[Xen-devel] [PATCH] blkback: Fix block I/O latency issue

["Vincent, Pradeep" <pradeepv@xxxxxxxxxx>]

In blkback driver, after I/O requests are submitted to Dom-0 block I/O subsystem, blkback goes to 'sleep' effectively without letting blkfront know about it (req_event isn't set appropriately). Hence blkfront doesn't notify blkback when it submits a new I/O thus delaying the 'dispatch' of the new I/O to Dom-0 block I/O subsystem. The new I/O is dispatched as soon as one of the previous I/Os completes.

As a result of this issue, the block I/O latency performance is degraded for some workloads on Xen guests using blkfront-blkback stack.

The following change addresses this issue:

Signed-off-by: Pradeep Vincent <pradeepv@xxxxxxxxxx>

diff --git a/drivers/xen/blkback/blkback.c b/drivers/xen/blkback/blkback.c

--- a/drivers/xen/blkback/blkback.c

+++ b/drivers/xen/blkback/blkback.c

@@ -383,6 +383,12 @@ static int do_block_io_op(blkif_t *blkif)

  cond_resched();

  }

 

+ /* If blkback might go to sleep (i.e. more_to_do == 0) then we better

+   let blkfront know about it (by setting req_event appropriately) so that

+   blkfront will bother to wake us up (via interrupt) when it submits a 

+   new I/O */

+        if (!more_to_do)

+                 RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);

  return more_to_do;

 }

 

Attachment: blkback-bugfix-reqevent-assignment.patch
Description: blkback-bugfix-reqevent-assignment.patch

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