[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: Andreas Mohr <andi@lisas.de> Signed-off-by: Jens Axboe <axboe@suse.de>
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@ -346,14 +346,16 @@ static int cfq_queue_empty(request_queue_t *q)
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/*
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* Lifted from AS - choose which of crq1 and crq2 that is best served now.
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* We choose the request that is closest to the head right now. Distance
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* behind the head are penalized and only allowed to a certain extent.
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* behind the head is penalized and only allowed to a certain extent.
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*/
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static struct cfq_rq *
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cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2)
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{
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sector_t last, s1, s2, d1 = 0, d2 = 0;
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int r1_wrap = 0, r2_wrap = 0; /* requests are behind the disk head */
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unsigned long back_max;
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#define CFQ_RQ1_WRAP 0x01 /* request 1 wraps */
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#define CFQ_RQ2_WRAP 0x02 /* request 2 wraps */
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unsigned wrap = 0; /* bit mask: requests behind the disk head? */
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if (crq1 == NULL || crq1 == crq2)
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return crq2;
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@ -385,29 +387,23 @@ cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2)
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else if (s1 + back_max >= last)
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d1 = (last - s1) * cfqd->cfq_back_penalty;
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else
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r1_wrap = 1;
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wrap |= CFQ_RQ1_WRAP;
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if (s2 >= last)
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d2 = s2 - last;
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else if (s2 + back_max >= last)
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d2 = (last - s2) * cfqd->cfq_back_penalty;
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else
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r2_wrap = 1;
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wrap |= CFQ_RQ2_WRAP;
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/* Found required data */
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if (!r1_wrap && r2_wrap)
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return crq1;
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else if (!r2_wrap && r1_wrap)
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return crq2;
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else if (r1_wrap && r2_wrap) {
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/* both behind the head */
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if (s1 <= s2)
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return crq1;
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else
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return crq2;
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}
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/* Both requests in front of the head */
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/*
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* By doing switch() on the bit mask "wrap" we avoid having to
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* check two variables for all permutations: --> faster!
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*/
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switch (wrap) {
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case 0: /* common case for CFQ: crq1 and crq2 not wrapped */
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if (d1 < d2)
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return crq1;
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else if (d2 < d1)
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@ -418,6 +414,24 @@ cfq_choose_req(struct cfq_data *cfqd, struct cfq_rq *crq1, struct cfq_rq *crq2)
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else
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return crq2;
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}
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case CFQ_RQ2_WRAP:
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return crq1;
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case CFQ_RQ1_WRAP:
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return crq2;
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case (CFQ_RQ1_WRAP|CFQ_RQ2_WRAP): /* both crqs wrapped */
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default:
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/*
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* Since both rqs are wrapped,
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* start with the one that's further behind head
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* (--> only *one* back seek required),
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* since back seek takes more time than forward.
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*/
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if (s1 <= s2)
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return crq1;
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else
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return crq2;
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}
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}
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/*
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