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authorMike Snitzer <snitzer@redhat.com>2015-02-26 00:50:28 -0500
committerMike Snitzer <snitzer@redhat.com>2015-04-15 12:10:15 -0400
commit0ce65797a77ee780f62909d3128bf08b9735718b (patch)
treecd83882e699392fafdd7cee5f9f34c6648f54555 /drivers/md/dm.c
parentb898320d683d54c2bc17b748b9742d2b601ad453 (diff)
dm: impose configurable deadline for dm_request_fn's merge heuristic
Otherwise, for sequential workloads, the dm_request_fn can allow excessive request merging at the expense of increased service time. Add a per-device sysfs attribute to allow the user to control how long a request, that is a reasonable merge candidate, can be queued on the request queue. The resolution of this request dispatch deadline is in microseconds (ranging from 1 to 100000 usecs), to set a 20us deadline: echo 20 > /sys/block/dm-7/dm/rq_based_seq_io_merge_deadline The dm_request_fn's merge heuristic and associated extra accounting is disabled by default (rq_based_seq_io_merge_deadline is 0). This sysfs attribute is not applicable to bio-based DM devices so it will only ever report 0 for them. By allowing a request to remain on the queue it will block others requests on the queue. But introducing a short dequeue delay has proven very effective at enabling certain sequential IO workloads on really fast, yet IOPS constrained, devices to build up slightly larger IOs -- yielding 90+% throughput improvements. Having precise control over the time taken to wait for larger requests to build affords control beyond that of waiting for certain IO sizes to accumulate (which would require a deadline anyway). This knob will only ever make sense with sequential IO workloads and the particular value used is storage configuration specific. Given the expected niche use-case for when this knob is useful it has been deemed acceptable to expose this relatively crude method for crafting optimal IO on specific storage -- especially given the solution is simple yet effective. In the context of DM multipath, it is advisable to tune this sysfs attribute to a value that offers the best performance for the common case (e.g. if 4 paths are expected active, tune for that; if paths fail then performance may be slightly reduced). Alternatives were explored to have request-based DM autotune this value (e.g. if/when paths fail) but they were quickly deemed too fragile and complex to warrant further design and development time. If this problem proves more common as faster storage emerges we'll have to look at elevating a generic solution into the block core. Tested-by: Shiva Krishna Merla <shivakrishna.merla@netapp.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'drivers/md/dm.c')
-rw-r--r--drivers/md/dm.c57
1 files changed, 53 insertions, 4 deletions
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 2ae78b31e4c0..5294e016e92b 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -21,6 +21,7 @@
#include <linux/delay.h>
#include <linux/wait.h>
#include <linux/kthread.h>
+#include <linux/ktime.h>
#include <linux/elevator.h> /* for rq_end_sector() */
#include <trace/events/block.h>
@@ -219,8 +220,10 @@ struct mapped_device {
struct task_struct *kworker_task;
/* for request-based merge heuristic in dm_request_fn() */
- sector_t last_rq_pos;
+ unsigned seq_rq_merge_deadline_usecs;
int last_rq_rw;
+ sector_t last_rq_pos;
+ ktime_t last_rq_start_time;
};
/*
@@ -1935,8 +1938,11 @@ static void dm_start_request(struct mapped_device *md, struct request *orig)
blk_start_request(orig);
atomic_inc(&md->pending[rq_data_dir(orig)]);
- md->last_rq_pos = rq_end_sector(orig);
- md->last_rq_rw = rq_data_dir(orig);
+ if (md->seq_rq_merge_deadline_usecs) {
+ md->last_rq_pos = rq_end_sector(orig);
+ md->last_rq_rw = rq_data_dir(orig);
+ md->last_rq_start_time = ktime_get();
+ }
/*
* Hold the md reference here for the in-flight I/O.
@@ -1948,6 +1954,45 @@ static void dm_start_request(struct mapped_device *md, struct request *orig)
dm_get(md);
}
+#define MAX_SEQ_RQ_MERGE_DEADLINE_USECS 100000
+
+ssize_t dm_attr_rq_based_seq_io_merge_deadline_show(struct mapped_device *md, char *buf)
+{
+ return sprintf(buf, "%u\n", md->seq_rq_merge_deadline_usecs);
+}
+
+ssize_t dm_attr_rq_based_seq_io_merge_deadline_store(struct mapped_device *md,
+ const char *buf, size_t count)
+{
+ unsigned deadline;
+
+ if (!dm_request_based(md))
+ return count;
+
+ if (kstrtouint(buf, 10, &deadline))
+ return -EINVAL;
+
+ if (deadline > MAX_SEQ_RQ_MERGE_DEADLINE_USECS)
+ deadline = MAX_SEQ_RQ_MERGE_DEADLINE_USECS;
+
+ md->seq_rq_merge_deadline_usecs = deadline;
+
+ return count;
+}
+
+static bool dm_request_peeked_before_merge_deadline(struct mapped_device *md)
+{
+ ktime_t kt_deadline;
+
+ if (!md->seq_rq_merge_deadline_usecs)
+ return false;
+
+ kt_deadline = ns_to_ktime((u64)md->seq_rq_merge_deadline_usecs * NSEC_PER_USEC);
+ kt_deadline = ktime_add_safe(md->last_rq_start_time, kt_deadline);
+
+ return !ktime_after(ktime_get(), kt_deadline);
+}
+
/*
* q->request_fn for request-based dm.
* Called with the queue lock held.
@@ -1990,7 +2035,8 @@ static void dm_request_fn(struct request_queue *q)
continue;
}
- if (md_in_flight(md) && rq->bio && rq->bio->bi_vcnt == 1 &&
+ if (dm_request_peeked_before_merge_deadline(md) &&
+ md_in_flight(md) && rq->bio && rq->bio->bi_vcnt == 1 &&
md->last_rq_pos == pos && md->last_rq_rw == rq_data_dir(rq))
goto delay_and_out;
@@ -2532,6 +2578,9 @@ static int dm_init_request_based_queue(struct mapped_device *md)
if (!q)
return 0;
+ /* disable dm_request_fn's merge heuristic by default */
+ md->seq_rq_merge_deadline_usecs = 0;
+
md->queue = q;
dm_init_md_queue(md);
blk_queue_softirq_done(md->queue, dm_softirq_done);