o There are situations where a queue gets expired very soon and it looks
as if time slice used by that queue is zero. For example, If an async
queue dispatches a bunch of requests and queue is expired before first
request completes. Another example is where a queue is expired as soon
as first request completes and queue has no more requests (sync queues
on SSD).
o Currently we just charge 25% of slice length in such cases. This patch tries
to improve on that approximation by keeping a track of average disk rate
and charging for time by nr_sectors/disk_rate.
o This is still experimental, not very sure if it gives measurable improvement
or not. May be a better scheme is to use something more granular than jiffies
for time keeping for io queues.
Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
---
block/elevator-fq.c | 97 +++++++++++++++++++++++++++++++++++++++++++++++----
block/elevator-fq.h | 11 ++++++
2 files changed, 101 insertions(+), 7 deletions(-)
diff --git a/block/elevator-fq.c b/block/elevator-fq.c
index 6f23d7e..67c02b9 100644
--- a/block/elevator-fq.c
+++ b/block/elevator-fq.c
@@ -23,6 +23,9 @@ const int elv_slice_async_rq = 2;
int elv_slice_idle = HZ / 125;
static struct kmem_cache *elv_ioq_pool;
+/* Maximum Window length for updating average disk rate */
+static int elv_rate_sampling_window = HZ / 10;
+
#define ELV_SLICE_SCALE (5)
#define ELV_HW_QUEUE_MIN (5)
@@ -941,6 +944,47 @@ static void elv_ioq_update_io_thinktime(struct io_queue *ioq)
ioq->ttime_mean = (ioq->ttime_total + 128) / ioq->ttime_samples;
}
+static void elv_update_io_rate(struct elv_fq_data *efqd, struct request *rq)
+{
+ long elapsed = jiffies - efqd->rate_sampling_start;
+ unsigned long total;
+
+ /* sampling window is off */
+ if (!efqd->rate_sampling_start)
+ return;
+
+ efqd->rate_sectors_current += blk_rq_sectors(rq);
+
+ if (efqd->rq_in_driver && (elapsed < elv_rate_sampling_window))
+ return;
+
+ efqd->rate_sectors = (7*efqd->rate_sectors +
+ 256*efqd->rate_sectors_current) / 8;
+
+ if (!elapsed) {
+ /*
+ * updating rate before a jiffy could complete. Could be a
+ * problem with fast queuing/non-queuing hardware. Should we
+ * look at higher resolution time source?
+ *
+ * In case of non-queuing hardware we will probably not try to
+ * dispatch from multiple queues and will be able to account
+ * for disk time used and will not need this approximation
+ * anyway?
+ */
+ elapsed = 1;
+ }
+
+ efqd->rate_time = (7*efqd->rate_time + 256*elapsed) / 8;
+ total = efqd->rate_sectors + (efqd->rate_time/2);
+ efqd->mean_rate = total/efqd->rate_time;
+
+ elv_log(efqd, "mean_rate=%d, t=%d s=%d", efqd->mean_rate,
+ elapsed, efqd->rate_sectors_current);
+ efqd->rate_sampling_start = 0;
+ efqd->rate_sectors_current = 0;
+}
+
/*
* Disable idle window if the process thinks too long.
* This idle flag can also be updated by io scheduler.
@@ -1231,6 +1275,34 @@ static void elv_del_ioq_busy(struct elevator_queue *e, struct io_queue *ioq,
}
/*
+ * Calculate the effective disk time used by the queue based on how many
+ * sectors queue has dispatched and what is the average disk rate
+ * Returns disk time in ms.
+ */
+static inline unsigned long elv_disk_time_used(struct request_queue *q,
+ struct io_queue *ioq)
+{
+ struct elv_fq_data *efqd = &q->elevator->efqd;
+ struct io_entity *entity = &ioq->entity;
+ unsigned long jiffies_used = 0;
+
+ if (!efqd->mean_rate)
+ return entity->budget/4;
+
+ /* Charge the queue based on average disk rate */
+ jiffies_used = ioq->nr_sectors/efqd->mean_rate;
+
+ if (!jiffies_used)
+ jiffies_used = 1;
+
+ elv_log_ioq(efqd, ioq, "disk time=%ldms sect=%lu rate=%ld",
+ jiffies_to_msecs(jiffies_used),
+ ioq->nr_sectors, efqd->mean_rate);
+
+ return jiffies_used;
+}
+
+/*
* Do the accounting. Determine how much service (in terms of time slices)
* current queue used and adjust the start, finish time of queue and vtime
* of the tree accordingly.
