linux-kernel - Re: [PATCH V6 4/5] virtio-scsi: introduce multiqueue support

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Date:	Wed, 20 Mar 2013 14:22:48 -0700
From:	Venkatesh Srinivas <venkateshs@...gle.com>
To:	Wanlong Gao <gaowanlong@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	mst@...hat.com, asias@...hat.com, JBottomley@...allels.com,
	pbonzini@...hat.com
Subject: Re: [PATCH V6 4/5] virtio-scsi: introduce multiqueue support

On Wed, Mar 20, 2013 at 03:01:23PM +0800, Wanlong Gao wrote:
>From: Paolo Bonzini <pbonzini@...hat.com>
>
>This patch adds queue steering to virtio-scsi.  When a target is sent
>multiple requests, we always drive them to the same queue so that FIFO
>processing order is kept.  However, if a target was idle, we can choose
>a queue arbitrarily.  In this case the queue is chosen according to the
>current VCPU, so the driver expects the number of request queues to be
>equal to the number of VCPUs.  This makes it easy and fast to select
>the queue, and also lets the driver optimize the IRQ affinity for the
>virtqueues (each virtqueue's affinity is set to the CPU that "owns"
>the queue).
>
>The speedup comes from improving cache locality and giving CPU affinity
>to the virtqueues, which is why this scheme was selected.  Assuming that
>the thread that is sending requests to the device is I/O-bound, it is
>likely to be sleeping at the time the ISR is executed, and thus executing
>the ISR on the same processor that sent the requests is cheap.
>
>However, the kernel will not execute the ISR on the "best" processor
>unless you explicitly set the affinity.  This is because in practice
>you will have many such I/O-bound processes and thus many otherwise
>idle processors.  Then the kernel will execute the ISR on a random
>processor, rather than the one that is sending requests to the device.
>
>The alternative to per-CPU virtqueues is per-target virtqueues.  To
>achieve the same locality, we could dynamically choose the virtqueue's
>affinity based on the CPU of the last task that sent a request.  This
>is less appealing because we do not set the affinity directly---we only
>provide a hint to the irqbalanced running in userspace.  Dynamically
>changing the affinity only works if the userspace applies the hint
>fast enough.

Looks good! Tested as V5.

Tested-by: Venkatesh Srinivas <venkateshs@...gle.com>

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