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

From: Jens Axboe
Date: Mon May 25 2009 - 03:54:33 EST

On Mon, May 25 2009, Christoph Hellwig wrote:
> 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.

That's a good idea and could kill one more alloc/free per IO. I'll add
that to the mix!

And in case anyone is interested, the patches that got mixed up with the
writeback patches are from the 'ssd' branch. It's basically a mix of
experimental patches for improving performance. Some are crap, some are
worth continuing with. There's been a steady influx of patches from
there to mainline, so it's a continually changing branch. Well not so
much lately since I spent most of the time in the writeback branch, but

Jens Axboe

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