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().