@@ -1248,8 +1320,10 @@ static void elv_del_ioq_busy(struct elevator_queue *e, struct io_queue *ioq,
* from next queue.
*
* Not sure how to determine the time consumed by queue in such scenarios.
- * Currently as a crude approximation, we are charging 25% of time slice
- * for such cases. A better mechanism is needed for accurate accounting.
+ * Currently as a crude approximation, try to keep track of average disk rate
+ * and charge the queue based on number of sectors transferred. If suffcient
+ * disk rate data is not available then we are charging 25% of time slice
+ * for such cases. A better mechanism, is needed for accurate accounting.
*/
void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
{
@@ -1270,9 +1344,9 @@ void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
* reuqest from the queue got completed. Of course we are not planning
* to idle on the queue otherwise we would not have expired it.
*
- * Charge for the 25% slice in such cases. This is not the best thing
- * to do but at the same time not very sure what's the next best
- * thing to do.
+ * Charge the queue based on average disk rate or the 25% slice if
+ * mean rate is 0. This is not the best thing to do but at the same
+ * time not very sure what's the next best thing to do.
*
* This arises from that fact that we don't have the notion of
* one queue being operational at one time. io scheduler can dispatch
@@ -1282,7 +1356,7 @@ void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
* the requests to finish. But this will reduce throughput.
*/
if (!ioq->slice_end)
- slice_used = entity->budget/4;
+ slice_used = elv_disk_time_used(q, ioq);
else {
if (time_after(ioq->slice_end, jiffies)) {
slice_unused = ioq->slice_end - jiffies;
@@ -1292,7 +1366,7 @@ void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
* completing first request. Charge 25% of
* slice.
*/
- slice_used = entity->budget/4;
+ slice_used = elv_disk_time_used(q, ioq);
} else
slice_used = entity->budget - slice_unused;
} else {
@@ -1310,6 +1384,8 @@ void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
BUG_ON(ioq != efqd->active_queue);
elv_reset_active_ioq(efqd);
+ /* Queue is being expired. Reset number of secotrs dispatched */
+ ioq->nr_sectors = 0;
if (!ioq->nr_queued)
elv_del_ioq_busy(q->elevator, ioq, 1);
else
@@ -1671,6 +1747,7 @@ void elv_fq_dispatched_request(struct elevator_queue *e, struct request *rq)
BUG_ON(!ioq);
elv_ioq_request_dispatched(ioq);
+ ioq->nr_sectors += blk_rq_sectors(rq);
elv_ioq_request_removed(e, rq);
elv_clear_ioq_must_dispatch(ioq);
}
@@ -1683,6 +1760,10 @@ void elv_fq_activate_rq(struct request_queue *q, struct request *rq)
return;
efqd->rq_in_driver++;
+
+ if (!efqd->rate_sampling_start)
+ efqd->rate_sampling_start = jiffies;
+
elv_log_ioq(efqd, rq->ioq, "activate rq, drv=%d",
efqd->rq_in_driver);
}
@@ -1746,6 +1827,8 @@ void elv_ioq_completed_request(struct request_queue *q, struct request *rq)
efqd->rq_in_driver--;
ioq->dispatched--;
+ elv_update_io_rate(efqd, rq);
+
if (sync)
ioq->last_end_request = jiffies;
diff --git a/block/elevator-fq.h b/block/elevator-fq.h
index a7cbc0f..4b69239 100644
--- a/block/elevator-fq.h
+++ b/block/elevator-fq.h
@@ -165,6 +165,9 @@ struct io_queue {
/* Requests dispatched from this queue */
int dispatched;
+ /* Number of sectors dispatched in current dispatch round */
+ unsigned long nr_sectors;
+
/* Keep a track of think time of processes in this queue */
unsigned long last_end_request;
unsigned long ttime_total;
@@ -228,6 +231,14 @@ struct elv_fq_data {
/* Base slice length for sync and async queues */
unsigned int elv_slice[2];
+
+ /* Fields for keeping track of average disk rate */
+ unsigned long rate_sectors; /* number of sectors finished */
+ unsigned long rate_time; /* jiffies elapsed */
+ unsigned long mean_rate; /* sectors per jiffy */
+ unsigned long long rate_sampling_start; /*sampling window start jifies*/
+ /* number of sectors finished io during current sampling window */
+ unsigned long rate_sectors_current;
};
/* Logging facilities. */
--
1.6.0.6
--
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