[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>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index bb43a1677626b0ab98b2ff9ebabdb0a995a9fee0..15152e2da0d28f7abe42831fc0159f7807587052 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -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)
-               return crq1;
-       else if (!r2_wrap && r1_wrap)
-               return crq2;
-       else if (r1_wrap && r2_wrap) {
-               /* both behind the head */
-               if (s1 <= s2)
+
+       /*
+        * 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
+               else if (d2 < d1)
                        return crq2;
-       }
+               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;