Re: bad performance with SSD since kernel version 2.6.32

[Mark Lord <kernel@teksavvy.com>]

On 02/22/10 14:05, Jens Axboe wrote:

--- a/block/blk-core.c
+++ b/block/blk-core.c
...
@@ -1857,8 +1857,15 @@ void blk_dequeue_request(struct request *rq)
          * and to it is freed is accounted as io that is in progress at
          * the driver side.
          */
-       if (blk_account_rq(rq))
+       if (blk_account_rq(rq)) {
                 q->in_flight[rq_is_sync(rq)]++;
+               /*
+                * Mark this device as supporting hardware queuing, if
+                * we have more IOs in flight than 4.
+                */
+               if (!blk_queue_queuing(q) && queue_in_flight(q) > 4)
+                       set_bit(QUEUE_FLAG_CQ, &q->queue_flags);
+       }
  }
...

Mmm.. So is this code actually trying to rely upon the software being quick
enough to queue five or more commands before the drive completes one of them?

Wouldn't a better way be to just look at the queue_depth, for SCSI/SATA at least?

-ml