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loader: Refactor readahead adjustment in bcache

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While I'm here, add an explanatory comment.

No functional change intended.

Reviewed by:	imp, tsoome (previous version)
MFC after:	1 week
Sponsored by:	https://patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D32249
This commit is contained in:
Colin Percival 2021-10-03 12:10:36 -07:00
parent 31776afdc7
commit b841148bbb
1 changed files with 14 additions and 4 deletions
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18
stand/common/bcache.c
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@ -238,17 +238,27 @@ read_strategy(void *devdata, int rw, daddr_t blk, size_t size,
if (BCACHE_LOOKUP(bc, (daddr_t)(blk + i))) { if (BCACHE_LOOKUP(bc, (daddr_t)(blk + i))) {
bcache_misses += (nblk - i); bcache_misses += (nblk - i);
complete = 0; complete = 0;
if (nblk - i > BCACHE_MINREADAHEAD && bc->ra > BCACHE_MINREADAHEAD)
bc->ra >>= 1; /* reduce read ahead */
break; break;
} else { } else {
bcache_hits++; bcache_hits++;
} }
} }
if (complete) { /* whole set was in cache, return it */ /*
* Adjust read-ahead size if appropriate. Subject to the requirement
* that bc->ra must stay in between MINREADAHEAD and READAHEAD, we
* increase it when we notice that readahead was useful and decrease
* it when we notice that readahead was not useful.
*/
if (complete) {
if (bc->ra < BCACHE_READAHEAD) if (bc->ra < BCACHE_READAHEAD)
bc->ra <<= 1; /* increase read ahead */ bc->ra <<= 1; /* increase read ahead */
} else {
if (nblk - i > BCACHE_MINREADAHEAD && bc->ra > BCACHE_MINREADAHEAD)
bc->ra >>= 1; /* reduce read ahead */
}
if (complete) { /* whole set was in cache, return it */
bcopy(bc->bcache_data + (bcache_blksize * BHASH(bc, blk)), buf, size); bcopy(bc->bcache_data + (bcache_blksize * BHASH(bc, blk)), buf, size);
goto done; goto done;
} }