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block, bfq: unconditionally plug I/O in asymmetric scenarios 

bfq detects the creation of multiple bfq_queues shortly after each
other, namely a burst of queue creations in the terminology used in the
code. If the burst is large, then no queue in the burst is granted
- either I/O-dispatch plugging when the queue remains temporarily idle
  while in service;
- or weight raising, because it causes even longer plugging.

In fact, such a plugging tends to lower throughput, while these bursts
are typically due to applications or services that spawn multiple
processes, to reach a common goal as soon as possible. Examples are a
"git grep" or the booting of a system.

Unfortunately, disabling plugging may cause a loss of service guarantees
in asymmetric scenarios, i.e., if queue weights are differentiated or if
more than one group is active.

This commit addresses this issue by no longer disabling I/O-dispatch
plugging for queues in large bursts.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2019-01-29 12:06:32 +01:00 committed by Jens Axboe
parent ac8b0cb415
commit 530c4cbb3c
1 changed files with 165 additions and 181 deletions
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346
block/bfq-iosched.c
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@ -3479,191 +3479,175 @@ static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd,
bfqd->wr_busy_queues == 0;
}
/*
* There is a case where idling must be performed not for
* throughput concerns, but to preserve service guarantees.
*
* To introduce this case, we can note that allowing the drive
* to enqueue more than one request at a time, and hence
* delegating de facto final scheduling decisions to the
* drive's internal scheduler, entails loss of control on the
* actual request service order. In particular, the critical
* situation is when requests from different processes happen
* to be present, at the same time, in the internal queue(s)
* of the drive. In such a situation, the drive, by deciding
* the service order of the internally-queued requests, does
* determine also the actual throughput distribution among
* these processes. But the drive typically has no notion or
* concern about per-process throughput distribution, and
* makes its decisions only on a per-request basis. Therefore,
* the service distribution enforced by the drive's internal
* scheduler is likely to coincide with the desired
* device-throughput distribution only in a completely
* symmetric scenario where:
* (i) each of these processes must get the same throughput as
* the others;
* (ii) the I/O of each process has the same properties, in
* terms of locality (sequential or random), direction
* (reads or writes), request sizes, greediness
* (from I/O-bound to sporadic), and so on.
* In fact, in such a scenario, the drive tends to treat
* the requests of each of these processes in about the same
* way as the requests of the others, and thus to provide
* each of these processes with about the same throughput
* (which is exactly the desired throughput distribution). In
* contrast, in any asymmetric scenario, device idling is
* certainly needed to guarantee that bfqq receives its
* assigned fraction of the device throughput (see [1] for
* details).
* The problem is that idling may significantly reduce
* throughput with certain combinations of types of I/O and
* devices. An important example is sync random I/O, on flash
* storage with command queueing. So, unless bfqq falls in the
* above cases where idling also boosts throughput, it would
* be important to check conditions (i) and (ii) accurately,
* so as to avoid idling when not strictly needed for service
* guarantees.
*
* Unfortunately, it is extremely difficult to thoroughly
* check condition (ii). And, in case there are active groups,
* it becomes very difficult to check condition (i) too. In
* fact, if there are active groups, then, for condition (i)
* to become false, it is enough that an active group contains
* more active processes or sub-groups than some other active
* group. More precisely, for condition (i) to hold because of
* such a group, it is not even necessary that the group is
* (still) active: it is sufficient that, even if the group
* has become inactive, some of its descendant processes still
* have some request already dispatched but still waiting for
* completion. In fact, requests have still to be guaranteed
* their share of the throughput even after being
* dispatched. In this respect, it is easy to show that, if a
* group frequently becomes inactive while still having
* in-flight requests, and if, when this happens, the group is
* not considered in the calculation of whether the scenario
* is asymmetric, then the group may fail to be guaranteed its
* fair share of the throughput (basically because idling may
* not be performed for the descendant processes of the group,
* but it had to be). We address this issue with the
* following bi-modal behavior, implemented in the function
* bfq_symmetric_scenario().
