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

[Jens Axboe]

On Tue, May 26 2009, FUJITA Tomonori wrote:
> On Mon, 25 May 2009 18:45:25 -0700
> Roland Dreier <rdreier@xxxxxxxxx> 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@xxxxxxxxxxxxxxx/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.

-- 
Jens Axboe

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