Histograms are a native prometheus type, and there are many functions
available that operate on them. There is extensive documentation about
them at https://prometheus.io/docs/practices/histograms/
One example is the function histogram_quantile(), that can extract
useful quantiles from the histograms. Currently, those functions don't
work well.
The reasons are twofold:
1) We are only exporting 16 metrics, starting from 1usec. That means
that the highest latency we can differentiate is 4ms. After that,
everything falls into the same bin.
2) The format that prometheus expects is that each bin will contain
the total number of points seen *up until that bin*, while we
currently export the total number of points that falls between bins.
IOW, it is a cummulative histogram.
About point two, granted it is a bit hidden in their website, but it is
there. The following phrase about a caveat make it clear:
"Note that we divide the sum of both buckets. The reason is that the
histogram buckets are cumulative. The le="0.3" bucket is also contained
in the le="1.2" bucket; dividing it by 2 corrects for that."
It is also not needed to accumulate things that fall over the last bin:
the _count component of the histogram will already account for that.
This patch changes the histogram format to be more in line with what
prometheus expect.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
Collect coordinator side read statistic per CF and use them in percentile
speculative read executor. Getting percentile from estimated_histogram
object is rather expensive, so cache it and recalculate only once per
second (or if requested percentile changes).
Fixes#2757
Message-Id: <20170911131752.27369-3-gleb@scylladb.com>
Currently overflow values are stored in incorrect bucket (last one
instead of special "overflow" one) and percentile() function throws
if there is overflow value. The patch fixes the code to store overflow
value in corespondent bucket and makes percentile() to take it into
account instead of throwing.
Message-Id: <20170911131752.27369-2-gleb@scylladb.com>
"This series reduce that effect in two ways:
1. Remove the latency counters from the system keyspaces
2. Reduce the histogram size by limiting the maximum number of buckets and
stop the last bucket."
Fixes#2650.
* 'amnon/remove_cf_latency_v2' of github.com:cloudius-systems/seastar-dev:
database: remove latency from the system table
estimated histogram: return a smaller histogram
The current histogram contains 91 buckets, this is a very high
resolution with a high upper limit.
To reduce traffic passed, between scylla and the prometheus, this patch
generate a smaller histogram.
It limit the number of buckets (16 by default), set a lower limit to the
lowest bucket, and uses 2 as the bucket coeficient.
Highest empty buckets will not be reported.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
estimated histogram
The metrics histogram is a struct that describe a histogram.
This patch adds a getter method that lets the estimated_histogram return
a metrics::histogram, this will allow to register it as a histogram
metrics.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
