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Message-ID: <4e5e476b0911171021r2c09e41bk88295082a6ad2715@mail.gmail.com>
Date: Tue, 17 Nov 2009 19:21:47 +0100
From: Corrado Zoccolo <czoccolo@...il.com>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: Linux-Kernel <linux-kernel@...r.kernel.org>,
Jens Axboe <jens.axboe@...cle.com>,
Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [RFC,RFT,PATCH] cfq: autotuning support
On Tue, Nov 17, 2009 at 5:52 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Tue, Nov 17, 2009 at 03:51:14PM +0100, Corrado Zoccolo wrote:
>> Hi, this is my first attempt at autotuning cfq parameters, and should apply on top of for-2.6.33 branch.
>> Jeff and Vivek, if you can test this on your NCQ SSDs, it will help me to have your feedback (please include
>> the output of: 'grep -r . /sys/block/sdX/queue/iosched' after you run your tests).
>
> Sure, I will do some tests on this patch, may be later today.
>
> I had a quick look at the patch. Had couple of questions.
>
> - Service time seems to be the measure of on an average how much time it
> took to service a read/write request (be it sequential or random read).
We sample both in the histogram, but then we try to extract the random
read service time.
> In auto tuning, you seem to be updating slice_idle dynamically based on
> service time. So the intent seems to be that is service times are higher
> then it is a slow media and we should have higher slice_idle otherwise
> a low slice_idle?
Yes. The target is to have slice_idle = the average seek cost.
So we will wait up to the average seek cost for a sequential request
before switching to an other queue and paying a possibly long seek.
For media that have really fast seek (less than 0.5ms), both when
reading and when writing, then we put slice_idle = 0. Both of your
SSDs should be in this category.
Thanks
Corrado
> Thanks
> Vivek
>
>
>>
>> The patch introduces code to sample the request service time distribution, and analyze it,
>> in order to compute the following cfq parameters:
>> * slice_idle, is computed as the expected service time of random request
>> * cfq_slice[1] (i.e. the slice for sync queues)
>> * cfq_slice[0] (i.e. the slice for async queues)
>>
>> The sync and async slices are scaled from default values proportionally to the new computed slice_idle.
>>
>> Autotuning will be enabled by default only on kernels compiled with HZ >= 500.
>> With smaller HZ, I don't think jiffies is reliable to estimate those parameters.
>>
>> Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>
>> ---
>> block/cfq-iosched.c | 166 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 files changed, 163 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>> index 6925ab9..d786a0b 100644
>> --- a/block/cfq-iosched.c
>> +++ b/block/cfq-iosched.c
>> @@ -32,6 +32,12 @@ static const int cfq_target_latency = HZ * 3/10; /* 300 ms */
>> static const int cfq_hist_divisor = 4;
>>
>> /*
>> + * Number of samples to collect before updating autotune
>> + * higher # makes the measurements more stable
>> + */
>> +#define CFQ_AUTOTUNE_SAMPLES (10)
>> +
>> +/*
>> * offset from end of service tree
>> */
>> #define CFQ_IDLE_DELAY (HZ / 5)
>> @@ -201,6 +207,21 @@ struct cfq_data {
>> unsigned int hw_tag_samples;
>>
>> /*
>> + * disk performance measurements
>> + */
>> + unsigned long observation_start;
>> + /*
>> + * measures are split (READ vs WRITE)
>> + */
>> + unsigned long processed_rq[2];
>> + /*
>> + * We store an histogram of samples for the service time
>> + * in log scale [0..5]; [6] is a count, that is reset every
>> + * time autotuning is done (i.e. every CFQ_AUTOTUNE_SAMPLES)
>> + */
