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block, bfq: tune service injection basing on request service times

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The processes associated with a bfq_queue, say Q, may happen to
generate their cumulative I/O at a lower rate than the rate at which
the device could serve the same I/O. This is rather probable, e.g., if
only one process is associated with Q and the device is an SSD. It
results in Q becoming often empty while in service. If BFQ is not
allowed to switch to another queue when Q becomes empty, then, during
the service of Q, there will be frequent "service holes", i.e., time
intervals during which Q gets empty and the device can only consume
the I/O already queued in its hardware queues. This easily causes
considerable losses of throughput.

To counter this problem, BFQ implements a request injection mechanism,
which tries to fill the above service holes with I/O requests taken
from other bfq_queues. The hard part in this mechanism is finding the
right amount of I/O to inject, so as to both boost throughput and not
break Q's bandwidth and latency guarantees. To this goal, the current
version of this mechanism measures the bandwidth enjoyed by Q while it
is being served, and tries to inject the maximum possible amount of
extra service that does not cause Q's bandwidth to decrease too
much.

This solution has an important shortcoming. For bandwidth measurements
to be stable and reliable, Q must remain in service for a much longer
time than that needed to serve a single I/O request. Unfortunately,
this does not hold with many workloads. This commit addresses this
issue by changing the way the amount of injection allowed is
dynamically computed. It tunes injection as a function of the service
times of single I/O requests of Q, instead of Q's
bandwidth. Single-request service times are evidently meaningful even
if Q gets very few I/O requests completed while it is in service.

As a testbed for this new solution, we measured the throughput reached
by BFQ for one of the nastiest workloads and configurations for this
scheduler: the workload generated by the dbench test (in the Phoronix
suite), with 6 clients, on a filesystem with journaling, and with the
journaling daemon enjoying a higher weight than normal processes.
With this commit, the throughput grows from ~100 MB/s to ~150 MB/s on
a PLEXTOR PX-256M5.

Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2019-03-12 09:59:29 +01:00 committed by Jens Axboe
parent fb53ac6cd0
commit 2341d662e9
2 changed files with 409 additions and 59 deletions
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417
block/bfq-iosched.c
View File

@ -1721,6 +1721,123 @@ static void bfq_add_request(struct request *rq)
bfqq->queued[rq_is_sync(rq)]++;
bfqd->queued++;
if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
/*
* Periodically reset inject limit, to make sure that
* the latter eventually drops in case workload
* changes, see step (3) in the comments on
* bfq_update_inject_limit().
*/
if (time_is_before_eq_jiffies(bfqq->decrease_time_jif +
msecs_to_jiffies(1000))) {
/* invalidate baseline total service time */
bfqq->last_serv_time_ns = 0;
/*
* Reset pointer in case we are waiting for
* some request completion.
*/
bfqd->waited_rq = NULL;
/*
* If bfqq has a short think time, then start
* by setting the inject limit to 0
* prudentially, because the service time of
* an injected I/O request may be higher than
* the think time of bfqq, and therefore, if
* one request was injected when bfqq remains
* empty, this injected request might delay
* the service of the next I/O request for
* bfqq significantly. In case bfqq can
* actually tolerate some injection, then the
* adaptive update will however raise the
* limit soon. This lucky circumstance holds
* exactly because bfqq has a short think
* time, and thus, after remaining empty, is
* likely to get new I/O enqueued---and then
* completed---before being expired. This is
* the very pattern that gives the
* limit-update algorithm the chance to
* measure the effect of injection on request
* service times, and then to update the limit
* accordingly.
