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

["Vincent, Pradeep" <pradeepv@amazon.com>]

[-- Attachment #1.1: Type: text/plain, Size: 1274 bytes --]

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@amazon.com>

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 #1.2: Type: text/html, Size: 2329 bytes --]

[-- Attachment #2: blkback-bugfix-reqevent-assignment.patch --]
[-- Type: application/octet-stream, Size: 662 bytes --]

Signed-off-by: Pradeep Vincent <pradeepv@amazon.com>

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 #3: Type: text/plain, Size: 138 bytes --]

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel