Re: [PATCH 09/60] dm: dm.c: replace 'bio->bi_vcnt == 1' with !bio_multiple_segments

[Ming Lei]

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