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

[Kent Overstreet <kent.overstreet@gmail.com>]

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.