/* on-disk position of the last served request */
sector_t last_position;
+ /* time of last request completion (ns) */
+ u64 last_completion;
+
+ /* time of first rq dispatch in current observation interval (ns) */
+ u64 first_dispatch;
+ /* time of last rq dispatch in current observation interval (ns) */
+ u64 last_dispatch;
+
/* beginning of the last budget */
ktime_t last_budget_start;
/* beginning of the last idle slice */
ktime_t last_idling_start;
- /* number of samples used to calculate @peak_rate */
+
+ /* number of samples in current observation interval */
int peak_rate_samples;
+ /* num of samples of seq dispatches in current observation interval */
+ u32 sequential_samples;
+ /* total num of sectors transferred in current observation interval */
+ u64 tot_sectors_dispatched;
+ /* max rq size seen during current observation interval (sectors) */
+ u32 last_rq_max_size;
+ /* time elapsed from first dispatch in current observ. interval (us) */
+ u64 delta_from_first;
/*
- * Peak read/write rate, observed during the service of a
- * budget [BFQ_RATE_SHIFT * sectors/usec]. The value is
- * left-shifted by BFQ_RATE_SHIFT to increase precision in
+ * Current estimate of the device peak rate, measured in
+ * [BFQ_RATE_SHIFT * sectors/usec]. The left-shift by
+ * BFQ_RATE_SHIFT is performed to increase precision in
* fixed-point calculations.
*/
- u64 peak_rate;
+ u32 peak_rate;
+
/* maximum budget allotted to a bfq_queue before rescheduling */
int bfq_max_budget;
static struct kmem_cache *bfq_pool;
-/* Below this threshold (in ms), we consider thinktime immediate. */
+/* Below this threshold (in ns), we consider thinktime immediate. */
#define BFQ_MIN_TT (2 * NSEC_PER_MSEC)
/* hw_tag detection: parallel requests threshold and min samples needed. */
#define BFQQ_CLOSE_THR (sector_t)(8 * 1024)
#define BFQQ_SEEKY(bfqq) (hweight32(bfqq->seek_history) > 32/8)
-/* Min samples used for peak rate estimation (for autotuning). */
-#define BFQ_PEAK_RATE_SAMPLES 32
+/* Min number of samples required to perform peak-rate update */
+#define BFQ_RATE_MIN_SAMPLES 32
+/* Min observation time interval required to perform a peak-rate update (ns) */
+#define BFQ_RATE_MIN_INTERVAL (300*NSEC_PER_MSEC)
+/* Target observation time interval for a peak-rate update (ns) */
+#define BFQ_RATE_REF_INTERVAL NSEC_PER_SEC
/* Shift used for peak rate fixed precision calculations. */
#define BFQ_RATE_SHIFT 16
return NULL;
}
+static sector_t get_sdist(sector_t last_pos, struct request *rq)
+{
+ if (last_pos)
+ return abs(blk_rq_pos(rq) - last_pos);
+
+ return 0;
+}
+
#if 0 /* Still not clear if we can do without next two functions */
static void bfq_activate_request(struct request_queue *q, struct request *rq)
{
struct bfq_data *bfqd = q->elevator->elevator_data;
bfqd->rq_in_driver++;
- bfqd->last_position = blk_rq_pos(rq) + blk_rq_sectors(rq);
- bfq_log(bfqd, "activate_request: new bfqd->last_position %llu",
- (unsigned long long)bfqd->last_position);
}
static void bfq_deactivate_request(struct request_queue *q, struct request *rq)
jiffies_to_msecs(bfqd->bfq_timeout * timeout_coeff));
}
+/*
+ * In autotuning mode, max_budget is dynamically recomputed as the
+ * amount of sectors transferred in timeout at the estimated peak
+ * rate. This enables BFQ to utilize a full timeslice with a full
+ * budget, even if the in-service queue is served at peak rate. And
+ * this maximises throughput with sequential workloads.
