Re: [PATCH v2 2/4] ceph: track average/stdev r/w/m latency

[Xiubo Li <xiubli@redhat.com>]

On 9/14/21 9:45 PM, Xiubo Li 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:
[...]
> In user space this is very easy to do, but not in kernel space, 
> especially there has no float computing.
>
As I remembered this is main reason why I was planing to send the raw 
metrics to MDS and let the MDS do the computing.

So if possible why not just send the raw data to MDS and let the MDS to 
do the stdev 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.
>
> 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;
>>>>
>>