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  1. 12 May, 2011 4 commits
  3. 11 May, 2011 1 commit
  5. 05 May, 2011 4 commits
  7. 27 Apr, 2011 1 commit
  9. 26 Apr, 2011 2 commits
    • Konrad Rzeszutek Wilk's avatar
      xen/blkback: Stick REQ_SYNC on WRITEs to deal with CFQ I/O scheduler. · 013c3ca1
      Konrad Rzeszutek Wilk authored 
      If one runs a simple fio request with random read/write with a
20%/80% ratio, the numbers are incredibly bad when using the CFQ scheduler.

IOmeter       |       |      |          |
64K, randrw   |  NOOP | CFQ  | deadline |
randrwmix=80  |       |      |          |
--------------+-------+------+----------+
blkback       |103/27 |32/10 | 102/27   |
--------------+-------+------+----------+
QEMU qdisk    |103/27 |102/27| 102/27   |

The problem as explained by Vivek Goyal was:

".. that difference is that sync vs async requests. In the case of
a kernel thread submitting IO, [..] all the WRITES might be being
considered as async and will go in a different queue. If you mix those
with some READS, they are always sync and will go in differnet queue.
In presence of sync queue, CFQ will idle and choke up WRITES in
an attempt to improve latencies of READs.

In case of AIO [note: this is what QEMU qdisk is doing] , [..]
it is direct IO and both READS and WRITES will be considered SYNC
and will go in a single queue and no choking of WRITES will take place."

The solution is quite simple, tack on REQ_SYNC (which is
what the WRITE_ODIRECT macro points to) and the numbers go
back up.

Suggested-by: Vivek Goyal <vgoyal@redhat.com
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
      013c3ca1
    • Konrad Rzeszutek Wilk's avatar
      xen/blkback: Move the plugging/unplugging to a higher level. · 97961ef4
      Konrad Rzeszutek Wilk authored 
      We used to the plug/unplug on the submit_bio. But that means
if within a stream of WRITE, WRITE, WRITE,...,WRITE we have
one READ, it could stall the pipeline (as the 'submio_bio'
could trigger the unplug_fnc to be called and stall/sync
when doing the READ). Instead we want to move the unplugging
when the whole (or as a much as possible) ring buffer has been
processed. This also eliminates us doing plug/unplug for
each request.
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
      97961ef4
  11. 20 Apr, 2011 4 commits
  13. 18 Apr, 2011 4 commits
  15. 15 Apr, 2011 5 commits
  17. 14 Apr, 2011 15 commits
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