On Tue, Feb 16, 2016 at 4:06 AM, Sagi Grimberg <sagig@xxxxxxxxxxxxxxxxxx> wrote:
>
>> Cc Kent and Keith.
>>
>> Follows another version which should be more efficient.
>> Kent and Keith, I appreciate much if you may give a review on it.
>>
>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>> index 56d2db8..ef45fec 100644
>> --- a/include/linux/bio.h
>> +++ b/include/linux/bio.h
>> @@ -278,11 +278,21 @@ static inline void bio_get_first_bvec(struct bio
>> *bio, struct bio_vec *bv)
>> */
>> static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec
>> *bv)
>> {
>> - struct bvec_iter iter;
>> + struct bvec_iter iter = bio->bi_iter;
>> + int idx;
>> +
>> + bio_advance_iter(bio, &iter, iter.bi_size);
>> +
>> + WARN_ON(!iter.bi_idx && !iter.bi_bvec_done);
>> +
>> + if (!iter.bi_bvec_done)
>> + idx = iter.bi_idx - 1;
>> + else /* in the middle of bvec */
>> + idx = iter.bi_idx;
>>
>> - bio_for_each_segment(*bv, bio, iter)
>> - if (bv->bv_len == iter.bi_size)
>> - break;
>> + *bv = bio->bi_io_vec[idx];
>> + if (iter.bi_bvec_done)
>> + bv->bv_len = iter.bi_bvec_done;
>> }
>>
>> /*
>>
>
> This looks good too.
>
>>
>>>
>>> However, given that it's a regression bug fix I'm not sure it's the best
>>> idea to add logic here.
>>
>>
>> But the issue is obviously in bio_will_gap(), isn't it?
>>
>> Simply reverting 52cc6eead9095(block: blk-merge: fast-clone bio when
>> splitting rw bios)
>> still might cause performance regression too.
>
>
> That's correct. I assume that the bio splitting code affects
> specific I/O pattern (gappy), however bio_will_gap is also tested
I don't understand why bio splitting affects specific I/O pattern, could you
explain a bit?
>From commit b54ffb73c(block: remove bio_get_nr_vecs()), the upper
layer(fs, dm, dio,...) creates bio with its max size, and splitting should
be triggered easily.
> for bio merges (even if the bios won't merge eventually). This means
As I mentioned, bio_will_gap() is only called for non-splitted bio.
> that each merge check will invoke bio_advance_iter() which is something
> I'd like to avoid...
One idea is to use original way to compute the last bvec for non-cloned
bio, and use the approach in this patch for cloned bio(often splitted bio).
I will take this way in v1 if no one objects.
thanks,
Ming
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