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block: Skip I/O merges when disabled

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The block I/O + elevator + I/O scheduler code spend a lot of time trying
to merge I/Os -- rightfully so under "normal" circumstances. However,
if one were to know that the incoming I/O stream was /very/ random in
nature, the cycles are wasted.

This patch adds a per-request_queue tunable that (when set) disables
merge attempts (beyond the simple one-hit cache check), thus freeing up
a non-trivial amount of CPU cycles.

Signed-off-by: Alan D. Brunelle <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Alan D. Brunelle 2008-04-29 14:44:19 +02:00 • committed by Jens Axboe
commit ac9fafa124
3 changed files with 31 additions and 0 deletions
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26
block/blk-sysfs.c
View file

@ -135,6 +135,25 @@ static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
return queue_var_show(max_hw_sectors_kb, (page));
}
static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
{
return queue_var_show(blk_queue_nomerges(q), page);
}
static ssize_t queue_nomerges_store(struct request_queue *q, const char *page,
size_t count)
{
unsigned long nm;
ssize_t ret = queue_var_store(&nm, page, count);
if (nm)
set_bit(QUEUE_FLAG_NOMERGES, &q->queue_flags);
else
clear_bit(QUEUE_FLAG_NOMERGES, &q->queue_flags);
return ret;
}
static struct queue_sysfs_entry queue_requests_entry = {
.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
@ -170,6 +189,12 @@ static struct queue_sysfs_entry queue_hw_sector_size_entry = {
.show = queue_hw_sector_size_show,
};
static struct queue_sysfs_entry queue_nomerges_entry = {
.attr = {.name = "nomerges", .mode = S_IRUGO | S_IWUSR },
.show = queue_nomerges_show,
.store = queue_nomerges_store,
};
static struct attribute *default_attrs[] = {
&queue_requests_entry.attr,
&queue_ra_entry.attr,
@ -177,6 +202,7 @@ static struct attribute *default_attrs[] = {
&queue_max_sectors_entry.attr,
&queue_iosched_entry.attr,
&queue_hw_sector_size_entry.attr,
&queue_nomerges_entry.attr,
NULL,
};

3
block/elevator.c
View file

@ -488,6 +488,9 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
}
}
if (blk_queue_nomerges(q))
return ELEVATOR_NO_MERGE;
/*
* See if our hash lookup can find a potential backmerge.
*/