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SubjectRE: [patch] speed up single bio_vec allocation
Jens Axboe wrote on Wednesday, December 06, 2006 2:09 AM
> > > I will try that too. I'm a bit touchy about sharing a cache line for
> > > different bio. But given that there are 200,000 I/O per second we are
> > > currently pushing the kernel, the chances of two cpu working on two
> > > bio that sits in the same cache line are pretty small.
> >
> > Yep I really think so. Besides, it's not like we are repeatedly writing
> > to these objects in the first place.
> This is what I had in mind, in case it wasn't completely clear. Not
> tested, other than it compiles. Basically it eliminates the small
> bio_vec pool, and grows the bio by 16-bytes on 64-bit archs, or by
> 12-bytes on 32-bit archs instead and uses the room at the end for the
> bio_vec structure.

Yeah, I had a very similar patch queued internally for the large benchmark
measurement. I will post the result as soon as I get it.

> I still can't help but think we can do better than this, and that this
> is nothing more than optimizing for a benchmark. For high performance
> I/O, you will be doing > 1 page bio's anyway and this patch wont help
> you at all. Perhaps we can just kill bio_vec slabs completely, and
> create bio slabs instead with differing sizes. So instead of having 1
> bio slab and 5 bio_vec slabs, change that to 5 bio slabs that leave room
> for the bio_vec list at the end. That would always eliminate the extra
> allocation, at the cost of blowing the 256-page case into a order 1 page
> allocation (256*16 + sizeof(*bio) > PAGE_SIZE) for the 4kb 64-bit archs,
> which is something I've always tried to avoid.

I took a quick query of biovec-* slab stats on various production machines,
majority of the allocation is on 1 and 4 segments, usages falls off quickly
on 16 or more. 256 segment biovec allocation is really rare. I think it
makes sense to heavily bias towards smaller biovec allocation and have
separate biovec allocation for really large ones.

- Ken
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