>> + unsigned int serv_time_samples[2][7];
>> +
>> + /*
>> * idle window management
>> */
>> struct timer_list idle_slice_timer;
>> @@ -228,6 +249,7 @@ struct cfq_data {
>> unsigned int cfq_slice_async_rq;
>> unsigned int cfq_slice_idle;
>> unsigned int cfq_latency;
>> + unsigned int cfq_autotune;
>>
>> struct list_head cic_list;
>>
>> @@ -891,6 +913,12 @@ static void cfq_activate_request(struct request_queue *q, struct request *rq)
>> {
>> struct cfq_data *cfqd = q->elevator->elevator_data;
>>
>> + if (!rq_in_driver(cfqd)) {
>> + cfqd->observation_start = jiffies;
>> + cfqd->processed_rq[0] = 0;
>> + cfqd->processed_rq[1] = 0;
>> + }
>> +
>> cfqd->rq_in_driver[rq_is_sync(rq)]++;
>> cfq_log_cfqq(cfqd, RQ_CFQQ(rq), "activate rq, drv=%d",
>> rq_in_driver(cfqd));
>> @@ -2562,14 +2590,120 @@ static void cfq_update_hw_tag(struct cfq_data *cfqd)
>> cfqd->hw_tag = 0;
>> }
>>
>> +
>> +/*
>> + * Update the histogram to compute service time
>> + * Returns true if we collected enough samples to re-run autotune
>> + */
>> +static bool cfq_update_stime(unsigned samples[6], unsigned long stime)
>> +{
>> + unsigned idx = (stime > 15) + (stime > 7) + (stime > 3)
>> + + (stime > 1) + (stime > 0);
>> + samples[idx]++;
>> + if (samples[idx] > (1U<<31))
>> + for (idx = 0; idx < 6; ++idx) {
>> + samples[idx]++;
>> + samples[idx] >>= 1;
>> + }
>> +
>> + if (samples[6]++ < CFQ_AUTOTUNE_SAMPLES)
>> + return false;
>> + samples[6] = 0;
>> + return true;
>> +}
>> +
>> +/*
>> + * Currently, we measure only service time for pure READ or WRITE requests
>> + * and we update autotune when we have collected enough READ requests
>> + */
>> +static bool cfq_update_perf_measures(struct cfq_data *cfqd, unsigned long now)
>> +{
>> + unsigned long tot_proc = cfqd->processed_rq[0] + cfqd->processed_rq[1];
>> + unsigned long obstime = now - cfqd->observation_start;
>> + unsigned long stime = obstime / tot_proc;
>> +
>> + cfqd->observation_start = now;
>> +
>> + if (!cfqd->processed_rq[READ])
>> + cfq_update_stime(cfqd->serv_time_samples[WRITE], stime);
>> + if (!cfqd->processed_rq[WRITE])
>> + return cfq_update_stime(cfqd->serv_time_samples[READ], stime);
>> + return false;
>> +}
>> +
>> +/*
>> + * Compute service time from the sampled distribution in the histogram
>> + * The real service time distribution is a super-position of two distinct
>> + * distributions:
>> + * the one for sequential requests (usually this has a small mean)
>> + * the one for random requests (usually with a larger mean)
>> + * and we want to identify the random request one
>> + */
>> +static unsigned cfq_service_time(unsigned samples[6])
>> +{
>> + unsigned last_max = 0, i;
>> + /* Random request service time corresponds to the
>> + * largest maximum in the histogram */
>> + for (i = 1; i < 6; ++i)
>> + if (samples[i] > samples[i-1])
>> + last_max = i;
>> + /*
>> + * Unfortunately, if sequential requests overwhelm
>> + * random ones, and the two peaks are too near,
>> + * the second peak could be masked by the tail of the first.
>> + * To catch this, we check if the tail has enough weight,
>> + * and in this case we take the next bin as maximum
>> + */
>> + if (last_max < 5) {
>> + unsigned total = 0;
>> + for (i = last_max + 1; i < 6; ++i)
>> + total += samples[i];
>> + if (total > samples[last_max])
>> + ++last_max;
>> + }
>> + if (!last_max)
>> + return 0;
>> + return 1U << last_max;
>> +}
>> +
>> +static void cfq_update_autotune(struct cfq_data *cfqd)
>> +{
>> + unsigned long base, baseW;
>> + if (!cfqd->cfq_autotune)
>> + return;
>> + base = cfq_service_time(cfqd->serv_time_samples[READ]);
>> + baseW = cfq_service_time(cfqd->serv_time_samples[WRITE]);
>> +
>> + /* Compute slice_idle */
