <kbusch@kernel.org>
To: Sumit Saxena <sumit.saxena@broadcom.com>
Cc: Christoph Hellwig <hch@infradead.org>,
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Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH v3 3/4] block: drop shared-tag fairness throttling
Date: Wed, 10 Jun 2026 10:35:59 -0600 [thread overview]
Message-ID: <aimSb9I0Vl-68hy9@kbusch-mbp> (raw)
In-Reply-To: <CAL2rwxr1uGshb1o=jvP2OnBffNz2cKXj8tHuAUCN5HFuy2vB_g@mail.gmail.com>
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@broadcom.com/
> ).
>
> 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.
next prev parent reply other threads:[~2026-06-10 16:36 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-09 12:17 [PATCH v3 0/4] scsi/block: NUMA-local scan allocations, shared-tag path cleanup, and SCSI I/O counters Sumit Saxena
2026-06-09 12:18 ` [PATCH v3 1/4] scsi: scan: allocate sdev and starget on the NUMA node of the host adapter Sumit Saxena
2026-06-10 6:00 ` Hannes Reinecke
2026-06-09 12:18 ` [PATCH v3 2/4] scsi: host: allocate struct Scsi_Host " Sumit Saxena
2026-06-09 13:03 ` John Garry
2026-06-10 5:59 ` Hannes Reinecke
2026-06-09 12:18 ` [PATCH v3 3/4] block: drop shared-tag fairness throttling Sumit Saxena
2026-06-10 6:14 ` Christoph Hellwig
2026-06-10 15:46 ` Sumit Saxena
2026-06-10 16:35 ` Keith Busch [this message]
2026-06-10 6:18 ` Hannes Reinecke
2026-06-09 12:18 ` [PATCH v3 4/4] scsi: use percpu counters for iostat counters in struct scsi_device Sumit Saxena
2026-06-10 6:21 ` Hannes Reinecke
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