*
* On the opposite end, if bfqq has a long
* think time, then start directly by 1,
* because:
* a) on the bright side, keeping at most one
* request in service in the drive is unlikely
* to cause any harm to the latency of bfqq's
* requests, as the service time of a single
* request is likely to be lower than the
* think time of bfqq;
* b) on the downside, after becoming empty,
* bfqq is likely to expire before getting its
* next request. With this request arrival
* pattern, it is very hard to sample total
* service times and update the inject limit
* accordingly (see comments on
* bfq_update_inject_limit()). So the limit is
* likely to be never, or at least seldom,
* updated. As a consequence, by setting the
* limit to 1, we avoid that no injection ever
* occurs with bfqq. On the downside, this
* proactive step further reduces chances to
* actually compute the baseline total service
* time. Thus it reduces chances to execute the
* limit-update algorithm and possibly raise the
* limit to more than 1.
*/
if (bfq_bfqq_has_short_ttime(bfqq))
bfqq->inject_limit = 0;
else
bfqq->inject_limit = 1;
bfqq->decrease_time_jif = jiffies;
}
/*
* The following conditions must hold to setup a new
* sampling of total service time, and then a new
* update of the inject limit:
* - bfqq is in service, because the total service
* time is evaluated only for the I/O requests of
* the queues in service;
* - this is the right occasion to compute or to
* lower the baseline total service time, because
* there are actually no requests in the drive,
* or
* the baseline total service time is available, and
* this is the right occasion to compute the other
* quantity needed to update the inject limit, i.e.,
* the total service time caused by the amount of
* injection allowed by the current value of the
* limit. It is the right occasion because injection
* has actually been performed during the service
* hole, and there are still in-flight requests,
* which are very likely to be exactly the injected
* requests, or part of them;
* - the minimum interval for sampling the total
* service time and updating the inject limit has
* elapsed.
*/
if (bfqq == bfqd->in_service_queue &&
(bfqd->rq_in_driver == 0 ||
(bfqq->last_serv_time_ns > 0 &&
bfqd->rqs_injected && bfqd->rq_in_driver > 0)) &&
time_is_before_eq_jiffies(bfqq->decrease_time_jif +
msecs_to_jiffies(100))) {
bfqd->last_empty_occupied_ns = ktime_get_ns();
/*
* Start the state machine for measuring the
* total service time of rq: setting
* wait_dispatch will cause bfqd->waited_rq to
* be set when rq will be dispatched.
*/
bfqd->wait_dispatch = true;
bfqd->rqs_injected = false;
}
}
elv_rb_add(&bfqq->sort_list, rq);
/*
@ -2566,6 +2683,8 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd)
sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);
bfqd->last_idling_start = ktime_get();
bfqd->last_idling_start_jiffies = jiffies;
hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),
HRTIMER_MODE_REL);
bfqg_stats_set_start_idle_time(bfqq_group(bfqq));
@ -3240,13 +3359,6 @@ static unsigned long bfq_bfqq_softrt_next_start(struct bfq_data *bfqd,
jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4);
}
static bool bfq_bfqq_injectable(struct bfq_queue *bfqq)
{
return BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
blk_queue_nonrot(bfqq->bfqd->queue) &&
bfqq->bfqd->hw_tag;
}
/**
* bfq_bfqq_expire - expire a queue.
* @bfqd: device owning the queue.
@ -3361,6 +3473,14 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
"expire (%d, slow %d, num_disp %d, short_ttime %d)", reason,
slow, bfqq->dispatched, bfq_bfqq_has_short_ttime(bfqq));
/*
* bfqq expired, so no total service time needs to be computed
* any longer: reset state machine for measuring total service
* times.
*/
bfqd->rqs_injected = bfqd->wait_dispatch = false;
bfqd->waited_rq = NULL;
/*
* Increase, decrease or leave budget unchanged according to
* reason.
@ -3372,8 +3492,6 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
if (ref == 1) /* bfqq is gone, no more actions on it */
return;
bfqq->injected_service = 0;
/* mark bfqq as waiting a request only if a bic still points to it */
if (!bfq_bfqq_busy(bfqq) &&
reason != BFQQE_BUDGET_TIMEOUT &&
@ -3767,26 +3885,98 @@ static bool bfq_bfqq_must_idle(struct bfq_queue *bfqq)
return RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_better_to_idle(bfqq);
}
static struct bfq_queue *bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
/*
* This function chooses the queue from which to pick the next extra
* I/O request to inject, if it finds a compatible queue. See the
* comments on bfq_update_inject_limit() for details on the injection
* mechanism, and for the definitions of the quantities mentioned
* below.