>> + if (!base)
>> + base = baseW;
>> + if (base > cfq_slice_idle)
>> + base = min_t(unsigned long,
>> + (base + cfq_slice_idle) / 2, 2 * cfq_slice_idle);
>> +
>> + cfqd->cfq_slice_idle = base;
>> +
>> + /* Compute derived cfq_slice[*]
>> + * Note: those cannot be 0
>> + */
>> + if (!base)
>> + base = 1;
>> +
>> + if (baseW)
>> + baseW = min(base, baseW * cfq_slice_sync / cfq_slice_async);
>> + else
>> + baseW = base;
>> +
>> + cfqd->cfq_slice[1] = base * cfq_slice_sync / cfq_slice_idle;
>> + cfqd->cfq_slice[0] = baseW * cfq_slice_async / cfq_slice_idle;
>> +}
>> +
>> static void cfq_completed_request(struct request_queue *q, struct request *rq)
>> {
>> struct cfq_queue *cfqq = RQ_CFQQ(rq);
>> struct cfq_data *cfqd = cfqq->cfqd;
>> const int sync = rq_is_sync(rq);
>> - unsigned long now;
>> -
>> - now = jiffies;
>> + unsigned long now = jiffies;
>> cfq_log_cfqq(cfqd, cfqq, "complete");
>>
>> cfq_update_hw_tag(cfqd);
>> @@ -2578,6 +2712,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
>> WARN_ON(!cfqq->dispatched);
>> cfqd->rq_in_driver[sync]--;
>> cfqq->dispatched--;
>> + cfqd->processed_rq[rq_data_dir(rq)]++;
>> +
>> + if (!rq_in_driver(cfqd) && cfq_update_perf_measures(cfqd, now))
>> + cfq_update_autotune(cfqd);
>>
>> if (cfq_cfqq_sync(cfqq))
>> cfqd->sync_flight--;
>> @@ -2966,6 +3104,7 @@ static void *cfq_init_queue(struct request_queue *q)
>> cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
>> cfqd->cfq_slice_idle = cfq_slice_idle;
>> cfqd->cfq_latency = 1;
>> + cfqd->cfq_autotune = (HZ >= 500);
>> cfqd->hw_tag = -1;
>> cfqd->last_end_sync_rq = jiffies;
>> return cfqd;
>> @@ -3002,6 +3141,23 @@ fail:
>> /*
>> * sysfs parts below -->
>> */
>> +static ssize_t autotune_stats_show(struct elevator_queue *e, char *page)
>> +{
>> + struct cfq_data *cfqd = e->elevator_data;
>> + int pos = 0, i, j;
>> +#if HZ < 500
>> + pos += sprintf(page, "Autotune may work incorrectly with HZ < 500\n");
>> +#endif
>> + for (j = 0; j < 2; ++j) {
>> + for (i = 0; i < 6; ++i)
>> + pos += sprintf(page+pos, "[%2u]",
>> + cfqd->serv_time_samples[j][i]);
>> + page[pos++] = '\n';
>> + }
>> + page[pos] = '\0';
>> + return pos;
>> +}
>> +
>> static ssize_t
>> cfq_var_show(unsigned int var, char *page)
>> {
>> @@ -3036,6 +3192,7 @@ SHOW_FUNCTION(cfq_slice_sync_show, cfqd->cfq_slice[1], 1);
>> SHOW_FUNCTION(cfq_slice_async_show, cfqd->cfq_slice[0], 1);
>> SHOW_FUNCTION(cfq_slice_async_rq_show, cfqd->cfq_slice_async_rq, 0);
>> SHOW_FUNCTION(cfq_low_latency_show, cfqd->cfq_latency, 0);
>> +SHOW_FUNCTION(cfq_autotune_show, cfqd->cfq_autotune, 0);
>> #undef SHOW_FUNCTION
>>
>> #define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV) \
>> @@ -3068,6 +3225,7 @@ STORE_FUNCTION(cfq_slice_async_store, &cfqd->cfq_slice[0], 1, UINT_MAX, 1);
>> STORE_FUNCTION(cfq_slice_async_rq_store, &cfqd->cfq_slice_async_rq, 1,
>> UINT_MAX, 0);
>> STORE_FUNCTION(cfq_low_latency_store, &cfqd->cfq_latency, 0, 1, 0);
>> +STORE_FUNCTION(cfq_autotune_store, &cfqd->cfq_autotune, 0, 1, 0);
>> #undef STORE_FUNCTION
>>
>> #define CFQ_ATTR(name) \
>> @@ -3084,6 +3242,8 @@ static struct elv_fs_entry cfq_attrs[] = {
>> CFQ_ATTR(slice_async_rq),
>> CFQ_ATTR(slice_idle),
>> CFQ_ATTR(low_latency),
>> + CFQ_ATTR(autotune),
>> + __ATTR_RO(autotune_stats),
>> __ATTR_NULL
>> };
>>
>> --
>> 1.6.2.5
>>
>
--
__________________________________________________________________________
dott. Corrado Zoccolo mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
--------------------------------------------------------------------------
The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
Tales of Power - C. Castaneda
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