+ */
+static unsigned long bfq_calc_max_budget(struct bfq_data *bfqd)
+{
+ return (u64)bfqd->peak_rate * USEC_PER_MSEC *
+ jiffies_to_msecs(bfqd->bfq_timeout)>>BFQ_RATE_SHIFT;
+}
+
+static void bfq_reset_rate_computation(struct bfq_data *bfqd,
+ struct request *rq)
+{
+ if (rq != NULL) { /* new rq dispatch now, reset accordingly */
+ bfqd->last_dispatch = bfqd->first_dispatch = ktime_get_ns();
+ bfqd->peak_rate_samples = 1;
+ bfqd->sequential_samples = 0;
+ bfqd->tot_sectors_dispatched = bfqd->last_rq_max_size =
+ blk_rq_sectors(rq);
+ } else /* no new rq dispatched, just reset the number of samples */
+ bfqd->peak_rate_samples = 0; /* full re-init on next disp. */
+
+ bfq_log(bfqd,
+ "reset_rate_computation at end, sample %u/%u tot_sects %llu",
+ bfqd->peak_rate_samples, bfqd->sequential_samples,
+ bfqd->tot_sectors_dispatched);
+}
+
+static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq)
+{
+ u32 rate, weight, divisor;
+
+ /*
+ * For the convergence property to hold (see comments on
+ * bfq_update_peak_rate()) and for the assessment to be
+ * reliable, a minimum number of samples must be present, and
+ * a minimum amount of time must have elapsed. If not so, do
+ * not compute new rate. Just reset parameters, to get ready
+ * for a new evaluation attempt.
+ */
+ if (bfqd->peak_rate_samples < BFQ_RATE_MIN_SAMPLES ||
+ bfqd->delta_from_first < BFQ_RATE_MIN_INTERVAL)
+ goto reset_computation;
+
+ /*
+ * If a new request completion has occurred after last
+ * dispatch, then, to approximate the rate at which requests
+ * have been served by the device, it is more precise to
+ * extend the observation interval to the last completion.
+ */
+ bfqd->delta_from_first =
+ max_t(u64, bfqd->delta_from_first,
+ bfqd->last_completion - bfqd->first_dispatch);
+
+ /*
+ * Rate computed in sects/usec, and not sects/nsec, for
+ * precision issues.
+ */
+ rate = div64_ul(bfqd->tot_sectors_dispatched<<BFQ_RATE_SHIFT,
+ div_u64(bfqd->delta_from_first, NSEC_PER_USEC));
+
+ /*
+ * Peak rate not updated if:
+ * - the percentage of sequential dispatches is below 3/4 of the
+ * total, and rate is below the current estimated peak rate
+ * - rate is unreasonably high (> 20M sectors/sec)
+ */
+ if ((bfqd->sequential_samples < (3 * bfqd->peak_rate_samples)>>2 &&
+ rate <= bfqd->peak_rate) ||
+ rate > 20<<BFQ_RATE_SHIFT)
+ goto reset_computation;
+
+ /*
+ * We have to update the peak rate, at last! To this purpose,
+ * we use a low-pass filter. We compute the smoothing constant
+ * of the filter as a function of the 'weight' of the new
+ * measured rate.
+ *
+ * As can be seen in next formulas, we define this weight as a
+ * quantity proportional to how sequential the workload is,
+ * and to how long the observation time interval is.
+ *
+ * The weight runs from 0 to 8. The maximum value of the
+ * weight, 8, yields the minimum value for the smoothing
+ * constant. At this minimum value for the smoothing constant,
+ * the measured rate contributes for half of the next value of
+ * the estimated peak rate.
+ *
+ * So, the first step is to compute the weight as a function
+ * of how sequential the workload is. Note that the weight
+ * cannot reach 9, because bfqd->sequential_samples cannot
+ * become equal to bfqd->peak_rate_samples, which, in its
+ * turn, holds true because bfqd->sequential_samples is not
+ * incremented for the first sample.
+ */
+ weight = (9 * bfqd->sequential_samples) / bfqd->peak_rate_samples;
+
+ /*
+ * Second step: further refine the weight as a function of the
+ * duration of the observation interval.
+ */
+ weight = min_t(u32, 8,
+ div_u64(weight * bfqd->delta_from_first,
+ BFQ_RATE_REF_INTERVAL));
+
+ /*
+ * Divisor ranging from 10, for minimum weight, to 2, for
+ * maximum weight.
