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

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

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().