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Subject[PATCH 03/18] io-controller: Charge for time slice based on average disk rate
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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.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
block/elevator-fq.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++++-
block/elevator-fq.h | 11 ++++++
2 files changed, 94 insertions(+), 2 deletions(-)
diff --git a/block/elevator-fq.c b/block/elevator-fq.c
index 9aea899..9f1fbb9 100644
--- a/block/elevator-fq.c
+++ b/block/elevator-fq.c
@@ -19,6 +19,9 @@ 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)
#define IO_SERVICE_TREE_INIT ((struct io_service_tree) \
@@ -1022,6 +1025,47 @@ 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 += rq->nr_sectors;
+
+ 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.
@@ -1312,6 +1356,34 @@ 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=%ld 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.
@@ -1363,7 +1435,7 @@ 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;
@@ -1373,7 +1445,7 @@ 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 {
@@ -1391,6 +1463,8 @@ 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
@@ -1725,6 +1799,7 @@ void elv_fq_dispatched_request(struct elevator_queue *e, struct request *rq)

BUG_ON(!ioq);
elv_ioq_request_dispatched(ioq);
+ ioq->nr_sectors += rq->nr_sectors;
elv_ioq_request_removed(e, rq);
elv_clear_ioq_must_dispatch(ioq);
}
@@ -1737,6 +1812,10 @@ 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(rq), "activate rq, drv=%d",
efqd->rq_in_driver);
}
@@ -1826,6 +1905,8 @@ 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 3bea279..ce2d671 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 */
+ int nr_sectors;
+
/* Keep a track of think time of processes in this queue */
unsigned long last_end_request;
unsigned long ttime_total;
@@ -223,6 +226,14 @@ struct elv_fq_data {
struct work_struct unplug_work;

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;
};

extern int elv_slice_idle;
--
1.6.0.1


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