+ */
+ divisor = 10 - weight;
+
+ /*
+ * Finally, update peak rate:
+ *
+ * peak_rate = peak_rate * (divisor-1) / divisor + rate / divisor
+ */
+ bfqd->peak_rate *= divisor-1;
+ bfqd->peak_rate /= divisor;
+ rate /= divisor; /* smoothing constant alpha = 1/divisor */
+
+ bfqd->peak_rate += rate;
+ if (bfqd->bfq_user_max_budget == 0)
+ bfqd->bfq_max_budget =
+ bfq_calc_max_budget(bfqd);
+
+reset_computation:
+ bfq_reset_rate_computation(bfqd, rq);
+}
+
+/*
+ * Update the read/write peak rate (the main quantity used for
+ * auto-tuning, see update_thr_responsiveness_params()).
+ *
+ * It is not trivial to estimate the peak rate (correctly): because of
+ * the presence of sw and hw queues between the scheduler and the
+ * device components that finally serve I/O requests, it is hard to
+ * say exactly when a given dispatched request is served inside the
+ * device, and for how long. As a consequence, it is hard to know
+ * precisely at what rate a given set of requests is actually served
+ * by the device.
+ *
+ * On the opposite end, the dispatch time of any request is trivially
+ * available, and, from this piece of information, the "dispatch rate"
+ * of requests can be immediately computed. So, the idea in the next
+ * function is to use what is known, namely request dispatch times
+ * (plus, when useful, request completion times), to estimate what is
+ * unknown, namely in-device request service rate.
+ *
+ * The main issue is that, because of the above facts, the rate at
+ * which a certain set of requests is dispatched over a certain time
+ * interval can vary greatly with respect to the rate at which the
+ * same requests are then served. But, since the size of any
+ * intermediate queue is limited, and the service scheme is lossless
+ * (no request is silently dropped), the following obvious convergence
+ * property holds: the number of requests dispatched MUST become
+ * closer and closer to the number of requests completed as the
+ * observation interval grows. This is the key property used in
+ * the next function to estimate the peak service rate as a function
+ * of the observed dispatch rate. The function assumes to be invoked
+ * on every request dispatch.
+ */
+static void bfq_update_peak_rate(struct bfq_data *bfqd, struct request *rq)
+{
+ u64 now_ns = ktime_get_ns();
+
+ if (bfqd->peak_rate_samples == 0) { /* first dispatch */
+ bfq_log(bfqd, "update_peak_rate: goto reset, samples %d",
+ bfqd->peak_rate_samples);
+ bfq_reset_rate_computation(bfqd, rq);
+ goto update_last_values; /* will add one sample */
+ }
+
+ /*
+ * Device idle for very long: the observation interval lasting
+ * up to this dispatch cannot be a valid observation interval
+ * for computing a new peak rate (similarly to the late-
+ * completion event in bfq_completed_request()). Go to
+ * update_rate_and_reset to have the following three steps
+ * taken:
+ * - close the observation interval at the last (previous)
+ * request dispatch or completion
+ * - compute rate, if possible, for that observation interval
+ * - start a new observation interval with this dispatch
+ */
+ if (now_ns - bfqd->last_dispatch > 100*NSEC_PER_MSEC &&
+ bfqd->rq_in_driver == 0)
+ goto update_rate_and_reset;
+
+ /* Update sampling information */
+ bfqd->peak_rate_samples++;
+
+ if ((bfqd->rq_in_driver > 0 ||
+ now_ns - bfqd->last_completion < BFQ_MIN_TT)
+ && get_sdist(bfqd->last_position, rq) < BFQQ_SEEK_THR)
+ bfqd->sequential_samples++;
+
+ bfqd->tot_sectors_dispatched += blk_rq_sectors(rq);
+
+ /* Reset max observed rq size every 32 dispatches */
+ if (likely(bfqd->peak_rate_samples % 32))
+ bfqd->last_rq_max_size =
+ max_t(u32, blk_rq_sectors(rq), bfqd->last_rq_max_size);
+ else
+ bfqd->last_rq_max_size = blk_rq_sectors(rq);
+
+ bfqd->delta_from_first = now_ns - bfqd->first_dispatch;
+
+ /* Target observation interval not yet reached, go on sampling */
+ if (bfqd->delta_from_first < BFQ_RATE_REF_INTERVAL)
+ goto update_last_values;
+
+update_rate_and_reset:
+ bfq_update_rate_reset(bfqd, rq);
+update_last_values:
+ bfqd->last_position = blk_rq_pos(rq) + blk_rq_sectors(rq);
+ bfqd->last_dispatch = now_ns;
+}
+
/*
* Remove request from internal lists.
