linux-kernel - Re: [PATCH] scsi_ram: a RAM-based SCSI driver

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Date:	Fri, 07 Dec 2012 23:24:34 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Stephen Cameron <stephenmcameron@...il.com>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi_ram: a RAM-based SCSI driver

Il 07/12/2012 12:20, Kirill A. Shutemov ha scritto:
>> > Is this that much faster than scsi-debug?  The discarding options surely
>> > can be added there.
> scsi_ram is about 9% faster (without fake_rw/throw_away_*) on my machine:

There are two main differences in the data path:

- scsi_debug uses locking.  Shouldn't be the culprit because otherwise
read and write would have different speed, but it's easy to add it to
scsi_ram and check.

- scsi_debug uses scsi_sg_copy_to_buffer, i.e. sg_miter_start/next/stop
in kernel/scatterlist.c.  Again, it should be easy to port the scsi_ram
code to scsi_debug or vice versa and verify if this is the culprit.

Having a completely different driver seems useless to me...

Paolo
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