On Tue, Mar 15, 2016 at 04:17:56PM +0100, Vitaly Kuznetsov wrote:
> The reason of the slowdown is the fact that bios don't get merged and we
> end up sending many short requests to the host. My investigation led me to
> the following code (__bvec_gap_to_prev()):
>
> return offset ||
> ((bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q));
>
> Here is an example: we have two bio_vec with the following content:
> bprv.bv_offset = 512
> bprv.bv_len = 512
>
> bnxt.bv_offset = 1024
> bnxt.bv_len = 512
>
> bprv.bv_page == bnxt.bv_page
> virt_boundary is set to PAGE_SIZE-1
>
> The above mentioned code will report that a gap will appear if we merge
> these two (as offset = 1024) but this doesn't look sane. On top of that,
> we have the following optimization in bio_add_pc_page():
>
> if (page == prev->bv_page &&
> offset == prev->bv_offset + prev->bv_len) {
> prev->bv_len += len;
> bio->bi_iter.bi_size += len;
> goto done;
> }
This part sounds odd. Why is a filesystem using bio_add_pc_page? Shouldn't
these go through "bio_add_page" instead? That already has an optimization
to combine bio's within the same page.
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