*/
static struct bfq_queue *
bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
{
struct bfq_queue *bfqq;
struct bfq_queue *bfqq, *in_serv_bfqq = bfqd->in_service_queue;
unsigned int limit = in_serv_bfqq->inject_limit;
/*
* If
* - bfqq is not weight-raised and therefore does not carry
* time-critical I/O,
* or
* - regardless of whether bfqq is weight-raised, bfqq has
* however a long think time, during which it can absorb the
* effect of an appropriate number of extra I/O requests
* from other queues (see bfq_update_inject_limit for
* details on the computation of this number);
* then injection can be performed without restrictions.
*/
bool in_serv_always_inject = in_serv_bfqq->wr_coeff == 1 ||
!bfq_bfqq_has_short_ttime(in_serv_bfqq);
/*
* A linear search; but, with a high probability, very few
* steps are needed to find a candidate queue, i.e., a queue
* with enough budget left for its next request. In fact:
* If
* - the baseline total service time could not be sampled yet,
* so the inject limit happens to be still 0, and
* - a lot of time has elapsed since the plugging of I/O
* dispatching started, so drive speed is being wasted
* significantly;
* then temporarily raise inject limit to one request.
*/
if (limit == 0 && in_serv_bfqq->last_serv_time_ns == 0 &&
bfq_bfqq_wait_request(in_serv_bfqq) &&
time_is_before_eq_jiffies(bfqd->last_idling_start_jiffies +
bfqd->bfq_slice_idle)
)
limit = 1;
if (bfqd->rq_in_driver >= limit)
return NULL;
/*
* Linear search of the source queue for injection; but, with
* a high probability, very few steps are needed to find a
* candidate queue, i.e., a queue with enough budget left for
* its next request. In fact:
* - BFQ dynamically updates the budget of every queue so as
* to accommodate the expected backlog of the queue;
* - if a queue gets all its requests dispatched as injected
* service, then the queue is removed from the active list
* (and re-added only if it gets new requests, but with
* enough budget for its new backlog).
* (and re-added only if it gets new requests, but then it
* is assigned again enough budget for its new backlog).
*/
list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
(in_serv_always_inject || bfqq->wr_coeff > 1) &&
bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
bfq_bfqq_budget_left(bfqq))
return bfqq;
bfq_bfqq_budget_left(bfqq)) {
/*
* Allow for only one large in-flight request
* on non-rotational devices, for the
* following reason. On non-rotationl drives,
* large requests take much longer than
* smaller requests to be served. In addition,
* the drive prefers to serve large requests
* w.r.t. to small ones, if it can choose. So,
* having more than one large requests queued
* in the drive may easily make the next first
* request of the in-service queue wait for so
* long to break bfqq's service guarantees. On
* the bright side, large requests let the
* drive reach a very high throughput, even if
* there is only one in-flight large request
* at a time.
*/
if (blk_queue_nonrot(bfqd->queue) &&
blk_rq_sectors(bfqq->next_rq) >=
BFQQ_SECT_THR_NONROT)
limit = min_t(unsigned int, 1, limit);
else
limit = in_serv_bfqq->inject_limit;
if (bfqd->rq_in_driver < limit) {
bfqd->rqs_injected = true;
return bfqq;
}
}
return NULL;
}
@ -3873,14 +4063,32 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
* for a new request, or has requests waiting for a completion and
* may idle after their completion, then keep it anyway.
*
* Yet, to boost throughput, inject service from other queues if
* possible.
* Yet, inject service from other queues if it boosts
* throughput and is possible.