*
* If there are groups with requests waiting for completion
* (as commented above, some of these groups may even be
* already inactive), then the scenario is tagged as
* asymmetric, conservatively, without checking any of the
* conditions (i) and (ii). So the device is idled for bfqq.
* This behavior matches also the fact that groups are created
* exactly if controlling I/O is a primary concern (to
* preserve bandwidth and latency guarantees).
*
* On the opposite end, if there are no groups with requests
* waiting for completion, then only condition (i) is actually
* controlled, i.e., provided that condition (i) holds, idling
* is not performed, regardless of whether condition (ii)
* holds. In other words, only if condition (i) does not hold,
* then idling is allowed, and the device tends to be
* prevented from queueing many requests, possibly of several
* processes. Since there are no groups with requests waiting
* for completion, then, to control condition (i) it is enough
* to check just whether all the queues with requests waiting
* for completion also have the same weight.
*
* Not checking condition (ii) evidently exposes bfqq to the
* risk of getting less throughput than its fair share.
* However, for queues with the same weight, a further
* mechanism, preemption, mitigates or even eliminates this
* problem. And it does so without consequences on overall
* throughput. This mechanism and its benefits are explained
* in the next three paragraphs.
*
* Even if a queue, say Q, is expired when it remains idle, Q
* can still preempt the new in-service queue if the next
* request of Q arrives soon (see the comments on
* bfq_bfqq_update_budg_for_activation). If all queues and
* groups have the same weight, this form of preemption,
* combined with the hole-recovery heuristic described in the
* comments on function bfq_bfqq_update_budg_for_activation,
* are enough to preserve a correct bandwidth distribution in
* the mid term, even without idling. In fact, even if not
* idling allows the internal queues of the device to contain
* many requests, and thus to reorder requests, we can rather
* safely assume that the internal scheduler still preserves a
* minimum of mid-term fairness.
*
* More precisely, this preemption-based, idleless approach
* provides fairness in terms of IOPS, and not sectors per
* second. This can be seen with a simple example. Suppose
* that there are two queues with the same weight, but that
* the first queue receives requests of 8 sectors, while the
* second queue receives requests of 1024 sectors. In
* addition, suppose that each of the two queues contains at
* most one request at a time, which implies that each queue
* always remains idle after it is served. Finally, after
* remaining idle, each queue receives very quickly a new
* request. It follows that the two queues are served
* alternatively, preempting each other if needed. This
* implies that, although both queues have the same weight,
* the queue with large requests receives a service that is
* 1024/8 times as high as the service received by the other
* queue.
*
* The motivation for using preemption instead of idling (for
* queues with the same weight) is that, by not idling,
* service guarantees are preserved (completely or at least in
* part) without minimally sacrificing throughput. And, if
* there is no active group, then the primary expectation for
* this device is probably a high throughput.
*
* We are now left only with explaining the additional
* compound condition that is checked below for deciding
* whether the scenario is asymmetric. To explain this
* compound condition, we need to add that the function
* bfq_symmetric_scenario checks the weights of only
* non-weight-raised queues, for efficiency reasons (see
* comments on bfq_weights_tree_add()). Then the fact that
* bfqq is weight-raised is checked explicitly here. More
* precisely, the compound condition below takes into account
* also the fact that, even if bfqq is being weight-raised,
* the scenario is still symmetric if all queues with requests
* waiting for completion happen to be
* weight-raised. Actually, we should be even more precise
* here, and differentiate between interactive weight raising
* and soft real-time weight raising.
*
* As a side note, it is worth considering that the above
* device-idling countermeasures may however fail in the
* following unlucky scenario: if idling is (correctly)
* disabled in a time period during which all symmetry
* sub-conditions hold, and hence the device is allowed to
* enqueue many requests, but at some later point in time some
* sub-condition stops to hold, then it may become impossible
* to let requests be served in the desired order until all
* the requests already queued in the device have been served.
