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

[FUJITA Tomonori]

On Tue, 26 May 2009 08:29:53 +0200
Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:

> 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.

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.
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