*/
if (bfq_bfqq_wait_request(bfqq) ||
(bfqq->dispatched != 0 && bfq_better_to_idle(bfqq))) {
if (bfq_bfqq_injectable(bfqq) &&
bfqq->injected_service * bfqq->inject_coeff <
bfqq->entity.service * 10)
struct bfq_queue *async_bfqq =
bfqq->bic && bfqq->bic->bfqq[0] &&
bfq_bfqq_busy(bfqq->bic->bfqq[0]) ?
bfqq->bic->bfqq[0] : NULL;
/*
* If the process associated with bfqq has also async
* I/O pending, then inject it
* unconditionally. Injecting I/O from the same
* process can cause no harm to the process. On the
* contrary, it can only increase bandwidth and reduce
* latency for the process.
*/
if (async_bfqq &&
icq_to_bic(async_bfqq->next_rq->elv.icq) == bfqq->bic &&
bfq_serv_to_charge(async_bfqq->next_rq, async_bfqq) <=
bfq_bfqq_budget_left(async_bfqq))
bfqq = bfqq->bic->bfqq[0];
else if (!idling_boosts_thr_without_issues(bfqd, bfqq) &&
(bfqq->wr_coeff == 1 || bfqd->wr_busy_queues > 1 ||
!bfq_bfqq_has_short_ttime(bfqq)))
bfqq = bfq_choose_bfqq_for_injection(bfqd);
else
bfqq = NULL;
@ -3972,15 +4180,15 @@ static struct request *bfq_dispatch_rq_from_bfqq(struct bfq_data *bfqd,
bfq_bfqq_served(bfqq, service_to_charge);
if (bfqq == bfqd->in_service_queue && bfqd->wait_dispatch) {
bfqd->wait_dispatch = false;
bfqd->waited_rq = rq;
}
bfq_dispatch_remove(bfqd->queue, rq);
if (bfqq != bfqd->in_service_queue) {
if (likely(bfqd->in_service_queue))
bfqd->in_service_queue->injected_service +=
bfq_serv_to_charge(rq, bfqq);
if (bfqq != bfqd->in_service_queue)
goto return_rq;
}
/*
* If weight raising has to terminate for bfqq, then next
@ -4411,13 +4619,6 @@ static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bfq_mark_bfqq_has_short_ttime(bfqq);
bfq_mark_bfqq_sync(bfqq);
bfq_mark_bfqq_just_created(bfqq);
/*
* Aggressively inject a lot of service: up to 90%.
* This coefficient remains constant during bfqq life,
* but this behavior might be changed, after enough
* testing and tuning.
*/
bfqq->inject_coeff = 1;
} else
bfq_clear_bfqq_sync(bfqq);
@ -4976,6 +5177,147 @@ static void bfq_finish_requeue_request_body(struct bfq_queue *bfqq)
bfq_put_queue(bfqq);
}
/*
* The processes associated with bfqq may happen to generate their
* cumulative I/O at a lower rate than the rate at which the device
* could serve the same I/O. This is rather probable, e.g., if only
* one process is associated with bfqq and the device is an SSD. It
* results in bfqq becoming often empty while in service. In this
* respect, if BFQ is allowed to switch to another queue when bfqq
* remains empty, then the device goes on being fed with I/O requests,
* and the throughput is not affected. In contrast, if BFQ is not
* allowed to switch to another queue---because bfqq is sync and
* I/O-dispatch needs to be plugged while bfqq is temporarily
* empty---then, during the service of bfqq, there will be frequent
* "service holes", i.e., time intervals during which bfqq gets empty
* and the device can only consume the I/O already queued in its
* hardware queues. During service holes, the device may even get to
* remaining idle. In the end, during the service of bfqq, the device
* is driven at a lower speed than the one it can reach with the kind
* of I/O flowing through bfqq.
*
* To counter this loss of throughput, BFQ implements a "request
* injection mechanism", which tries to fill the above service holes
* with I/O requests taken from other queues. The hard part in this
* mechanism is finding the right amount of I/O to inject, so as to
* both boost throughput and not break bfqq's bandwidth and latency
* guarantees. In this respect, the mechanism maintains a per-queue
* inject limit, computed as below. While bfqq is empty, the injection
* mechanism dispatches extra I/O requests only until the total number
* of I/O requests in flight---i.e., already dispatched but not yet
* completed---remains lower than this limit.
