Re: [PATCH V4] block: optimize for small block size IO

[Ming Lei <ming.lei@redhat.com>]

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