On Wed, Nov 2, 2016 at 10:24 PM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > On Wed, Nov 02 2016 at 3:56am -0400, > Ming Lei <tom.leiming@xxxxxxxxx> wrote: > >> On Wed, Nov 2, 2016 at 11:09 AM, Kent Overstreet >> <kent.overstreet@xxxxxxxxx> wrote: >> > On Mon, Oct 31, 2016 at 08:29:01AM -0700, Christoph Hellwig wrote: >> >> On Sat, Oct 29, 2016 at 04:08:08PM +0800, Ming Lei wrote: >> >> > Avoid to access .bi_vcnt directly, because it may be not what >> >> > the driver expected any more after supporting multipage bvec. >> >> > >> >> > Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx> >> >> >> >> It would be really nice to have a comment in the code why it's >> >> even checking for multiple segments. >> > >> > Or ideally refactor the code to not care about multiple segments at all. >> >> The check on 'bio->bi_vcnt == 1' is introduced in commit de3ec86dff160(dm: >> don't start current request if it would've merged with the previous), which >> fixed one performance issue.[1] >> >> Looks the idea of the patch is to delay dispatching the rq if it >> would've merged with previous request and the rq is small(single bvec). >> I guess the motivation is to try to increase chance of merging with the delay. >> >> But why does the code check on 'bio->bi_vcnt == 1'? Once the bio is >> submitted, .bi_vcnt isn't changed any more and merging doesn't change >> it too. So should the check have been on blk_rq_bytes(rq)? >> >> Mike, please correct me if my understanding is wrong. >> >> >> [1] https://www.redhat.com/archives/dm-devel/2015-March/msg00014.html > > The patch was labored over for quite a while and is based on suggestions I > got from Jens when discussing a very problematic aspect of old > .request_fn request-based DM performance for a multi-threaded (64 > threads) sequential IO benchmark (vdbench IIRC). The issue was reported > by NetApp. > > The patch in question fixed the lack of merging that was seen with this > interleaved sequential IO benchmark. The lack of merging was made worse > if a DM multipath device had more underlying paths (e.g. 4 instead of 2). > > As for your question, about using blk_rq_bytes(rq) vs 'bio->bi_vcnt == 1' > .. not sure how that would be a suitable replacement. But it has been a > while since I've delved into these block core merge details of old Just last year, looks not long enough, :-) > .request_fn but _please_ don't change the logic of this code simply As I explained before, neither .bi_vcnt will be changed after submitting, nor be changed during merging, so I think the checking is wrong, could you explain what is your initial motivation of checking on 'bio->bi_vcnt == 1'? > because it is proving itself to be problematic for your current > patchset's cleanliness. Could you explain what is problematic for the cleanliness? Thanks, Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
