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

[Christoph Hellwig <hch@infradead.org>]

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.