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Date: Wed, 24 Jan 2024 08:41:38 -0700
From: Keith Busch <kbusch@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: linux-block@vger.kernel.org, virtualization@lists.linux.dev,
	linux-nvme@lists.infradead.org
Subject: Re: [Report] requests are submitted to hardware in reverse order
 from nvme/virtio-blk queue_rqs()
Message-ID: <ZbEvstiLSMwtFb8m@kbusch-mbp.dhcp.thefacebook.com>
References: <ZbD7ups50ryrlJ/G@fedora>
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In-Reply-To: <ZbD7ups50ryrlJ/G@fedora>

On Wed, Jan 24, 2024 at 07:59:54PM +0800, Ming Lei wrote:
> Requests are added to plug list in reverse order, and both virtio-blk
> and nvme retrieves request from plug list in order, so finally requests
> are submitted to hardware in reverse order via nvme_queue_rqs() or
> virtio_queue_rqs, see:
> 
> 	io_uring       submit_bio  vdb      6302096     4096
> 	io_uring       submit_bio  vdb     12235072     4096
> 	io_uring       submit_bio  vdb      7682280     4096
> 	io_uring       submit_bio  vdb     11912464     4096
> 	io_uring virtio_queue_rqs  vdb     11912464     4096
> 	io_uring virtio_queue_rqs  vdb      7682280     4096
> 	io_uring virtio_queue_rqs  vdb     12235072     4096
> 	io_uring virtio_queue_rqs  vdb      6302096     4096
> 
> 
> May this reorder be one problem for virtio-blk and nvme-pci?

For nvme, it depends. Usually it's probably not a problem, though some
pci ssd's have optimizations for sequential IO that might not work if
these get reordered.