*/
static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
struct bfq_queue *bfqq)
{
/*
* There is a case where idling must be performed not for
* throughput concerns, but to preserve service guarantees.
*
* To introduce this case, we can note that allowing the drive
* to enqueue more than one request at a time, and thereby
* delegating de facto final scheduling decisions to the
* drive's internal scheduler, entails loss of control on the
* actual request service order. In particular, the critical
* situation is when requests from different processes happen
* to be present, at the same time, in the internal queue(s)
* of the drive. In such a situation, the drive, by deciding
* the service order of the internally-queued requests, does
* determine also the actual throughput distribution among
* these processes. But the drive typically has no notion or
* concern about per-process throughput distribution, and
* makes its decisions only on a per-request basis. Therefore,
* the service distribution enforced by the drive's internal
* scheduler is likely to coincide with the desired
* device-throughput distribution only in a completely
* symmetric scenario where:
* (i) each of these processes must get the same throughput as
* the others;
* (ii) the I/O of each process has the same properties, in
* terms of locality (sequential or random), direction
* (reads or writes), request sizes, greediness
* (from I/O-bound to sporadic), and so on.
* In fact, in such a scenario, the drive tends to treat
* the requests of each of these processes in about the same
* way as the requests of the others, and thus to provide
* each of these processes with about the same throughput
* (which is exactly the desired throughput distribution). In
* contrast, in any asymmetric scenario, device idling is
* certainly needed to guarantee that bfqq receives its
* assigned fraction of the device throughput (see [1] for
* details).
* The problem is that idling may significantly reduce
* throughput with certain combinations of types of I/O and
* devices. An important example is sync random I/O, on flash
* storage with command queueing. So, unless bfqq falls in the
* above cases where idling also boosts throughput, it would
* be important to check conditions (i) and (ii) accurately,
* so as to avoid idling when not strictly needed for service
* guarantees.
*
* Unfortunately, it is extremely difficult to thoroughly
* check condition (ii). And, in case there are active groups,
* it becomes very difficult to check condition (i) too. In
* fact, if there are active groups, then, for condition (i)
* to become false, it is enough that an active group contains
* more active processes or sub-groups than some other active
* group. More precisely, for condition (i) to hold because of
* such a group, it is not even necessary that the group is
* (still) active: it is sufficient that, even if the group
* has become inactive, some of its descendant processes still
* have some request already dispatched but still waiting for
* completion. In fact, requests have still to be guaranteed
* their share of the throughput even after being
* dispatched. In this respect, it is easy to show that, if a
* group frequently becomes inactive while still having
* in-flight requests, and if, when this happens, the group is
* not considered in the calculation of whether the scenario
* is asymmetric, then the group may fail to be guaranteed its
* fair share of the throughput (basically because idling may
* not be performed for the descendant processes of the group,
* but it had to be). We address this issue with the
* following bi-modal behavior, implemented in the function
* bfq_symmetric_scenario().
*
* If there are groups with requests waiting for completion
* (as commented above, some of these groups may even be
* already inactive), then the scenario is tagged as
* asymmetric, conservatively, without checking any of the
* conditions (i) and (ii). So the device is idled for bfqq.
* This behavior matches also the fact that groups are created
* exactly if controlling I/O is a primary concern (to
* preserve bandwidth and latency guarantees).
*
* On the opposite end, if there are no groups with requests
* waiting for completion, then only condition (i) is actually
* controlled, i.e., provided that condition (i) holds, idling
* is not performed, regardless of whether condition (ii)
* holds. In other words, only if condition (i) does not hold,
* then idling is allowed, and the device tends to be
* prevented from queueing many requests, possibly of several
* processes. Since there are no groups with requests waiting
* for completion, then, to control condition (i) it is enough
* to check just whether all the queues with requests waiting
* for completion also have the same weight.
*
* Not checking condition (ii) evidently exposes bfqq to the
* risk of getting less throughput than its fair share.
