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SubjectRe: [PATCH] 2.4.20-rmap15a
On Sun, 1 Dec 2002, Marc-Christian Petersen wrote:

> > update). The only thing left out of the merge for now is
> > Andrew Morton's read_latency patch, both because I'm not sure
> > how needed it is with the elevator updates

> Well, it is needed. This makes a difference for the I/O pauses noticed
> in 2.4.19 and 2.4.20. Anyway, readlatency-2 won't make them go away, but
> those stops/pauses are a bit less than before.

That was my gut feeling as well, but I guess it's a good thing
to quantify how much of a difference it makes. I wonder if we
could convince Con to test a kernel both with and without this
patch and look at the difference.

> So, here my patch proposal. Ontop of 2.4.20-rmap15a.

Looks good, now lets test it. If the patch is as needed as you
say we should push it to marcelo ;)

regards,

Rik
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--- linux-akpm/drivers/block/elevator.c~read-latency2 Sun Nov 10 19:53:53 2002
+++ linux-akpm-akpm/drivers/block/elevator.c Sun Nov 10 19:59:21 2002
@@ -80,25 +80,38 @@ int elevator_linus_merge(request_queue_t
struct buffer_head *bh, int rw,
int max_sectors)
{
- struct list_head *entry = &q->queue_head;
- unsigned int count = bh->b_size >> 9, ret = ELEVATOR_NO_MERGE;
+ struct list_head *entry;
+ unsigned int count = bh->b_size >> 9;
+ unsigned int ret = ELEVATOR_NO_MERGE;
+ int merge_only = 0;
+ const int max_bomb_segments = q->elevator.max_bomb_segments;
struct request *__rq;
+ int passed_a_read = 0;
+
+ entry = &q->queue_head;

while ((entry = entry->prev) != head) {
__rq = blkdev_entry_to_request(entry);

- /*
- * we can't insert beyond a zero sequence point
- */
- if (__rq->elevator_sequence <= 0)
- break;
+ if (__rq->elevator_sequence-- <= 0) {
+ /*
+ * OK, we've exceeded someone's latency limit.
+ * But we still continue to look for merges,
+ * because they're so much better than seeks.
+ */
+ merge_only = 1;
+ }

if (__rq->waiting)
continue;
if (__rq->rq_dev != bh->b_rdev)
continue;
- if (!*req && bh_rq_in_between(bh, __rq, &q->queue_head))
+ if (!*req && !merge_only &&
+ bh_rq_in_between(bh, __rq, &q->queue_head)) {
*req = __rq;
+ }
+ if (__rq->cmd != WRITE)
+ passed_a_read = 1;
if (__rq->cmd != rw)
continue;
if (__rq->nr_sectors + count > max_sectors)
@@ -129,6 +142,57 @@ int elevator_linus_merge(request_queue_t
}
}

+ /*
+ * If we failed to merge a read anywhere in the request
+ * queue, we really don't want to place it at the end
+ * of the list, behind lots of writes. So place it near
+ * the front.
+ *
+ * We don't want to place it in front of _all_ writes: that
+ * would create lots of seeking, and isn't tunable.
+ * We try to avoid promoting this read in front of existing
+ * reads.
+ *
+ * max_bomb_segments becomes the maximum number of write
+ * requests which we allow to remain in place in front of
+ * a newly introduced read. We weight things a little bit,
+ * so large writes are more expensive than small ones, but it's
+ * requests which count, not sectors.
+ */
+ if (max_bomb_segments && rw == READ && !passed_a_read &&
+ ret == ELEVATOR_NO_MERGE) {
+ int cur_latency = 0;
+ struct request * const cur_request = *req;
+
+ entry = head->next;
+ while (entry != &q->queue_head) {
+ struct request *__rq;
+
+ if (entry == &q->queue_head)
+ BUG();
+ if (entry == q->queue_head.next &&
+ q->head_active && !q->plugged)
+ BUG();
+ __rq = blkdev_entry_to_request(entry);
+
+ if (__rq == cur_request) {
+ /*
+ * This is where the old algorithm placed it.
+ * There's no point pushing it further back,
+ * so leave it here, in sorted order.
+ */
+ break;
+ }
+ if (__rq->cmd == WRITE) {
+ cur_latency += 1 + __rq->nr_sectors / 64;
+ if (cur_latency >= max_bomb_segments) {
+ *req = __rq;
+ break;
+ }
+ }
+ entry = entry->next;
+ }
+ }
return ret;
}

@@ -186,7 +250,7 @@ int blkelvget_ioctl(elevator_t * elevato
output.queue_ID = elevator->queue_ID;
output.read_latency = elevator->read_latency;
output.write_latency = elevator->write_latency;
- output.max_bomb_segments = 0;
+ output.max_bomb_segments = elevator->max_bomb_segments;

if (copy_to_user(arg, &output, sizeof(blkelv_ioctl_arg_t)))
return -EFAULT;
@@ -205,9 +269,12 @@ int blkelvset_ioctl(elevator_t * elevato
return -EINVAL;
if (input.write_latency < 0)
return -EINVAL;
+ if (input.max_bomb_segments < 0)
+ return -EINVAL;

elevator->read_latency = input.read_latency;
elevator->write_latency = input.write_latency;
+ elevator->max_bomb_segments = input.max_bomb_segments;
return 0;
}

--- linux-akpm/drivers/block/ll_rw_blk.c~read-latency2 Sun Nov 10 19:53:53 2002
+++ linux-akpm-akpm/drivers/block/ll_rw_blk.c Sun Nov 10 19:53:53 2002
@@ -432,9 +432,11 @@ static void blk_init_free_list(request_q

si_meminfo(&si);
megs = si.totalram >> (20 - PAGE_SHIFT);
- nr_requests = 128;
- if (megs < 32)
- nr_requests /= 2;
+ nr_requests = (megs * 2) & ~15; /* One per half-megabyte */
+ if (nr_requests < 32)
+ nr_requests = 32;
+ if (nr_requests > 1024)
+ nr_requests = 1024;
blk_grow_request_list(q, nr_requests);

init_waitqueue_head(&q->wait_for_requests[0]);
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