Hi,
Looking at the second loop in elevator_linus_merge(), it is possible for
requests to have their elevator_sequence go negative. This can cause a
v long latency before the request is finally serviced.
Say, for example, a request (in the queue) is jumped in the first loop
in elevator_linus_merge() as "cmd != rw", even though its
elevator_sequence is zero. If it is found that the new request will
merge, the walking back over requests which were jumped makes no test for
an already zeroed elevator_sequence. Hence it zero values can occur.
With high default values for read/wite_latency, this hardly ever occurs.
A simple fix for this is to test for zero before decrementing (patch
below) in the second loop.
Alternatively, should testing in the first loop be modified?
Mark
diff -u --recursive --new-file -X dontdiff linux-2.4.0-test12/drivers/block/elevator.c markhe-2.4.0-test12/drivers/block/elevator.c
--- linux-2.4.0-test12/drivers/block/elevator.c Tue Dec 5 23:05:26 2000
+++ markhe-2.4.0-test12/drivers/block/elevator.c Mon Dec 18 17:50:19 2000
@@ -90,6 +90,9 @@
if (ret != ELEVATOR_NO_MERGE && *req) {
while ((entry = entry->next) != &q->queue_head) {
struct request *tmp = blkdev_entry_to_request(entry);
+
+ if (!tmp->elevator_sequence)
+ continue;
tmp->elevator_sequence--;
}
}
On Mon, Dec 18 2000, Mark Hemment wrote:
> Hi,
>
> Looking at the second loop in elevator_linus_merge(), it is possible for
> requests to have their elevator_sequence go negative. This can cause a
> v long latency before the request is finally serviced.
>
> Say, for example, a request (in the queue) is jumped in the first loop
> in elevator_linus_merge() as "cmd != rw", even though its
> elevator_sequence is zero. If it is found that the new request will
> merge, the walking back over requests which were jumped makes no test for
> an already zeroed elevator_sequence. Hence it zero values can occur.
>
> With high default values for read/wite_latency, this hardly ever occurs.
>
> A simple fix for this is to test for zero before decrementing (patch
> below) in the second loop.
The merge part was original deliberate, as not to account successful
merges as much as a new request added (and thus an implied seek). But
you did uncover a problem, btw this is also fixed in the blk-12 patch
that also does better accounting to avoid indefinite starvation.
> Alternatively, should testing in the first loop be modified?
To stay with the original design, yes.
--
* Jens Axboe <[email protected]>
* SuSE Labs