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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 19:23:40 +0530	[thread overview]
Message-ID: <CACPzV1mKmmFttb8X05ePa5WyQN-EFWoH-n9XfEwY5fraush8+A@mail.gmail.com> (raw)
In-Reply-To: <537fc647-249b-d4e8-2139-90cda31b634c@redhat.com>

On Tue, Sep 14, 2021 at 7:16 PM Xiubo Li <xiubli@redhat.com> wrote:
>
>
> On 9/14/21 9:30 PM, Venky Shankar wrote:
> > 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.
>
> In user space this is very easy to do, but not in kernel space,
> especially there has no float computing.
>
> Currently the kclient is doing the avg computing by:
>
> avg(n) = (avg(n-1) + latency(n)) / (n), IMO this should be closer to the
> real avg(n) = sum(latency(n), latency(n-1), ..., latency(1)) / n.

That's how is done in libcephfs too.

>
> Because it's hard to record all the latency values, this is also many
> other user space tools doing to count the avg.
>
>
> >> 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


  parent reply	other threads:[~2021-09-14 13:54 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
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 [this message]
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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