On Tue, Sep 14, 2021 at 6:39 PM Xiubo Li <xiubli@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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