* However, for queues with the same weight, a further
* mechanism, preemption, mitigates or even eliminates this
* problem. And it does so without consequences on overall
* throughput. This mechanism and its benefits are explained
* in the next three paragraphs.
*
* Even if a queue, say Q, is expired when it remains idle, Q
* can still preempt the new in-service queue if the next
* request of Q arrives soon (see the comments on
* bfq_bfqq_update_budg_for_activation). If all queues and
* groups have the same weight, this form of preemption,
* combined with the hole-recovery heuristic described in the
* comments on function bfq_bfqq_update_budg_for_activation,
* are enough to preserve a correct bandwidth distribution in
* the mid term, even without idling. In fact, even if not
* idling allows the internal queues of the device to contain
* many requests, and thus to reorder requests, we can rather
* safely assume that the internal scheduler still preserves a
* minimum of mid-term fairness.
*
* More precisely, this preemption-based, idleless approach
* provides fairness in terms of IOPS, and not sectors per
* second. This can be seen with a simple example. Suppose
* that there are two queues with the same weight, but that
* the first queue receives requests of 8 sectors, while the
* second queue receives requests of 1024 sectors. In
* addition, suppose that each of the two queues contains at
* most one request at a time, which implies that each queue
* always remains idle after it is served. Finally, after
* remaining idle, each queue receives very quickly a new
* request. It follows that the two queues are served
* alternatively, preempting each other if needed. This
* implies that, although both queues have the same weight,
* the queue with large requests receives a service that is
* 1024/8 times as high as the service received by the other
* queue.
*
* The motivation for using preemption instead of idling (for
* queues with the same weight) is that, by not idling,
* service guarantees are preserved (completely or at least in
* part) without minimally sacrificing throughput. And, if
* there is no active group, then the primary expectation for
* this device is probably a high throughput.
*
* We are now left only with explaining the additional
* compound condition that is checked below for deciding
* whether the scenario is asymmetric. To explain this
* compound condition, we need to add that the function
* bfq_symmetric_scenario checks the weights of only
* non-weight-raised queues, for efficiency reasons (see
* comments on bfq_weights_tree_add()). Then the fact that
* bfqq is weight-raised is checked explicitly here. More
* precisely, the compound condition below takes into account
* also the fact that, even if bfqq is being weight-raised,
* the scenario is still symmetric if all queues with requests
* waiting for completion happen to be
* weight-raised. Actually, we should be even more precise
* here, and differentiate between interactive weight raising
* and soft real-time weight raising.
*
* As a side note, it is worth considering that the above
* device-idling countermeasures may however fail in the
* following unlucky scenario: if idling is (correctly)
* disabled in a time period during which all symmetry
* sub-conditions hold, and hence the device is allowed to
* enqueue many requests, but at some later point in time some
* sub-condition stops to hold, then it may become impossible
* to let requests be served in the desired order until all
* the requests already queued in the device have been served.
*/
bool asymmetric_scenario = (bfqq->wr_coeff > 1 &&
bfqd->wr_busy_queues <
bfq_tot_busy_queues(bfqd)) ||
return (bfqq->wr_coeff > 1 &&
bfqd->wr_busy_queues <
bfq_tot_busy_queues(bfqd)) ||
!bfq_symmetric_scenario(bfqd);
/*
* Finally, there is a case where maximizing throughput is the
* best choice even if it may cause unfairness toward
* bfqq. Such a case is when bfqq became active in a burst of
* queue activations. Queues that became active during a large
* burst benefit only from throughput, as discussed in the
* comments on bfq_handle_burst. Thus, if bfqq became active
* in a burst and not idling the device maximizes throughput,
* then the device must no be idled, because not idling the
* device provides bfqq and all other queues in the burst with
* maximum benefit. Combining this and the above case, we can
* now establish when idling is actually needed to preserve
* service guarantees.
*/
return asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq);
}
/*