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From: Venky Shankar <vshankar@redhat.com>
To: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@redhat.com>,
	Patrick Donnelly <pdonnell@redhat.com>,
	ceph-devel <ceph-devel@vger.kernel.org>
Subject: Re: [PATCH v2 2/4] ceph: track average/stdev r/w/m latency
Date: Tue, 14 Sep 2021 19:00:41 +0530	[thread overview]
Message-ID: <CACPzV1=A4CYvvrkBM1KKMrzYHJFyrMshQ_Qg9F=tXGQdfERK7g@mail.gmail.com> (raw)
In-Reply-To: <15cd06ae-fe96-c376-854d-738d4a2e70bf@redhat.com>

On Tue, Sep 14, 2021 at 6:39 PM Xiubo Li <xiubli@redhat.com> wrote:
>
>
> On 9/14/21 4:49 PM, Venky Shankar wrote:
> > The math involved in tracking average and standard deviation
> > for r/w/m latencies looks incorrect. Fix that up. Also, change
> > the variable name that tracks standard deviation (*_sq_sum) to
> > *_stdev.
> >
> > Signed-off-by: Venky Shankar <vshankar@redhat.com>
> > ---
> >   fs/ceph/debugfs.c | 14 +++++-----
> >   fs/ceph/metric.c  | 70 ++++++++++++++++++++++-------------------------
> >   fs/ceph/metric.h  |  9 ++++--
> >   3 files changed, 45 insertions(+), 48 deletions(-)
> >
> > diff --git a/fs/ceph/debugfs.c b/fs/ceph/debugfs.c
> > index 38b78b45811f..3abfa7ae8220 100644
> > --- a/fs/ceph/debugfs.c
> > +++ b/fs/ceph/debugfs.c
> > @@ -152,7 +152,7 @@ static int metric_show(struct seq_file *s, void *p)
> >       struct ceph_mds_client *mdsc = fsc->mdsc;
> >       struct ceph_client_metric *m = &mdsc->metric;
> >       int nr_caps = 0;
> > -     s64 total, sum, avg, min, max, sq;
> > +     s64 total, sum, avg, min, max, stdev;
> >       u64 sum_sz, avg_sz, min_sz, max_sz;
> >
> >       sum = percpu_counter_sum(&m->total_inodes);
> > @@ -175,9 +175,9 @@ static int metric_show(struct seq_file *s, void *p)
> >       avg = total > 0 ? DIV64_U64_ROUND_CLOSEST(sum, total) : 0;
> >       min = m->read_latency_min;
> >       max = m->read_latency_max;
> > -     sq = m->read_latency_sq_sum;
> > +     stdev = m->read_latency_stdev;
> >       spin_unlock(&m->read_metric_lock);
> > -     CEPH_LAT_METRIC_SHOW("read", total, avg, min, max, sq);
> > +     CEPH_LAT_METRIC_SHOW("read", total, avg, min, max, stdev);
> >
> >       spin_lock(&m->write_metric_lock);
> >       total = m->total_writes;
> > @@ -185,9 +185,9 @@ static int metric_show(struct seq_file *s, void *p)
> >       avg = total > 0 ? DIV64_U64_ROUND_CLOSEST(sum, total) : 0;
> >       min = m->write_latency_min;
> >       max = m->write_latency_max;
> > -     sq = m->write_latency_sq_sum;
> > +     stdev = m->write_latency_stdev;
> >       spin_unlock(&m->write_metric_lock);
> > -     CEPH_LAT_METRIC_SHOW("write", total, avg, min, max, sq);
> > +     CEPH_LAT_METRIC_SHOW("write", total, avg, min, max, stdev);
> >
> >       spin_lock(&m->metadata_metric_lock);
> >       total = m->total_metadatas;
> > @@ -195,9 +195,9 @@ static int metric_show(struct seq_file *s, void *p)
> >       avg = total > 0 ? DIV64_U64_ROUND_CLOSEST(sum, total) : 0;
> >       min = m->metadata_latency_min;
> >       max = m->metadata_latency_max;
> > -     sq = m->metadata_latency_sq_sum;
> > +     stdev = m->metadata_latency_stdev;
> >       spin_unlock(&m->metadata_metric_lock);
> > -     CEPH_LAT_METRIC_SHOW("metadata", total, avg, min, max, sq);
