Messages in this thread | | | Date | Thu, 7 Dec 2006 14:33:46 -0800 | From | "Nate Diller" <> | Subject | Re: [patch] speed up single bio_vec allocation |
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On 12/7/06, Chen, Kenneth W <kenneth.w.chen@intel.com> wrote: > Nate Diller wrote on Thursday, December 07, 2006 1:46 PM > > the current code is straightforward and obviously correct. you want > > to make the alloc/dealloc paths more complex, by special-casing for an > > arbitrary limit of "small" I/O, AFAICT. of *course* you can expect > > less overhead when you're doing one large I/O vs. two small ones, > > that's the whole reason we have all this code to try to coalesce > > contiguous I/O, do readahead, swap page clustering, etc. we *want* > > more complexity if it will get us bigger I/Os. I don't see why we > > want more complexity to reduce the *inherent* penalty of doing smaller > > ones. > > You should check out the latest proposal from Jens Axboe which treats > all biovec size the same and stuff it along with struct bio. I think > it is a better approach than my first cut of special casing 1 segment > biovec. His patch will speed up all sized I/O.
i rather agree with his reservations on that, since we'd be making the allocator's job harder by requesting order 1 pages for all allocations on x86_64 large I/O patterns. but it reduces complexity instead of increasing it ... can you produce some benchmarks not just for your workload but for one that triggers the order 1 case? biovec-(256) transfers are more common than you seem to think, and if the allocator can't do it, that forces the bio code to fall back to 2 x biovec-128, which, as you indicated above, would show a real penalty.
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