On 2012-09-07 16:25, Kent Overstreet wrote: > On Fri, Sep 07, 2012 at 04:06:45PM -0600, Jens Axboe wrote: >> On 2012-09-07 15:55, Jens Axboe wrote: >>> On 2012-09-07 14:58, Kent Overstreet wrote: >>>> On Thu, Sep 06, 2012 at 07:34:18PM -0600, Jens Axboe wrote: >>>>> On 2012-09-06 16:34, Kent Overstreet wrote: >>>>>> Reusing bios is something that's been highly frowned upon in the past, >>>>>> but driver code keeps doing it anyways. If it's going to happen anyways, >>>>>> we should provide a generic method. >>>>>> >>>>>> This'll help with getting rid of bi_destructor - drivers/block/pktcdvd.c >>>>>> was open coding it, by doing a bio_init() and resetting bi_destructor. >>>>>> >>>>>> This required reordering struct bio, but the block layer is not yet >>>>>> nearly fast enough for any cacheline effects to matter here. >>>>> >>>>> That's an odd and misplaced comment. Was just doing testing today at 5M >>>>> IOPS, and even years back we've had cache effects for O_DIRECT in higher >>>>> speed setups. >>>> >>>> Ah, I wasn't aware that you were pushing that many iops through the >>>> block layer - most I've tested myself was around 1M. It wouldn't >>>> surprise me if cache effects in struct bio mattered around 5M... >>> >>> 5M is nothing, just did 13.5M :-) >>> >>> But we can reshuffle for now. As mentioned, we're way overdue for a >>> decent look at cache profiling in any case. >> >> No ill effects seen so far, fwiw: >> >> read : io=1735.8GB, bw=53690MB/s, iops=13745K, runt= 33104msec > > Cool! > > I'd be really curious to see a profile. Of the patches I've got queued > up I don't think anything's going to significantly affect performance > yet, but I'm hoping the cleanups/immutable bvec stuff/efficient bio > splitting enables some performance gains. Got more work to do, but certainly not a problem sharing. > Well, it certainly will for stacking drivers, but I'm less sure what > it's going to look like running on just a raw flash device. > > My end goal is making generic_make_request handle arbitrary sized bios, > and have (efficient) splitting happen as required. This'll get rid of a > bunch of code and complexity in the upper layers, in bio_add_page() and > elsewhere. More in the stacking drivers - merge_bvec_fn is horrendous to > support. It is a nasty interface, in retrospect probably a mistake. As long as we don't split ever on non-stacking drivers, I don't care too much. And it would get rid of complexity in those drivers, so that's a nice win. merge_bvec_fn not only a bad interface, it's also pretty slow... > I think I might be able to efficiently get rid of the > segments-after-merging precalculating, and just have segments merged > once. That'd get rid of a couple fields in struct bio, and get it under > 2 cachelines last I counted. It's 2 cachelines now, but reducing is always a great thing. Getting rid of the repeated recalculate after merge would be a nice win. > Course, all this doesn't matter as much for 4k bios so it may just be a > wash for you. Right, for me it doesn't matter. As long as you don't slow me down :-) -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
