Re: [PATCH v3 2/4] ceph: track average/stdev r/w/m latency

[Xiubo Li <xiubli@redhat.com>]

On 9/22/21 9:44 PM, Venky Shankar wrote:
> On Wed, Sep 22, 2021 at 6:03 PM Venky Shankar <vshankar@redhat.com> wrote:
>> On Wed, Sep 22, 2021 at 5:47 PM Jeff Layton <jlayton@redhat.com> wrote:
>>> On Tue, 2021-09-21 at 18:37 +0530, Venky Shankar wrote:
>>>> Update the math involved to closely mimic how its done in
>>>> user land. This does not make a lot of difference to the
>>>> execution speed.
>>>>
>>>> Signed-off-by: Venky Shankar <vshankar@redhat.com>
>>>> ---
>>>>   fs/ceph/metric.c | 63 +++++++++++++++++++++---------------------------
>>>>   fs/ceph/metric.h |  3 +++
>>>>   2 files changed, 31 insertions(+), 35 deletions(-)
>>>>
>>>> diff --git a/fs/ceph/metric.c b/fs/ceph/metric.c
>>>> index 226dc38e2909..ca758bff69ca 100644
>>>> --- a/fs/ceph/metric.c
>>>> +++ b/fs/ceph/metric.c
>>>> @@ -245,6 +245,7 @@ int ceph_metric_init(struct ceph_client_metric *m)
>>>>
>>>>        spin_lock_init(&m->read_metric_lock);
>>>>        m->read_latency_sq_sum = 0;
>>>> +     m->avg_read_latency = 0;
>>>>        m->read_latency_min = KTIME_MAX;
>>>>        m->read_latency_max = 0;
>>>>        m->total_reads = 0;
>>>> @@ -255,6 +256,7 @@ int ceph_metric_init(struct ceph_client_metric *m)
>>>>
>>>>        spin_lock_init(&m->write_metric_lock);
>>>>        m->write_latency_sq_sum = 0;
>>>> +     m->avg_write_latency = 0;
>>>>        m->write_latency_min = KTIME_MAX;
>>>>        m->write_latency_max = 0;
>>>>        m->total_writes = 0;
>>>> @@ -265,6 +267,7 @@ int ceph_metric_init(struct ceph_client_metric *m)
>>>>
>>>>        spin_lock_init(&m->metadata_metric_lock);
>>>>        m->metadata_latency_sq_sum = 0;
>>>> +     m->avg_metadata_latency = 0;
>>>>        m->metadata_latency_min = KTIME_MAX;
>>>>        m->metadata_latency_max = 0;
>>>>        m->total_metadatas = 0;
>>>> @@ -322,20 +325,25 @@ 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 *sum_sq, ktime_t lat)
>>>>   {
>>>> -     ktime_t avg, sq;
>>>> +     ktime_t total, avg;
>>>>
>>>> -     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;
>>>> +             *sum_sq = 0;
>>>> +     } else {
>>>> +             avg = *lavg + div64_s64(lat - *lavg, total);
>>>> +             *sum_sq += (lat - *lavg)*(lat - avg);
>>>> +             *lavg = avg;
>>>> +     }
>>>>   }
>>>>
>>>>   void ceph_update_read_metrics(struct ceph_client_metric *m,
>>>> @@ -343,23 +351,18 @@ 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_sq_sum, lat);
>>> Do we really need to calculate the std deviation on every update? We
>>> have to figure that in most cases, this stuff will be collected but only
>>> seldom viewed.
>>>
>>> ISTM that we ought to collect just the bare minimum of info on each
>>> update, and save the more expensive calculations for the tool presenting
>>> this info.
>> Yeh, that's probably the plan we want going forward when introducing
>> new metrics.
>>
>> FWIW, we could start doing it with this itself. It's just that the
>> user land PRs are approved and those do the way it is done here.
>>
>> I'm ok with moving math crunching to the tool and it should not be a
>> major change to this patchset.
> So, I kind of recall why we did it this way -- metrics exchange
> between MDS and ceph-mgr is restricted to std::pair<uint64_t,
> uint64_t>.

Before I pushed one patch to improve this and switched it to a list or 
something else, but revert it dues to some reason.


> That would probably need to be expanded to carry variable
> sized objects. I remember seeing a PR that does something like that.
> Need to dig it up...
>
> The other way would be to exchange the necessary information needed
> for the calculation as a separate types but that's not really clean
> and results in unnecessary bloating.
>
>>>>        spin_unlock(&m->read_metric_lock);
>>>>   }
>>>>
>>>> @@ -368,23 +371,18 @@ 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_sq_sum, lat);
>>>>        spin_unlock(&m->write_metric_lock);
>>>>   }
>>>>
>>>> @@ -393,18 +391,13 @@ 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_sq_sum, lat);
>>>>        spin_unlock(&m->metadata_metric_lock);
>>>>   }
>>>> diff --git a/fs/ceph/metric.h b/fs/ceph/metric.h
>>>> index 103ed736f9d2..0af02e212033 100644
>>>> --- a/fs/ceph/metric.h
>>>> +++ b/fs/ceph/metric.h
>>>> @@ -138,6 +138,7 @@ struct ceph_client_metric {
>>>>        u64 read_size_min;
>>>>        u64 read_size_max;
>>>>        ktime_t read_latency_sum;
>>>> +     ktime_t avg_read_latency;
>>>>        ktime_t read_latency_sq_sum;
>>>>        ktime_t read_latency_min;
>>>>        ktime_t read_latency_max;
>>>> @@ -148,6 +149,7 @@ struct ceph_client_metric {
>>>>        u64 write_size_min;
>>>>        u64 write_size_max;
>>>>        ktime_t write_latency_sum;
>>>> +     ktime_t avg_write_latency;
>>>>        ktime_t write_latency_sq_sum;
>>>>        ktime_t write_latency_min;
>>>>        ktime_t write_latency_max;
>>>> @@ -155,6 +157,7 @@ struct ceph_client_metric {
>>>>        spinlock_t metadata_metric_lock;
>>>>        u64 total_metadatas;
>>>>        ktime_t metadata_latency_sum;
>>>> +     ktime_t avg_metadata_latency;
>>>>        ktime_t metadata_latency_sq_sum;
>>>>        ktime_t metadata_latency_min;
>>>>        ktime_t metadata_latency_max;
>>> --
>>> Jeff Layton <jlayton@redhat.com>
>>>
>>
>> --
>> Cheers,
>> Venky
>
>