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-rw-r--r--block/bfq-iosched.c120
1 files changed, 85 insertions, 35 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 61d880b90882..dce273b91015 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -753,6 +753,13 @@ static const int bfq_stats_min_budgets = 194;
/* Default maximum budget values, in sectors and number of requests. */
static const int bfq_default_max_budget = 16 * 1024;
+/*
+ * Async to sync throughput distribution is controlled as follows:
+ * when an async request is served, the entity is charged the number
+ * of sectors of the request, multiplied by the factor below
+ */
+static const int bfq_async_charge_factor = 10;
+
/* Default timeout values, in jiffies, approximating CFQ defaults. */
static const int bfq_timeout = HZ / 8;
@@ -1571,22 +1578,52 @@ static void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
}
/**
- * bfq_bfqq_charge_full_budget - set the service to the entity budget.
+ * bfq_bfqq_charge_time - charge an amount of service equivalent to the length
+ * of the time interval during which bfqq has been in
+ * service.
+ * @bfqd: the device
* @bfqq: the queue that needs a service update.
+ * @time_ms: the amount of time during which the queue has received service
*
- * When it's not possible to be fair in the service domain, because
- * a queue is not consuming its budget fast enough (the meaning of
- * fast depends on the timeout parameter), we charge it a full
- * budget. In this way we should obtain a sort of time-domain
- * fairness among all the seeky/slow queues.
+ * If a queue does not consume its budget fast enough, then providing
+ * the queue with service fairness may impair throughput, more or less
+ * severely. For this reason, queues that consume their budget slowly
+ * are provided with time fairness instead of service fairness. This
+ * goal is achieved through the BFQ scheduling engine, even if such an
+ * engine works in the service, and not in the time domain. The trick
+ * is charging these queues with an inflated amount of service, equal
+ * to the amount of service that they would have received during their
+ * service slot if they had been fast, i.e., if their requests had
+ * been dispatched at a rate equal to the estimated peak rate.
+ *
+ * It is worth noting that time fairness can cause important
+ * distortions in terms of bandwidth distribution, on devices with
+ * internal queueing. The reason is that I/O requests dispatched
+ * during the service slot of a queue may be served after that service
+ * slot is finished, and may have a total processing time loosely
+ * correlated with the duration of the service slot. This is
+ * especially true for short service slots.
*/
-static void bfq_bfqq_charge_full_budget(struct bfq_queue *bfqq)
+static void bfq_bfqq_charge_time(struct bfq_data *bfqd, struct bfq_queue *bfqq,
+ unsigned long time_ms)
{
struct bfq_entity *entity = &bfqq->entity;
+ int tot_serv_to_charge = entity->service;
+ unsigned int timeout_ms = jiffies_to_msecs(bfq_timeout);
+
+ if (time_ms > 0 && time_ms < timeout_ms)
+ tot_serv_to_charge =
+ (bfqd->bfq_max_budget * time_ms) / timeout_ms;
- bfq_log_bfqq(bfqq->bfqd, bfqq, "charge_full_budget");
+ if (tot_serv_to_charge < entity->service)
+ tot_serv_to_charge = entity->service;
- bfq_bfqq_served(bfqq, entity->budget - entity->service);
+ /* Increase budget to avoid inconsistencies */
+ if (tot_serv_to_charge > entity->budget)
+ entity->budget = tot_serv_to_charge;
+
+ bfq_bfqq_served(bfqq,
+ max_t(int, 0, tot_serv_to_charge - entity->service));
}
static void bfq_update_fin_time_enqueue(struct bfq_entity *entity,
@@ -3572,10 +3609,14 @@ static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
return bfq_choose_req(bfqd, next, prev, blk_rq_pos(last));
}
+/* see the definition of bfq_async_charge_factor for details */
static unsigned long bfq_serv_to_charge(struct request *rq,
struct bfq_queue *bfqq)
{
- return blk_rq_sectors(rq);
+ if (bfq_bfqq_sync(bfqq))
+ return blk_rq_sectors(rq);
+
+ return blk_rq_sectors(rq) * bfq_async_charge_factor;
}
/**
@@ -4676,28 +4717,24 @@ static unsigned long bfq_smallest_from_now(void)
* @compensate: if true, compensate for the time spent idling.