*
* A first definition comes in handy to introduce the algorithm by
* which the inject limit is computed. We define as first request for
* bfqq, an I/O request for bfqq that arrives while bfqq is in
* service, and causes bfqq to switch from empty to non-empty. The
* algorithm updates the limit as a function of the effect of
* injection on the service times of only the first requests of
* bfqq. The reason for this restriction is that these are the
* requests whose service time is affected most, because they are the
* first to arrive after injection possibly occurred.
*
* To evaluate the effect of injection, the algorithm measures the
* "total service time" of first requests. We define as total service
* time of an I/O request, the time that elapses since when the
* request is enqueued into bfqq, to when it is completed. This
* quantity allows the whole effect of injection to be measured. It is
* easy to see why. Suppose that some requests of other queues are
* actually injected while bfqq is empty, and that a new request R
* then arrives for bfqq. If the device does start to serve all or
* part of the injected requests during the service hole, then,
* because of this extra service, it may delay the next invocation of
* the dispatch hook of BFQ. Then, even after R gets eventually
* dispatched, the device may delay the actual service of R if it is
* still busy serving the extra requests, or if it decides to serve,
* before R, some extra request still present in its queues. As a
* conclusion, the cumulative extra delay caused by injection can be
* easily evaluated by just comparing the total service time of first
* requests with and without injection.
*
* The limit-update algorithm works as follows. On the arrival of a
* first request of bfqq, the algorithm measures the total time of the
* request only if one of the three cases below holds, and, for each
* case, it updates the limit as described below:
*
* (1) If there is no in-flight request. This gives a baseline for the
* total service time of the requests of bfqq. If the baseline has
* not been computed yet, then, after computing it, the limit is
* set to 1, to start boosting throughput, and to prepare the
* ground for the next case. If the baseline has already been
* computed, then it is updated, in case it results to be lower
* than the previous value.
*
* (2) If the limit is higher than 0 and there are in-flight
* requests. By comparing the total service time in this case with
* the above baseline, it is possible to know at which extent the
* current value of the limit is inflating the total service
* time. If the inflation is below a certain threshold, then bfqq
* is assumed to be suffering from no perceivable loss of its
* service guarantees, and the limit is even tentatively
* increased. If the inflation is above the threshold, then the
* limit is decreased. Due to the lack of any hysteresis, this
* logic makes the limit oscillate even in steady workload
* conditions. Yet we opted for it, because it is fast in reaching
* the best value for the limit, as a function of the current I/O
* workload. To reduce oscillations, this step is disabled for a
* short time interval after the limit happens to be decreased.
*
* (3) Periodically, after resetting the limit, to make sure that the
* limit eventually drops in case the workload changes. This is
* needed because, after the limit has gone safely up for a
* certain workload, it is impossible to guess whether the
* baseline total service time may have changed, without measuring
* it again without injection. A more effective version of this
* step might be to just sample the baseline, by interrupting
* injection only once, and then to reset/lower the limit only if
* the total service time with the current limit does happen to be
* too large.
*
* More details on each step are provided in the comments on the
* pieces of code that implement these steps: the branch handling the
* transition from empty to non empty in bfq_add_request(), the branch
* handling injection in bfq_select_queue(), and the function
* bfq_choose_bfqq_for_injection(). These comments also explain some
* exceptions, made by the injection mechanism in some special cases.
*/
static void bfq_update_inject_limit(struct bfq_data *bfqd,
struct bfq_queue *bfqq)
{
u64 tot_time_ns = ktime_get_ns() - bfqd->last_empty_occupied_ns;
unsigned int old_limit = bfqq->inject_limit;
if (bfqq->last_serv_time_ns > 0) {
u64 threshold = (bfqq->last_serv_time_ns * 3)>>1;
if (tot_time_ns >= threshold && old_limit > 0) {
bfqq->inject_limit--;
bfqq->decrease_time_jif = jiffies;
} else if (tot_time_ns < threshold &&
old_limit < bfqd->max_rq_in_driver<<1)
bfqq->inject_limit++;
}
/*
* Either we still have to compute the base value for the
* total service time, and there seem to be the right
* conditions to do it, or we can lower the last base value
* computed.