> > +     CEPH_LAT_METRIC_SHOW("metadata", total, avg, min, max, stdev);
> >
> >       seq_printf(s, "\n");
> >       seq_printf(s, "item          total       avg_sz(bytes)   min_sz(bytes)   max_sz(bytes)  total_sz(bytes)\n");
> > diff --git a/fs/ceph/metric.c b/fs/ceph/metric.c
> > index 226dc38e2909..6b774b1a88ce 100644
> > --- a/fs/ceph/metric.c
> > +++ b/fs/ceph/metric.c
> > @@ -244,7 +244,8 @@ int ceph_metric_init(struct ceph_client_metric *m)
> >               goto err_i_caps_mis;
> >
> >       spin_lock_init(&m->read_metric_lock);
> > -     m->read_latency_sq_sum = 0;
> > +     m->read_latency_stdev = 0;
> > +     m->avg_read_latency = 0;
> >       m->read_latency_min = KTIME_MAX;
> >       m->read_latency_max = 0;
> >       m->total_reads = 0;
> > @@ -254,7 +255,8 @@ int ceph_metric_init(struct ceph_client_metric *m)
> >       m->read_size_sum = 0;
> >
> >       spin_lock_init(&m->write_metric_lock);
> > -     m->write_latency_sq_sum = 0;
> > +     m->write_latency_stdev = 0;
> > +     m->avg_write_latency = 0;
> >       m->write_latency_min = KTIME_MAX;
> >       m->write_latency_max = 0;
> >       m->total_writes = 0;
> > @@ -264,7 +266,8 @@ int ceph_metric_init(struct ceph_client_metric *m)
> >       m->write_size_sum = 0;
> >
> >       spin_lock_init(&m->metadata_metric_lock);
> > -     m->metadata_latency_sq_sum = 0;
> > +     m->metadata_latency_stdev = 0;
> > +     m->avg_metadata_latency = 0;
> >       m->metadata_latency_min = KTIME_MAX;
> >       m->metadata_latency_max = 0;
> >       m->total_metadatas = 0;
> > @@ -322,20 +325,26 @@ void ceph_metric_destroy(struct ceph_client_metric *m)
> >               max = new;                      \
> >   }
> >
> > -static inline void __update_stdev(ktime_t total, ktime_t lsum,
> > -                               ktime_t *sq_sump, ktime_t lat)
> > +static inline void __update_latency(ktime_t *ctotal, ktime_t *lsum,
> > +                                 ktime_t *lavg, ktime_t *min, ktime_t *max,
> > +                                 ktime_t *lstdev, ktime_t lat)
> >   {
> > -     ktime_t avg, sq;
> > +     ktime_t total, avg, stdev;
> >
> > -     if (unlikely(total == 1))
> > -             return;
> > +     total = ++(*ctotal);
> > +     *lsum += lat;
> > +
> > +     METRIC_UPDATE_MIN_MAX(*min, *max, lat);
> >
> > -     /* the sq is (lat - old_avg) * (lat - new_avg) */
> > -     avg = DIV64_U64_ROUND_CLOSEST((lsum - lat), (total - 1));
> > -     sq = lat - avg;
> > -     avg = DIV64_U64_ROUND_CLOSEST(lsum, total);
> > -     sq = sq * (lat - avg);
> > -     *sq_sump += sq;
> > +     if (unlikely(total == 1)) {
> > +             *lavg = lat;
> > +             *lstdev = 0;
> > +     } else {
> > +             avg = *lavg + div64_s64(lat - *lavg, total);
> > +             stdev = *lstdev + (lat - *lavg)*(lat - avg);
> > +             *lstdev = int_sqrt(div64_u64(stdev, total - 1));
> > +             *lavg = avg;
> > +     }
>
> IMO, this is incorrect, the math formula please see:
>
> https://www.investopedia.com/ask/answers/042415/what-difference-between-standard-error-means-and-standard-deviation.asp
>
> The most accurate result should be:
>
> stdev = int_sqrt(sum((X(n) - avg)^2, (X(n-1) - avg)^2, ..., (X(1) -
> avg)^2) / (n - 1)).
>
> While you are computing it:
>
> stdev_n = int_sqrt(stdev_(n-1) + (X(n-1) - avg)^2)

