linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Message-ID: <20090525075008.GA23413@infradead.org>
Date:	Mon, 25 May 2009 03:50:08 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
	buffer

On Mon, May 25, 2009 at 09:46:47AM +0200, Jens Axboe wrote:
> > But that patch looks good to me, avoiding one allocation for each
> > command and simplifying the code.  I try to remember why these were
> > two slabs to start with but can't find any reason.
> > 
> > Btw, we might just want to declare the sense buffer directly as a sized
> > array in the scsi command as there really doesn't seem to be a reason
> > not to allocate it.
> 
> That is also a workable solution. I've been trying to cut down on the
> number of allocations required per-IO, and there's definitely still some
> low hanging fruit there. Some of it is already included, like the inline
> io_vecs in the bio.

Btw, one thing I wanted to do for years is to add ->alloc_cmnd and
->destroy_cmnd method to the host template which optionally move the
command allocation to the LLDD.  That way we can embedd the scsi_cmnd
into the drivers per-commad structure and eliminate another memory
allocation.  Also this would naturally extend the keep one cmnd pool
to drivers without requiring additional code.  As a second step it
would also allow killing the scsi_host_cmd_pool byt just having
a set of library routines that drivers which need SLAB_CACHE_DMA can
use.

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