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

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

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?


Thanks,
Ming