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From: Paul Brook <paul@codesourcery.com>
To: Christoph Hellwig <hch@lst.de>
Cc: qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>,
	kvm@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Hannes Reinecke <hare@suse.de>
Subject: Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
Date: Wed, 29 Apr 2009 12:37:20 +0100	[thread overview]
Message-ID: <200904291237.21558.paul@codesourcery.com> (raw)
In-Reply-To: <20090429112130.GA11241@lst.de>

On Wednesday 29 April 2009, Christoph Hellwig wrote:
> On Wed, Apr 29, 2009 at 12:11:19PM +0100, Paul Brook wrote:
> > Is this actually measurably faster, or just infinitesimally faster in
> > theory?
>
> On normal disks it's rather theoretical.  On high-end SSDs and arrays the
> impact is noticeable, mostly due to the additional latency.

How exactly does it introduce additional latency? A scsi command block is 
hardly large or complicated. Are you suggesting that a 16/32byte scsi command 
takes significantly longer to process than a 16byte virtio command 
descriptor? I'd expect any extra processing to be a small fraction of the 
host syscall latency, let alone the latency of the physical host adapter. It 
probably even fits on the same CPU cache line.

Paul

WARNING: multiple messages have this Message-ID (diff)
From: Paul Brook <paul@codesourcery.com>
To: Christoph Hellwig <hch@lst.de>
Cc: kvm@vger.kernel.org, Rusty Russell <rusty@rustcorp.com.au>,
	qemu-devel@nongnu.org,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Hannes Reinecke <hare@suse.de>
Subject: Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
Date: Wed, 29 Apr 2009 12:37:20 +0100	[thread overview]
Message-ID: <200904291237.21558.paul@codesourcery.com> (raw)
In-Reply-To: <20090429112130.GA11241@lst.de>

On Wednesday 29 April 2009, Christoph Hellwig wrote:
> On Wed, Apr 29, 2009 at 12:11:19PM +0100, Paul Brook wrote:
> > Is this actually measurably faster, or just infinitesimally faster in
> > theory?
>
> On normal disks it's rather theoretical.  On high-end SSDs and arrays the
> impact is noticeable, mostly due to the additional latency.

How exactly does it introduce additional latency? A scsi command block is 
hardly large or complicated. Are you suggesting that a 16/32byte scsi command 
takes significantly longer to process than a 16byte virtio command 
descriptor? I'd expect any extra processing to be a small fraction of the 
host syscall latency, let alone the latency of the physical host adapter. It 
probably even fits on the same CPU cache line.

Paul

  reply	other threads:[~2009-04-29 11:37 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-04-27  8:26 [PATCH] virtio-blk: add SGI_IO passthru support Christoph Hellwig
2009-04-27  8:29 ` Christoph Hellwig
2009-04-27  8:29   ` [Qemu-devel] " Christoph Hellwig
2009-04-27  9:15   ` Avi Kivity
2009-04-27  9:15     ` [Qemu-devel] " Avi Kivity
2009-04-28  9:47     ` Christoph Hellwig
2009-04-28  9:47       ` Christoph Hellwig
2009-04-27 14:36   ` Anthony Liguori
2009-04-27 14:36     ` [Qemu-devel] " Anthony Liguori
2009-04-28  9:51     ` Christoph Hellwig
2009-04-28  9:51       ` Christoph Hellwig
2009-04-28 16:37       ` Anthony Liguori
2009-04-28 16:52         ` Christian Borntraeger
2009-04-28 16:52           ` Christian Borntraeger
2009-04-29 10:48         ` Christoph Hellwig
2009-04-29 10:48           ` Christoph Hellwig
2009-04-29 11:11           ` Paul Brook
2009-04-29 11:11             ` Paul Brook
2009-04-29 11:19             ` Christian Borntraeger
2009-04-29 11:19               ` Christian Borntraeger
2009-04-29 11:29               ` Paul Brook
2009-04-29 11:29                 ` Paul Brook
2009-04-29 11:21             ` Christoph Hellwig
2009-04-29 11:21               ` Christoph Hellwig
2009-04-29 11:37               ` Paul Brook [this message]
2009-04-29 11:37                 ` Paul Brook
2009-04-30 20:13                 ` Christoph Hellwig
2009-04-30 20:13                   ` Christoph Hellwig
2009-04-30 20:55                   ` Nicholas A. Bellinger
2009-04-30 20:55                     ` Nicholas A. Bellinger
2009-04-30 21:49                   ` Paul Brook
2009-04-30 21:49                     ` Paul Brook
2009-04-30 21:56                     ` Javier Guerra
2009-04-30 21:56                       ` Javier Guerra
2009-05-01  7:24                     ` Christoph Hellwig
2009-05-01  7:24                       ` Christoph Hellwig
2009-05-01 11:08                       ` Jamie Lokier
2009-05-01 11:08                         ` Jamie Lokier
2009-05-01 14:28                         ` Christoph Hellwig
2009-05-01 14:28                           ` Christoph Hellwig
2009-05-05  5:21                   ` Rusty Russell
2009-05-05  5:21                     ` Rusty Russell
2009-04-28 19:09       ` Christian Borntraeger
2009-04-28 19:09         ` Christian Borntraeger
2009-04-29 10:50         ` Christoph Hellwig
2009-04-29 10:50           ` Christoph Hellwig
2009-04-29 11:07           ` Christian Borntraeger
2009-04-29 11:07             ` Christian Borntraeger
2009-04-28  9:57   ` [PATCH 2/2] virtio-blk: add SG_IO " Christoph Hellwig
2009-04-28  9:57     ` [Qemu-devel] " Christoph Hellwig

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