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SubjectRE: [patch] speed up single bio_vec allocation
Jens Axboe wrote on Monday, December 04, 2006 12:07 PM
> On Mon, Dec 04 2006, Chen, Kenneth W wrote:
> > On 64-bit arch like x86_64, struct bio is 104 byte. Since bio slab is
> > created with SLAB_HWCACHE_ALIGN flag, there are usually spare memory
> > available at the end of bio. I think we can utilize that memory for
> > bio_vec allocation. The purpose is not so much on saving memory consumption
> > for bio_vec, instead, I'm attempting to optimize away a call to bvec_alloc_bs.
> >
> > So here is a patch to do just that for 1 segment bio_vec (we currently only
> > have space for 1 on 2.6.19). And the detection whether there are spare space
> > available is dynamically calculated at compile time. If there are no space
> > available, there will be no run time cost at all because gcc simply optimize
> > away all the code added in this patch. If there are space available, the only
> > run time check is to see what the size of iovec is and we do appropriate
> > assignment to bio->bi_io_Vec etc. The cost is minimal and we gain a whole
> > lot back from not calling bvec_alloc_bs() function.
> >
> > I tried to use cache_line_size() to find out the alignment of struct bio, but
> > stumbled on that it is a runtime function for x86_64. So instead I made bio
> > to hint to the slab allocator to align on 32 byte (slab will use the larger
> > value of hw cache line and caller hints of "align"). I think it is a sane
> > number for majority of the CPUs out in the world.
> Any benchmarks for this one?

About 0.2% on database transaction processing benchmark. It was done a while
back on top of a major Linux vendor kernel. I will retest it again for 2.6.19.

> [...]
> Another idea would be to kill SLAB_HWCACHE_ALIGN (it's pretty pointless,
> I bet), and always alloc sizeof(*bio) + sizeof(*bvl) in one go when a
> bio is allocated. It doesn't add a lot of overhead even for the case
> where we do > 1 page bios, and it gets rid of the dual allocation for
> the 1 page bio.

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.

- Ken
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