On Mon, Nov 04, 2019 at 07:49:49PM -0700, Jens Axboe wrote: > On 11/4/19 7:46 PM, Ming Lei wrote: > > On Mon, Nov 04, 2019 at 09:30:02PM -0500, Kent Overstreet wrote: > >> On Tue, Nov 05, 2019 at 10:20:46AM +0800, Ming Lei wrote: > >>> On Mon, Nov 04, 2019 at 09:11:30PM -0500, Kent Overstreet wrote: > >>>> On Tue, Nov 05, 2019 at 09:11:35AM +0800, Ming Lei wrote: > >>>>> On Mon, Nov 04, 2019 at 01:42:17PM -0500, Kent Overstreet wrote: > >>>>>> On Mon, Nov 04, 2019 at 11:23:42AM -0700, Jens Axboe wrote: > >>>>>>> On 11/4/19 11:17 AM, Kent Overstreet wrote: > >>>>>>>> On Mon, Nov 04, 2019 at 10:15:41AM -0800, Christoph Hellwig wrote: > >>>>>>>>> On Mon, Nov 04, 2019 at 01:14:03PM -0500, Kent Overstreet wrote: > >>>>>>>>>> On Sat, Nov 02, 2019 at 03:29:11PM +0800, Ming Lei wrote: > >>>>>>>>>>> __blk_queue_split() may be a bit heavy for small block size(such as > >>>>>>>>>>> 512B, or 4KB) IO, so introduce one flag to decide if this bio includes > >>>>>>>>>>> multiple page. And only consider to try splitting this bio in case > >>>>>>>>>>> that the multiple page flag is set. > >>>>>>>>>> > >>>>>>>>>> So, back in the day I had an alternative approach in mind: get rid of > >>>>>>>>>> blk_queue_split entirely, by pushing splitting down to the request layer - when > >>>>>>>>>> we map the bio/request to sgl, just have it map as much as will fit in the sgl > >>>>>>>>>> and if it doesn't entirely fit bump bi_remaining and leave it on the request > >>>>>>>>>> queue. > >>>>>>>>>> > >>>>>>>>>> This would mean there'd be no need for counting segments at all, and would cut a > >>>>>>>>>> fair amount of code out of the io path. > >>>>>>>>> > >>>>>>>>> I thought about that to, but it will take a lot more effort. Mostly > >>>>>>>>> because md/dm heavily rely on splitting as well. I still think it is > >>>>>>>>> worthwhile, it will just take a significant amount of time and we > >>>>>>>>> should have the quick improvement now. > >>>>>>>> > >>>>>>>> We can do it one driver at a time - driver sets a flag to disable > >>>>>>>> blk_queue_split(). Obvious one to do first would be nvme since that's where it > >>>>>>>> shows up the most. > >>>>>>>> > >>>>>>>> And md/md do splitting internally, but I'm not so sure they need > >>>>>>>> blk_queue_split(). > >>>>>>> > >>>>>>> I'm a big proponent of doing something like that instead, but it is a > >>>>>>> lot of work. I absolutely hate the splitting we're doing now, even > >>>>>>> though the original "let's work as hard as we add add page time to get > >>>>>>> things right" was pretty abysmal as well. > >>>>>> > >>>>>> Last I looked I don't think it was going to be that bad, just needed a bit of > >>>>>> finesse. We just need to be able to partially process a request in e.g. > >>>>>> nvme_map_data(), and blk_rq_map_sg() needs to be modified to only map as much as > >>>>>> will fit instead of popping an assertion. > >>>>> > >>>>> I think it may not be doable. > >>>>> > >>>>> blk_rq_map_sg() is called by drivers and has to work on single request, however > >>>>> more requests have to be involved if we delay the splitting to blk_rq_map_sg(). > >>>>> Cause splitting means that two bios can't be submitted in single IO request. > >>>> > >>>> Of course it's doable, do I have to show you how? > >>> > >>> No, you don't have to, could you just point out where my above words is wrong? > >> > >> blk_rq_map_sg() _currently_ works on a single request, but as I said from the > >> start that this would involve changing it to only process as much of a request > >> as would fit on an sglist. > > > >> Drivers will have to be modified, but the changes to driver code should be > >> pretty easy. What will be slightly trickier will be changing blk-mq to handle > >> requests that are only partially completed; that will be harder than it would > >> have been before blk-mq, since the old request queue code used to handle > >> partially completed requests - not much work would have to be done that code. > > > > Looks you are suggesting partial request completion. > > > > Then the biggest effect could be in performance, this change will cause the > > whole FS bio is handled part by part serially, instead of submitting all > > splitted bios(part) concurrently. > > > > So sounds you are suggesting to fix one performance issue by causing new perf > > issue, is that doable? > > It does seem like a rat hole of sorts. Because then you start adding code to > guesstimate how big the request could roughly be, and if you miss a bit, > you get a request that's tiny in between the normal sized ones. > > Or you'd clone, and then you could still have them inflight in parallel. > But then you're paying the cost of that... However, IO latency is often much longer than latency of bio fast clone, which is just one fix-sized slab allocation, mostly should hit cache. I understand the current issue should be the cost of blk_queue_split() in case of no splitting is required. blk_bio_segment_split() has been simple enough, looks we need to investigate why it is so slow since it shouldn't be such in-efficient. Thanks, Ming
