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

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, May 26 2009, FUJITA Tomonori wrote:
> On Tue, 26 May 2009 08:29:53 +0200
> Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> > On Tue, May 26 2009, FUJITA Tomonori wrote:
> > > On Mon, 25 May 2009 18:45:25 -0700
> > > Roland Dreier <rdreier@cisco.com> wrote:
> > > 
> > > >  > Ideally there should be a MACRO that is defined to WORD_SIZE on cache-coherent
> > > >  > ARCHs and to SMP_CACHE_BYTES on none-cache-coherent systems and use that size
> > > >  > at the __align() attribute. (So only stupid ARCHES get hurt)
> > > > 
> > > > this seems to come up repeatedly -- I had a proposal a _long_ time ago
> > > > that never quite got merged, cf http://lwn.net/Articles/2265/ and
> > > > http://lwn.net/Articles/2269/ -- from 2002 (!?).  The idea is to go a
> > > 
> > > Yeah, I think that Benjamin did last time:
> > > 
> > > http://www.mail-archive.com/linux-scsi@vger.kernel.org/msg12632.html
> > > 
> > > IIRC, James didn't like it so I wrote the current code. I didn't see
> > > any big performance difference with scsi_debug:
> > > 
> > > http://marc.info/?l=linux-scsi&m=120038907123706&w=2
> > > 
> > > Jens, you see the performance difference due to this unification?
> > 
> > Yes, it's definitely a worth while optimization. The problem isn't as
> > such this specific allocation, it's the total number of allocations we
> > do for a piece of IO. This sense buffer one is just one of many, I'm
> > continually working to reduce them. If we get rid of this one and add
> > the ->alloc_cmd() stuff, we can kill one more. The bio path already lost
> > one. So in the IO stack, we went from 6 allocations to 3 for a piece of
> > IO. And then it starts to add up. Even at just 30-50k iops, that's more
> > than 1% of time in the testing I did.
> 
> I see, thanks. Hmm, possibly slab becomes slower. ;)
> 
> Then I think that we need something like the ->alloc_cmd()
> method. Let's ask James. 
> 
> I don't think that it's just about simply adding the hook; there are
> some issues that we need to think about. Though Boaz worries too much
> a bit, I think.
> 
> I'm not sure about this patch if we add ->alloc_cmd(). I doubt that
> there are any llds don't use ->alloc_cmd() worry about the overhead of
> the separated sense buffer allocation. If a lld doesn't define the own
> alloc_cmd, then I think it's fine to use the generic command
> allocator that does the separate sense buffer allocation.

I think we should do the two things seperately. If we can safely inline
the sense buffer in the command by doing the right alignment, then lets
do that. The ->alloc_cmd() approach will be easier to do with an inline
sense buffer.

But there's really no reason to tie the two things together.

-- 
Jens Axboe