* @reason: the reason causing the expiration.
*
+ * If the process associated with bfqq does slow I/O (e.g., because it
+ * issues random requests), we charge bfqq with the time it has been
+ * in service instead of the service it has received (see
+ * bfq_bfqq_charge_time for details on how this goal is achieved). As
+ * a consequence, bfqq will typically get higher timestamps upon
+ * reactivation, and hence it will be rescheduled as if it had
+ * received more service than what it has actually received. In the
+ * end, bfqq receives less service in proportion to how slowly its
+ * associated process consumes its budgets (and hence how seriously it
+ * tends to lower the throughput). In addition, this time-charging
+ * strategy guarantees time fairness among slow processes. In
+ * contrast, if the process associated with bfqq is not slow, we
+ * charge bfqq exactly with the service it has received.
*
- * If the process associated with the queue is slow (i.e., seeky), or
- * in case of budget timeout, or, finally, if it is async, we
- * artificially charge it an entire budget (independently of the
- * actual service it received). As a consequence, the queue will get
- * higher timestamps than the correct ones upon reactivation, and
- * hence it will be rescheduled as if it had received more service
- * than what it actually received. In the end, this class of processes
- * will receive less service in proportion to how slowly they consume
- * their budgets (and hence how seriously they tend to lower the
- * throughput).
- *
- * In contrast, when a queue expires because it has been idling for
- * too much or because it exhausted its budget, we do not touch the
- * amount of service it has received. Hence when the queue will be
- * reactivated and its timestamps updated, the latter will be in sync
- * with the actual service received by the queue until expiration.
- *
- * Charging a full budget to the first type of queues and the exact
- * service to the others has the effect of using the WF2Q+ policy to
- * schedule the former on a timeslice basis, without violating the
- * service domain guarantees of the latter.
+ * Charging time to the first type of queues and the exact service to
+ * the other has the effect of using the WF2Q+ policy to schedule the
+ * former on a timeslice basis, without violating service domain
+ * guarantees among the latter.
*/
static void bfq_bfqq_expire(struct bfq_data *bfqd,
struct bfq_queue *bfqq,
@@ -4715,11 +4752,24 @@ static void bfq_bfqq_expire(struct bfq_data *bfqd,
slow = bfq_bfqq_is_slow(bfqd, bfqq, compensate, reason, &delta);
/*
- * As above explained, 'punish' slow (i.e., seeky), timed-out
- * and async queues, to favor sequential sync workloads.
+ * As above explained, charge slow (typically seeky) and
+ * timed-out queues with the time and not the service
+ * received, to favor sequential workloads.
+ *
+ * Processes doing I/O in the slower disk zones will tend to
+ * be slow(er) even if not seeky. Therefore, since the
+ * estimated peak rate is actually an average over the disk
+ * surface, these processes may timeout just for bad luck. To
+ * avoid punishing them, do not charge time to processes that
+ * succeeded in consuming at least 2/3 of their budget. This
+ * allows BFQ to preserve enough elasticity to still perform
+ * bandwidth, and not time, distribution with little unlucky
+ * or quasi-sequential processes.
*/
- if (slow || reason == BFQQE_BUDGET_TIMEOUT)
- bfq_bfqq_charge_full_budget(bfqq);
+ if (slow ||
+ (reason == BFQQE_BUDGET_TIMEOUT &&
+ bfq_bfqq_budget_left(bfqq) >= entity->budget / 3))
+ bfq_bfqq_charge_time(bfqd, bfqq, delta);
if (reason == BFQQE_TOO_IDLE &&
entity->service <= 2 * entity->budget / 10)