lkml.org 
[lkml]   [2016]   [Mar]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH RFC] block: fix bio merge checks when virt_boundary is set
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.

\
 
 \ /
  Last update: 2016-03-15 17:41    [W:0.100 / U:0.896 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site