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Commit e8a99053 authored by Andreas Mohr's avatar Andreas Mohr Committed by Jens Axboe
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[PATCH] cfq-iosched: small cfq_choose_req() optimization 



this is a small optimization to cfq_choose_req() in the CFQ I/O scheduler
(this function is a semi-often invoked candidate in an oprofile log):
by using a bit mask variable, we can use a simple switch() to check
the various cases instead of having to query two variables for each check.
Benefit: 251 vs. 285 bytes footprint of cfq_choose_req().
Also, common case 0 (no request wrapping) is now checked first in code.

Signed-off-by: default avatarAndreas Mohr <andi@lisas.de>
Signed-off-by: default avatarJens Axboe <axboe@suse.de>
parent e2d74ac0

block/cfq-iosched.c

+32 −18
Original line number Diff line number Diff line
@@ -346,14 +346,16 @@ static int cfq_queue_empty(request_queue_t *q) 
/*

 * Lifted from AS - choose which of crq1 and crq2 that is best served now.

 * We choose the request that is closest to the head right now. Distance

 * behind the head are penalized and only allowed to a certain extent.

 * behind the head is penalized and only allowed to a certain extent.

 */

static struct cfq_rq *

cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2)

{

	sector_t last, s1, s2, d1 = 0, d2 = 0;

	int r1_wrap = 0, r2_wrap = 0;	/* requests are behind the disk head */

	unsigned long back_max;

#define CFQ_RQ1_WRAP	0x01 /* request 1 wraps */

#define CFQ_RQ2_WRAP	0x02 /* request 2 wraps */

	unsigned wrap = 0; /* bit mask: requests behind the disk head? */



	if (crq1 == NULL || crq1 == crq2)

		return crq2;
@@ -385,35 +387,47 @@ cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2) 
	else if (s1 + back_max >= last)

		d1 = (last - s1) * cfqd->cfq_back_penalty;

	else

		r1_wrap = 1;

		wrap |= CFQ_RQ1_WRAP;



	if (s2 >= last)

		d2 = s2 - last;

	else if (s2 + back_max >= last)

		d2 = (last - s2) * cfqd->cfq_back_penalty;

	else

		r2_wrap = 1;

		wrap |= CFQ_RQ2_WRAP;



	/* Found required data */

	if (!r1_wrap && r2_wrap)



	/*

	 * By doing switch() on the bit mask "wrap" we avoid having to

	 * check two variables for all permutations: --> faster!

	 */

	switch (wrap) {

	case 0: /* common case for CFQ: crq1 and crq2 not wrapped */

		if (d1 < d2)

			return crq1;

	else if (!r2_wrap && r1_wrap)

		else if (d2 < d1)

			return crq2;

	else if (r1_wrap && r2_wrap) {

		/* both behind the head */

		if (s1 <= s2)

		else {

			if (s1 >= s2)

				return crq1;

			else

				return crq2;

		}



	/* Both requests in front of the head */

	if (d1 < d2)

	case CFQ_RQ2_WRAP:

		return crq1;

	else if (d2 < d1)

	case CFQ_RQ1_WRAP:

		return crq2;

	else {

		if (s1 >= s2)

	case (CFQ_RQ1_WRAP|CFQ_RQ2_WRAP): /* both crqs wrapped */

	default:

		/*

		 * Since both rqs are wrapped,

		 * start with the one that's further behind head

		 * (--> only *one* back seek required),

		 * since back seek takes more time than forward.

		 */

		if (s1 <= s2)

			return crq1;

		else

			return crq2;