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

[FUJITA Tomonori]

On Tue, 26 May 2009 09:47:02 -0500
James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:

> On Tue, 2009-05-26 at 16:38 +0900, FUJITA Tomonori wrote:
> > On Tue, 26 May 2009 09:32:29 +0200
> > Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> > 
> > > On Tue, May 26 2009, FUJITA Tomonori wrote:
> > > > 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.
> > > 
> > > 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.
> > 
> > James rejected this in the past. Let's wait for his verdict.
> 
> OK, so the reason for the original problems where the sense buffer was
> inlined with the scsi_command was that we need to DMA to the sense
> buffer but not to the command.  Plus the command is in fairly constant
> use so we get cacheline interference unless they're always in separate
> caches.  This necessitates opening up a hole in the command to achieve
> this (you can separate to the next cache line if you can guarantee that
> the command always begins on a cacheline.  If not, it has to be
> 2*cacheline).  The L1 cacheline can be up to 128 bytes on some
> architectures, so we'd need to know the waste of space is worth it in
> terms of speed.  The other problem is that the entire command now has to
> be allocated in DMAable memory, which restricts the allocation on some
> systems.

Yeah, I think that there are good reasons why we shouldn't inline the
sense buffer. As I already wrote, seems that the DMA requirement
wasn't properly understood; it's not about the alignment.


> > Yeah, we can inline the sense buffer but as we discussed in the past
> > several times, there are some good reasons that we should not do so, I
> > think.
> 
> There are several other approaches:
> 
>      1. Keep the sense buffer packed in the command but disallow DMA to
>         it, which fixes all the alignment problems.  Then we supply a
>         set of rotating DMA buffers to drivers which need to do the DMA
>         (which isn't the majority).

Can we just fix some drivers not to do the DMA with the sense buffer in
scsi_cmnd? IIRC, there are only five or six drivers that do such.
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