*/
* happens to be taken into account.
*/
bfqq->dispatched++;
+ bfq_update_peak_rate(q->elevator->elevator_data, rq);
bfq_remove_request(q, rq);
}
bfqq->entity.budget);
}
-static unsigned long bfq_calc_max_budget(u64 peak_rate, u64 timeout)
-{
- unsigned long max_budget;
-
- /*
- * The max_budget calculated when autotuning is equal to the
- * amount of sectors transferred in timeout at the estimated
- * peak rate. To get this value, peak_rate is, first,
- * multiplied by 1000, because timeout is measured in ms,
- * while peak_rate is measured in sectors/usecs. Then the
- * result of this multiplication is right-shifted by
- * BFQ_RATE_SHIFT, because peak_rate is equal to the value of
- * the peak rate left-shifted by BFQ_RATE_SHIFT.
- */
- max_budget = (unsigned long)(peak_rate * 1000 *
- timeout >> BFQ_RATE_SHIFT);
-
- return max_budget;
-}
-
/*
- * In addition to updating the peak rate, checks whether the process
- * is "slow", and returns 1 if so. This slow flag is used, in addition
- * to the budget timeout, to reduce the amount of service provided to
- * seeky processes, and hence reduce their chances to lower the
- * throughput. See the code for more details.
+ * Return true if the process associated with bfqq is "slow". The slow
+ * flag is used, in addition to the budget timeout, to reduce the
+ * amount of service provided to seeky processes, and thus reduce
+ * their chances to lower the throughput. More details in the comments
+ * on the function bfq_bfqq_expire().
+ *
+ * An important observation is in order: as discussed in the comments
+ * on the function bfq_update_peak_rate(), with devices with internal
+ * queues, it is hard if ever possible to know when and for how long
+ * an I/O request is processed by the device (apart from the trivial
+ * I/O pattern where a new request is dispatched only after the
+ * previous one has been completed). This makes it hard to evaluate
+ * the real rate at which the I/O requests of each bfq_queue are
+ * served. In fact, for an I/O scheduler like BFQ, serving a
+ * bfq_queue means just dispatching its requests during its service
+ * slot (i.e., until the budget of the queue is exhausted, or the
+ * queue remains idle, or, finally, a timeout fires). But, during the
+ * service slot of a bfq_queue, around 100 ms at most, the device may
+ * be even still processing requests of bfq_queues served in previous
+ * service slots. On the opposite end, the requests of the in-service
+ * bfq_queue may be completed after the service slot of the queue
+ * finishes.
+ *
+ * Anyway, unless more sophisticated solutions are used
+ * (where possible), the sum of the sizes of the requests dispatched
+ * during the service slot of a bfq_queue is probably the only
+ * approximation available for the service received by the bfq_queue
+ * during its service slot. And this sum is the quantity used in this
+ * function to evaluate the I/O speed of a process.
*/
-static bool bfq_update_peak_rate(struct bfq_data *bfqd, struct bfq_queue *bfqq,
- bool compensate)
+static bool bfq_bfqq_is_slow(struct bfq_data *bfqd, struct bfq_queue *bfqq,
+ bool compensate, enum bfqq_expiration reason,
+ unsigned long *delta_ms)
{
- u64 bw, usecs, expected, timeout;
- ktime_t delta;
- int update = 0;
+ ktime_t delta_ktime;
+ u32 delta_usecs;
+ bool slow = BFQQ_SEEKY(bfqq); /* if delta too short, use seekyness */
- if (!bfq_bfqq_sync(bfqq) || bfq_bfqq_budget_new(bfqq))
+ if (!bfq_bfqq_sync(bfqq))
return false;
if (compensate)
- delta = bfqd->last_idling_start;
+ delta_ktime = bfqd->last_idling_start;
else
- delta = ktime_get();
- delta = ktime_sub(delta, bfqd->last_budget_start);
- usecs = ktime_to_us(delta);
+ delta_ktime = ktime_get();
+ delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
+ delta_usecs = ktime_to_us(delta_ktime);
/* don't use too short time intervals */
- if (usecs < 1000)
- return false;
-
- /*
- * Calculate the bandwidth for the last slice. We use a 64 bit
- * value to store the peak rate, in sectors per usec in fixed
- * point math. We do so to have enough precision in the estimate
- * and to avoid overflows.