*/
if ((bfqq->last_serv_time_ns == 0 && bfqd->rq_in_driver == 0) ||
tot_time_ns < bfqq->last_serv_time_ns) {
bfqq->last_serv_time_ns = tot_time_ns;
/*
* Now we certainly have a base value: make sure we
* start trying injection.
*/
bfqq->inject_limit = max_t(unsigned int, 1, old_limit);
}
/* update complete, not waiting for any request completion any longer */
bfqd->waited_rq = NULL;
}
/*
* Handle either a requeue or a finish for rq. The things to do are
* the same in both cases: all references to rq are to be dropped. In
@ -5020,6 +5362,9 @@ static void bfq_finish_requeue_request(struct request *rq)
spin_lock_irqsave(&bfqd->lock, flags);
if (rq == bfqd->waited_rq)
bfq_update_inject_limit(bfqd, bfqq);
bfq_completed_request(bfqq, bfqd);
bfq_finish_requeue_request_body(bfqq);

51
block/bfq-iosched.h
View File

@ -240,6 +240,13 @@ struct bfq_queue {
/* next ioprio and ioprio class if a change is in progress */
unsigned short new_ioprio, new_ioprio_class;
/* last total-service-time sample, see bfq_update_inject_limit() */
u64 last_serv_time_ns;
/* limit for request injection */
unsigned int inject_limit;
/* last time the inject limit has been decreased, in jiffies */
unsigned long decrease_time_jif;
/*
* Shared bfq_queue if queue is cooperating with one or more
* other queues.
@ -357,29 +364,6 @@ struct bfq_queue {
/* max service rate measured so far */
u32 max_service_rate;
/*
* Ratio between the service received by bfqq while it is in
* service, and the cumulative service (of requests of other
* queues) that may be injected while bfqq is empty but still
* in service. To increase precision, the coefficient is
* measured in tenths of unit. Here are some example of (1)
* ratios, (2) resulting percentages of service injected
* w.r.t. to the total service dispatched while bfqq is in
* service, and (3) corresponding values of the coefficient:
* 1 (50%) -> 10
* 2 (33%) -> 20
* 10 (9%) -> 100
* 9.9 (9%) -> 99
* 1.5 (40%) -> 15
* 0.5 (66%) -> 5
* 0.1 (90%) -> 1
*
* So, if the coefficient is lower than 10, then
* injected service is more than bfqq service.
*/
unsigned int inject_coeff;
/* amount of service injected in current service slot */
unsigned int injected_service;
};
/**
@ -544,6 +528,26 @@ struct bfq_data {
/* time of last request completion (ns) */
u64 last_completion;
/* time of last transition from empty to non-empty (ns) */
u64 last_empty_occupied_ns;
/*
* Flag set to activate the sampling of the total service time
* of a just-arrived first I/O request (see
* bfq_update_inject_limit()). This will cause the setting of
* waited_rq when the request is finally dispatched.
*/
bool wait_dispatch;
/*
* If set, then bfq_update_inject_limit() is invoked when
* waited_rq is eventually completed.
*/
struct request *waited_rq;
/*
* True if some request has been injected during the last service hole.
*/
bool rqs_injected;
/* time of first rq dispatch in current observation interval (ns) */
u64 first_dispatch;
/* time of last rq dispatch in current observation interval (ns) */
@ -553,6 +557,7 @@ struct bfq_data {
ktime_t last_budget_start;
/* beginning of the last idle slice */
ktime_t last_idling_start;
unsigned long last_idling_start_jiffies;
/* number of samples in current observation interval */
int peak_rate_samples;