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Date: Thu, 14 Nov 2024 05:16:03 +0100
From: Christoph Hellwig <hch@lst.de>
To: Jens Axboe <axboe@kernel.dk>
Cc: Chaitanya Kulkarni <chaitanyak@nvidia.com>,
	Christoph Hellwig <hch@lst.de>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>, Keith Busch <kbusch@kernel.org>,
	Sagi Grimberg <sagi@grimberg.me>,
	Pavel Begunkov <asml.silence@gmail.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"virtualization@lists.linux.dev" <virtualization@lists.linux.dev>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	"io-uring@vger.kernel.org" <io-uring@vger.kernel.org>
Subject: Re: don't reorder requests passed to ->queue_rqs
Message-ID: <20241114041603.GA8971@lst.de>
References: <20241113152050.157179-1-hch@lst.de> <eb2faaba-c261-48de-8316-c8e34fdb516c@nvidia.com> <2f7fa13a-71d9-4a8d-b8f4-5f657fdaab60@kernel.dk>
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On Wed, Nov 13, 2024 at 01:51:48PM -0700, Jens Axboe wrote:
> Thanks for testing, but you can't verify any kind of perf change with
> that kind of setup. I'll be willing to bet that it'll be 1-2% drop at
> higher rates, which is substantial. But the reordering is a problem, not
> just for zoned devices, which is why I chose to merge this.

So I did not see any variation, but I also don't have access to a really
beefy setup right now.  If there is a degradation it probably is that
touching rq_next for each request actually has an effect if the list is
big enough and they aren't cache hot any more.  I can cook up a patch
that goes back to the scheme currently used upstream in nvme and virtio
that just cuts of the list at a hctx change instead of moving the
requests one by one now that the block layer doesn't mess up the order.