Hmm. The int_sqrt() is probably not needed at this point and can be
done when sending the metric. That would avoid some cycles.

Also, the way avg is calculated not totally incorrect, however, I
would like to keep it similar to how its done is libcephfs.

>
> Though current stdev computing method is not exactly the same the math
> formula does, but it's closer to it, because the kernel couldn't record
> all the latency value and do it whenever needed, which will occupy a
> large amount of memories and cpu resources.

The approach is to calculate the running variance, I.e., compute the
variance as  data (latency) arrive one at a time.

>
>
> >   }
> >
> >   void ceph_update_read_metrics(struct ceph_client_metric *m,
> > @@ -343,23 +352,18 @@ void ceph_update_read_metrics(struct ceph_client_metric *m,
> >                             unsigned int size, int rc)
> >   {
> >       ktime_t lat = ktime_sub(r_end, r_start);
> > -     ktime_t total;
> >
> >       if (unlikely(rc < 0 && rc != -ENOENT && rc != -ETIMEDOUT))
> >               return;
> >
> >       spin_lock(&m->read_metric_lock);
> > -     total = ++m->total_reads;
> >       m->read_size_sum += size;
> > -     m->read_latency_sum += lat;
> >       METRIC_UPDATE_MIN_MAX(m->read_size_min,
> >                             m->read_size_max,
> >                             size);
> > -     METRIC_UPDATE_MIN_MAX(m->read_latency_min,
> > -                           m->read_latency_max,
> > -                           lat);
> > -     __update_stdev(total, m->read_latency_sum,
> > -                    &m->read_latency_sq_sum, lat);
> > +     __update_latency(&m->total_reads, &m->read_latency_sum,
> > +                      &m->avg_read_latency, &m->read_latency_min,
> > +                      &m->read_latency_max, &m->read_latency_stdev, lat);
> >       spin_unlock(&m->read_metric_lock);
> >   }
> >
> > @@ -368,23 +372,18 @@ void ceph_update_write_metrics(struct ceph_client_metric *m,
> >                              unsigned int size, int rc)
> >   {
> >       ktime_t lat = ktime_sub(r_end, r_start);
> > -     ktime_t total;
> >
> >       if (unlikely(rc && rc != -ETIMEDOUT))
> >               return;
> >
> >       spin_lock(&m->write_metric_lock);
> > -     total = ++m->total_writes;
> >       m->write_size_sum += size;
> > -     m->write_latency_sum += lat;
> >       METRIC_UPDATE_MIN_MAX(m->write_size_min,
> >                             m->write_size_max,
> >                             size);
> > -     METRIC_UPDATE_MIN_MAX(m->write_latency_min,
> > -                           m->write_latency_max,
> > -                           lat);
> > -     __update_stdev(total, m->write_latency_sum,
> > -                    &m->write_latency_sq_sum, lat);
> > +     __update_latency(&m->total_writes, &m->write_latency_sum,
> > +                      &m->avg_write_latency, &m->write_latency_min,
> > +                      &m->write_latency_max, &m->write_latency_stdev, lat);
> >       spin_unlock(&m->write_metric_lock);
> >   }
> >
> > @@ -393,18 +392,13 @@ void ceph_update_metadata_metrics(struct ceph_client_metric *m,
> >                                 int rc)
> >   {
> >       ktime_t lat = ktime_sub(r_end, r_start);
> > -     ktime_t total;
> >
> >       if (unlikely(rc && rc != -ENOENT))
> >               return;
> >
> >       spin_lock(&m->metadata_metric_lock);
> > -     total = ++m->total_metadatas;
> > -     m->metadata_latency_sum += lat;
> > -     METRIC_UPDATE_MIN_MAX(m->metadata_latency_min,
> > -                           m->metadata_latency_max,
> > -                           lat);
> > -     __update_stdev(total, m->metadata_latency_sum,
> > -                    &m->metadata_latency_sq_sum, lat);
> > +     __update_latency(&m->total_metadatas, &m->metadata_latency_sum,
> > +                      &m->avg_metadata_latency, &m->metadata_latency_min,
> > +                      &m->metadata_latency_max, &m->metadata_latency_stdev, lat);
> >       spin_unlock(&m->metadata_metric_lock);
> >   }
> > diff --git a/fs/ceph/metric.h b/fs/ceph/metric.h
> > index 103ed736f9d2..a5da21b8f8ed 100644
> > --- a/fs/ceph/metric.h
> > +++ b/fs/ceph/metric.h
> > @@ -138,7 +138,8 @@ struct ceph_client_metric {
> >       u64 read_size_min;
> >       u64 read_size_max;
> >       ktime_t read_latency_sum;
> > -     ktime_t read_latency_sq_sum;
> > +     ktime_t avg_read_latency;
> > +     ktime_t read_latency_stdev;
> >       ktime_t read_latency_min;
> >       ktime_t read_latency_max;
> >
> > @@ -148,14 +149,16 @@ struct ceph_client_metric {
> >       u64 write_size_min;
> >       u64 write_size_max;
> >       ktime_t write_latency_sum;
> > -     ktime_t write_latency_sq_sum;
> > +     ktime_t avg_write_latency;
> > +     ktime_t write_latency_stdev;
> >       ktime_t write_latency_min;
> >       ktime_t write_latency_max;
> >
> >       spinlock_t metadata_metric_lock;
> >       u64 total_metadatas;
> >       ktime_t metadata_latency_sum;
> > -     ktime_t metadata_latency_sq_sum;
> > +     ktime_t avg_metadata_latency;
> > +     ktime_t metadata_latency_stdev;
> >       ktime_t metadata_latency_min;
> >       ktime_t metadata_latency_max;
> >
>


