Messages in this thread Patches in this message | | | From | "Chen, Kenneth W" <> | Subject | RE: Cache queue_congestion_on/off_threshold | Date | Thu, 6 May 2004 13:29:06 -0700 |
| |
>>>> Andrew Morton wrote on Wednesday, May 05, 2004 11:34 PM > Jens Axboe <axboe@suse.de> wrote: > > > > Do you have any numbers at all for this? I'd say these calculations are > > severly into the noise area when submitting io. > > The difference will not be measurable, but I think the patch makes sense > regardless of what the numbers say.
Even though it is in the noise range that can't be easily measured, they are indeed in the positive territory. If I stack 5 of these little things, we actually measured positive gain on a large db workload.
There isn't anything absurd in 2.6 kernel, however, I hate to say that we consistently see performance regression with latest 2.6 kernel compare to best 2.4 based kernel under heavy db workload on 4-way SMP platform. (2.6 rocks on numa platform that 2.4 doesn't even have a chance to compete).
Some of the examples are:
(1) it's cheaper to find out whether a queue is empty or not by calling elv_queue_empty() instead of using heavier elv_next_request(). (2) it's better to check queue empty before calling into q->request_fn()
diff -Nurp linux-2.6.6-rc3/drivers/block/ll_rw_blk.c linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c --- linux-2.6.6-rc3/drivers/block/ll_rw_blk.c 2004-05-06 13:03:14.000000000 -0700 +++ linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c 2004-05-06 13:04:04.000000000 -0700 @@ -1128,7 +1128,7 @@ static inline void __generic_unplug_devi /* * was plugged, fire request_fn if queue has stuff to do */ - if (elv_next_request(q)) + if (!elv_queue_empty(q)) q->request_fn(q); }
@@ -1237,7 +1237,8 @@ void blk_run_queue(struct request_queue
spin_lock_irqsave(q->queue_lock, flags); blk_remove_plug(q); - q->request_fn(q); + if (!elv_queue_empty(q)) + q->request_fn(q); spin_unlock_irqrestore(q->queue_lock, flags); }
(3) can we allocate request structure up front in __make_request? For I/O that cannot be merged, the elevator code executes twice in __make_request.
diff -Nurp linux-2.6.6-rc3/drivers/block/ll_rw_blk.c linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c --- linux-2.6.6-rc3/drivers/block/ll_rw_blk.c 2004-05-06 13:03:14.000000000 -0700 +++ linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c 2004-05-06 13:11:39.000000000 -0700 @@ -2154,15 +2154,14 @@ static int __make_request(request_queue_
ra = bio->bi_rw & (1 << BIO_RW_AHEAD);
+ /* Grab a free request from the freelist */ + freereq = get_request(q, rw, GFP_ATOMIC); + again: spin_lock_irq(q->queue_lock);
- if (elv_queue_empty(q)) { + if (elv_queue_empty(q)) blk_plug_device(q); - goto get_rq; - } - if (barrier) - goto get_rq;
el_ret = elv_merge(q, &req, bio); switch (el_ret) {
Some more, I will post in another thread.
- Ken
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |