Re: Cache queue_congestion_on/off_threshold

Jens Axboe
2004-05-06T06:20:36Z

On Wed, May 05 2004, Chen, Kenneth W wrote:
> It's kind of redundant that queue_congestion_on/off_threshold gets
> calculated on every I/O and they produce the same number over and
> over again unless q->nr_requests gets changed (which is probably a
> very rare event). Can we cache those values in the request_queue
> structure?
>
> - Ken
>
>
> diff -Nurp linux-2.6.6-rc3/drivers/block/ll_rw_blk.c linux-2.6.6-rc3.blk/drivers/block/ll_rw_blk.c
> --- linux-2.6.6-rc3/drivers/block/ll_rw_blk.c 2004-05-05 14:32:31.000000000 -0700
> +++ linux-2.6.6-rc3.blk/drivers/block/ll_rw_blk.c 2004-05-05 15:04:59.000000000 -0700
> @@ -70,14 +70,7 @@ EXPORT_SYMBOL(blk_max_pfn);
> */
> static inline int queue_congestion_on_threshold(struct request_queue *q)
> {
> - int ret;
> -
> - ret = q->nr_requests - (q->nr_requests / 8) + 1;
> -
> - if (ret > q->nr_requests)
> - ret = q->nr_requests;
> -
> - return ret;
> + return q->nr_congestion_on;
> }
>
> /*
> @@ -85,14 +78,22 @@ static inline int queue_congestion_on_th
> */
> static inline int queue_congestion_off_threshold(struct request_queue *q)
> {
> - int ret;
> + return q->nr_congestion_off;
> +}
>
> - ret = q->nr_requests - (q->nr_requests / 8) - 1;
> +static inline void blk_queue_congestion_threshold(struct request_queue *q)
> +{
> + int nr;
>
> - if (ret < 1)
> - ret = 1;
> + nr = q->nr_requests - (q->nr_requests / 8) + 1;
> + if (nr > q->nr_requests)
> + nr = q->nr_requests;
> + q->nr_congestion_on = nr;
>
> - return ret;
> + nr = q->nr_requests - (q->nr_requests / 8) - 1;
> + if (nr < 1)
> + nr = 1;
> + q->nr_congestion_off = nr;
> }
>
> void clear_backing_dev_congested(struct backing_dev_info *bdi, int rw)
> @@ -235,6 +236,7 @@ void blk_queue_make_request(request_queu
> blk_queue_max_sectors(q, MAX_SECTORS);
> blk_queue_hardsect_size(q, 512);
> blk_queue_dma_alignment(q, 511);
> + blk_queue_congestion_threshold(q);
>
> q->unplug_thresh = 4; /* hmm */
> q->unplug_delay = (3 * HZ) / 1000; /* 3 milliseconds */
> @@ -2953,6 +2955,7 @@ queue_requests_store(struct request_queu
> int ret = queue_var_store(&q->nr_requests, page, count);
> if (q->nr_requests < BLKDEV_MIN_RQ)
> q->nr_requests = BLKDEV_MIN_RQ;
> + blk_queue_congestion_threshold(q);
>
> if (rl->count[READ] >= queue_congestion_on_threshold(q))
> set_queue_congested(q, READ);
> diff -Nurp linux-2.6.6-rc3/include/linux/blkdev.h linux-2.6.6-rc3.blk/include/linux/blkdev.h
> --- linux-2.6.6-rc3/include/linux/blkdev.h 2004-04-27 18:35:21.000000000 -0700
> +++ linux-2.6.6-rc3.blk/include/linux/blkdev.h 2004-05-05 15:04:59.000000000 -0700
> @@ -334,6 +334,8 @@ struct request_queue
> * queue settings
> */
> unsigned long nr_requests; /* Max # of requests */
> + unsigned int nr_congestion_on;
> + unsigned int nr_congestion_off;
>
> unsigned short max_sectors;
> unsigned short max_phys_segments;

Do you have any numbers at all for this? I'd say these calculations are
severly into the noise area when submitting io.

--
Jens Axboe