On Mon, Feb 23 2015 at 4:19pm -0500, Benjamin Marzinski <bmarzins@xxxxxxxxxx> wrote: > On Mon, Feb 23, 2015 at 02:56:03PM -0500, Mike Snitzer wrote: > > On Mon, Feb 23 2015 at 1:34pm -0500, > > Benjamin Marzinski <bmarzins@xxxxxxxxxx> wrote: > > > > > On Mon, Feb 23, 2015 at 11:18:36AM -0600, Mike Christie wrote: > > > > > > > > If the device/transport is fast or the workload is low, the multipath_busy > > > > never returns busy, then we can hit Hannes's issue. For 4 paths, we just > > > > might not be able to fill up the paths and hit the busy check. With only 2 > > > > paths, we might be slow enough or the workload is heavy enough to keep the > > > > paths busy and so we hit the busy check and do more merging. > > > > > > Netapp is seeing this same issue. It seems like we might want to make > > > multipath_busy more aggressive about returning busy, which would > > > probably require multipath tracking the size and frequency of the > > > requests. If it determines that it's getting a lot of requests that > > > could have been merged, it could start throttling how fast requests are > > > getting pulled off the queue, even there underlying paths aren't busy. > > > > the ->busy() checks are just an extra check the shouldn't be the primary > > method for governing the effectiveness of the DM-mpath queue's elevator. > > > > I need to get back to basics to appreciate how the existing block layer > > is able to have an effective elevator regardless of the device's speed. > > And why isn't request-based DM able to just take advantage of it? > > I always thought that at least one of the schedulers always kept > incoming requests on an interal queue for at least a little bit to see > if any merging could happen, even if they could otherwise just be added > to the request queue. but I admit to being a little vague on how exactly > they all work. CFQ has idling, etc. Which promotes merging. > Another place where we could break out of constantly pulling requests of > the queue before they're merged is in dm_prep_fn(). If we thought that > we should break and let merging happen, we could return BLKPREP_DEFER. It is blk_queue_bio(), via q->make_request_fn, that is intended to actually do the merging. What I'm hearing is that we're only getting some small amount of merging if: 1) the 2 path case is used and therefore ->busy hook within q->request_fn is not taking the request off the queue, so there is more potential for later merging 2) the 4 path case IFF nr_requests is reduced to induce ->busy, which only promoted merging as a side-effect like 1) above The reality is we aren't getting merging where it _should_ be happening (in blk_queue_bio). We need to understand why that is. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
