RE: Processes stuck on D state on Dual Opteron

["Chen, Kenneth W" <kenneth.w.chen@intel.com>]

On Tue, Apr 12 2005, Nick Piggin wrote:
> Actually the patches I have sent you do fix real bugs, but they also
> make the block layer less likely to recurse into page reclaim, so it
> may be eg. hiding the problem that Neil's patch fixes.

Jens Axboe wrote on Tuesday, April 12, 2005 12:08 AM
> Can you push those to Andrew? I'm quite happy with the way they turned
> out. It would be nice if Ken would bench 2.6.12-rc2 with and without
> those patches.


I like the patch a lot and already did bench it on our db setup.  However,
I'm seeing a negative regression compare to a very very crappy patch (see
attached, you can laugh at me for doing things like that :-).

My first reaction is that the overhead is in wait queue setup and tear down
in get_request_wait function. Throwing the following patch on top does improve
things a bit, but we are still in the negative territory.  I can't explain why.
Everything suppose to be faster.  So I'm staring at the execution profile at
the moment.


diff -Nru a/drivers/block/ll_rw_blk.c b/drivers/block/ll_rw_blk.c
--- a/drivers/block/ll_rw_blk.c	2005-04-12 00:48:12 -07:00
+++ b/drivers/block/ll_rw_blk.c	2005-04-12 00:48:12 -07:00
@@ -1740,10 +1740,35 @@
  */
 static struct request *get_request_wait(request_queue_t *q, int rw)
 {
-	DEFINE_WAIT(wait);
 	struct request *rq;
+	struct request_list *rl = &q->rq;
+	int gfp_flag = GFP_ATOMIC;
+
+	if (rl->count[rw] < queue_congestion_off_threshold(q)) {
+		rq = kmem_cache_alloc(request_cachep, gfp_flag);
+		if (rq) {
+			if (!elv_set_request(q, rq, gfp_flag)) {
+
+				rl->count[rw]++;
+				INIT_LIST_HEAD(&rq->queuelist);
+				rq->flags = rw;
+				rq->rq_status = RQ_ACTIVE;
+				rq->ref_count = 1;
+				rq->q = q;
+				rq->rl = rl;
+				rq->special = NULL;
+				rq->data_len = 0;
+				rq->data = NULL;
+				rq->sense = NULL;
+
+				return rq;
+			}
+			kmem_cache_free(request_cachep, rq);
+		}
+	}

 	do {
+		DEFINE_WAIT(wait);
 		struct request_list *rl = &q->rq;

 		prepare_to_wait_exclusive(&rl->wait[rw], &wait,



begin 666 old_freereq.patch
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`
end

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