- */
- bw = (u64)bfqq->entity.service << BFQ_RATE_SHIFT;
- do_div(bw, (unsigned long)usecs);
+ if (delta_usecs < 1000) {
+ if (blk_queue_nonrot(bfqd->queue))
+ /*
+ * give same worst-case guarantees as idling
+ * for seeky
+ */
+ *delta_ms = BFQ_MIN_TT / NSEC_PER_MSEC;
+ else /* charge at least one seek */
+ *delta_ms = bfq_slice_idle / NSEC_PER_MSEC;
+
+ return slow;
+ }
- timeout = jiffies_to_msecs(bfqd->bfq_timeout);
+ *delta_ms = delta_usecs / USEC_PER_MSEC;
/*
- * Use only long (> 20ms) intervals to filter out spikes for
- * the peak rate estimation.
+ * Use only long (> 20ms) intervals to filter out excessive
+ * spikes in service rate estimation.
*/
- if (usecs > 20000) {
- if (bw > bfqd->peak_rate) {
- bfqd->peak_rate = bw;
- update = 1;
- bfq_log(bfqd, "new peak_rate=%llu", bw);
- }
-
- update |= bfqd->peak_rate_samples == BFQ_PEAK_RATE_SAMPLES - 1;
-
- if (bfqd->peak_rate_samples < BFQ_PEAK_RATE_SAMPLES)
- bfqd->peak_rate_samples++;
-
- if (bfqd->peak_rate_samples == BFQ_PEAK_RATE_SAMPLES &&
- update && bfqd->bfq_user_max_budget == 0) {
- bfqd->bfq_max_budget =
- bfq_calc_max_budget(bfqd->peak_rate,
- timeout);
- bfq_log(bfqd, "new max_budget=%d",
- bfqd->bfq_max_budget);
- }
+ if (delta_usecs > 20000) {
+ /*
+ * Caveat for rotational devices: processes doing I/O
+ * in the slower disk zones tend to be slow(er) even
+ * if not seeky. In this respect, the estimated peak
+ * rate is likely to be an average over the disk
+ * surface. Accordingly, to not be too harsh with
+ * unlucky processes, a process is deemed slow only if
+ * its rate has been lower than half of the estimated
+ * peak rate.
+ */
+ slow = bfqq->entity.service < bfqd->bfq_max_budget / 2;
}
- /*
- * A process is considered ``slow'' (i.e., seeky, so that we
- * cannot treat it fairly in the service domain, as it would
- * slow down too much the other processes) if, when a slice
- * ends for whatever reason, it has received service at a
- * rate that would not be high enough to complete the budget
- * before the budget timeout expiration.
- */
- expected = bw * 1000 * timeout >> BFQ_RATE_SHIFT;
+ bfq_log_bfqq(bfqd, bfqq, "bfq_bfqq_is_slow: slow %d", slow);
- /*
- * Caveat: processes doing IO in the slower disk zones will
- * tend to be slow(er) even if not seeky. And the estimated
- * peak rate will actually be an average over the disk
- * surface. Hence, to not be too harsh with unlucky processes,
- * we keep a budget/3 margin of safety before declaring a
- * process slow.
- */
- return expected > (4 * bfqq->entity.budget) / 3;
+ return slow;
}
/*
enum bfqq_expiration reason)
{
bool slow;
+ unsigned long delta = 0;
+ struct bfq_entity *entity = &bfqq->entity;
int ref;
/*
- * Update device peak rate for autotuning and check whether the
- * process is slow (see bfq_update_peak_rate).
+ * Check whether the process is slow (see bfq_bfqq_is_slow).
