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.