-- 
Cheers,
Venky


  reply	other threads:[~2021-09-14 13:31 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-14  8:48 [PATCH v2 0/4] ceph: forward average read/write/metadata latency Venky Shankar
2021-09-14  8:48 ` [PATCH v2 1/4] ceph: use "struct ceph_timespec" for r/w/m latencies Venky Shankar
2021-09-14  8:49 ` [PATCH v2 2/4] ceph: track average/stdev r/w/m latency Venky Shankar
2021-09-14 12:52   ` Xiubo Li
2021-09-14 13:03     ` Venky Shankar
2021-09-14 13:09   ` Xiubo Li
2021-09-14 13:30     ` Venky Shankar [this message]
2021-09-14 13:45       ` Xiubo Li
2021-09-14 13:52         ` Xiubo Li
2021-09-14 14:00           ` Venky Shankar
2021-09-14 14:10             ` Xiubo Li
2021-09-14 13:53         ` Venky Shankar
2021-09-14 13:58           ` Xiubo Li
2021-09-14 13:13   ` Xiubo Li
2021-09-14 13:32     ` Jeff Layton
2021-09-14 13:32     ` Venky Shankar
2021-09-14  8:49 ` [PATCH v2 3/4] ceph: include average/stddev r/w/m latency in mds metrics Venky Shankar
2021-09-14 13:57   ` Xiubo Li
2021-09-14  8:49 ` [PATCH v2 4/4] ceph: use tracked average r/w/m latencies to display metrics in debugfs Venky Shankar

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