*/
- slow = bfq_update_peak_rate(bfqd, bfqq, compensate);
+ slow = bfq_bfqq_is_slow(bfqd, bfqq, compensate, reason, &delta);
/*
* As above explained, 'punish' slow (i.e., seeky), timed-out
bfq_bfqq_charge_full_budget(bfqq);
if (reason == BFQQE_TOO_IDLE &&
- bfqq->entity.service <= 2 * bfqq->entity.budget / 10)
+ entity->service <= 2 * entity->budget / 10)
bfq_clear_bfqq_IO_bound(bfqq);
bfq_log_bfqq(bfqd, bfqq,
bfq_update_io_seektime(struct bfq_data *bfqd, struct bfq_queue *bfqq,
struct request *rq)
{
- sector_t sdist = 0;
-
- if (bfqq->last_request_pos) {
- if (bfqq->last_request_pos < blk_rq_pos(rq))
- sdist = blk_rq_pos(rq) - bfqq->last_request_pos;
- else
- sdist = bfqq->last_request_pos - blk_rq_pos(rq);
- }
-
bfqq->seek_history <<= 1;
- bfqq->seek_history |= sdist > BFQQ_SEEK_THR &&
+ bfqq->seek_history |=
+ get_sdist(bfqq->last_request_pos, rq) > BFQQ_SEEK_THR &&
(!blk_queue_nonrot(bfqd->queue) ||
blk_rq_sectors(rq) < BFQQ_SECT_THR_NONROT);
}
static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
{
+ u64 now_ns;
+ u32 delta_us;
+
bfq_update_hw_tag(bfqd);
bfqd->rq_in_driver--;
bfqq->dispatched--;
- bfqq->ttime.last_end_request = ktime_get_ns();
+ now_ns = ktime_get_ns();
+
+ bfqq->ttime.last_end_request = now_ns;
+
+ /*
+ * Using us instead of ns, to get a reasonable precision in
+ * computing rate in next check.
+ */
+ delta_us = div_u64(now_ns - bfqd->last_completion, NSEC_PER_USEC);
+
+ /*
+ * If the request took rather long to complete, and, according
+ * to the maximum request size recorded, this completion latency
+ * implies that the request was certainly served at a very low
+ * rate (less than 1M sectors/sec), then the whole observation
+ * interval that lasts up to this time instant cannot be a
+ * valid time interval for computing a new peak rate. Invoke
+ * bfq_update_rate_reset to have the following three steps
+ * taken:
+ * - close the observation interval at the last (previous)
+ * request dispatch or completion
+ * - compute rate, if possible, for that observation interval
+ * - reset to zero samples, which will trigger a proper
+ * re-initialization of the observation interval on next
+ * dispatch
+ */
+ if (delta_us > BFQ_MIN_TT/NSEC_PER_USEC &&
+ (bfqd->last_rq_max_size<<BFQ_RATE_SHIFT)/delta_us <
+ 1UL<<(BFQ_RATE_SHIFT - 10))
+ bfq_update_rate_reset(bfqd, NULL);
+ bfqd->last_completion = now_ns;
/*
* If this is the in-service queue, check if it needs to be expired,
UINT_MAX);
#undef USEC_STORE_FUNCTION
-static unsigned long bfq_estimated_max_budget(struct bfq_data *bfqd)
-{
- u64 timeout = jiffies_to_msecs(bfqd->bfq_timeout);
-
- if (bfqd->peak_rate_samples >= BFQ_PEAK_RATE_SAMPLES)
- return bfq_calc_max_budget(bfqd->peak_rate, timeout);
- else
- return bfq_default_max_budget;
-}
-
static ssize_t bfq_max_budget_store(struct elevator_queue *e,
const char *page, size_t count)
{
int ret = bfq_var_store(&__data, (page), count);
if (__data == 0)
- bfqd->bfq_max_budget = bfq_estimated_max_budget(bfqd);
+ bfqd->bfq_max_budget = bfq_calc_max_budget(bfqd);
else {
if (__data > INT_MAX)
__data = INT_MAX;
bfqd->bfq_timeout = msecs_to_jiffies(__data);
if (bfqd->bfq_user_max_budget == 0)
- bfqd->bfq_max_budget = bfq_estimated_max_budget(bfqd);
+ bfqd->bfq_max_budget = bfq_calc_max_budget(bfqd);
return ret;
}
projects / users / jedix / linux-maple.git / commitdiff