[couchdb] 01/01: Use milliseconds for fsync time histogram

[va...@apache.org]

This is an automated email from the ASF dual-hosted git repository.

vatamane pushed a commit to branch use-milliseconds-for-fsync-histogram
in repository https://gitbox.apache.org/repos/asf/couchdb.git

commit c68291cd7ab7f4762b106379e5d808414a60fab8
Author: Nick Vatamaniuc <va...@gmail.com>
AuthorDate: Mon May 22 22:57:12 2023 -0400

    Use milliseconds for fsync time histogram
    
    Most of our histograms are in milliseconds. Don't deviate from that with the
    new fsync.time metric so make it use milliseconds as well. It turns out
    fractional histogram values are allowed, so we can still get the
    sub-millisecond precision for faster disks that way.
    
    Example of usage during: `fabric_bench:go(#{docs=>50000, batch_size=>100}).`
    
    ```
    couch_stats:sample([fsync, time]).
    [{min,8.303},
     {max,134.124},
     {arithmetic_mean,49.3490064102564},
     {geometric_mean,42.939511193319994},
     {harmonic_mean,37.05011500299417},
     {median,43.198},
     {variance,654.1673972451199},
     {standard_deviation,25.57669637082006},
     {skewness,0.7276971171556265},
     {kurtosis,-0.021701320829371706},
     {percentile,[{50,43.198},
                  {75,63.858},
                  {90,81.22},
                  {95,97.231},
                  {99,114.86},
                  {999,134.124}]},
     {histogram,[{25.303,48},
                 {48.303,40},
                 {68.303,38},
                 {78.303,4},
                 {98.303,18},
                 {118.303,7},
                 {128.303,0},
                 {148.303,1}]},
     {n,156}]
    ```
---
 src/couch/src/couch_file.erl | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/couch/src/couch_file.erl b/src/couch/src/couch_file.erl
index d68f8c49c..51b0e69e0 100644
--- a/src/couch/src/couch_file.erl
+++ b/src/couch/src/couch_file.erl
@@ -643,10 +643,13 @@ format_status(_Opt, [PDict, #file{} = File]) ->
     [{data, [{"State", File}, {"InitialFilePath", FilePath}]}].
 
 fsync(Fd) ->
+    % Our metrics histograms are in milliseconds so stick to that pattern.
+    % However we measure the time delta in microseconds so we get fractional
+    % microseconds for cases when the disk drives are faster.
     T0 = erlang:monotonic_time(microsecond),
     Res = file:sync(Fd),
-    Dt = erlang:monotonic_time(microsecond) - T0,
-    couch_stats:update_histogram([fsync, time], Dt),
+    DtUSec = erlang:monotonic_time(microsecond) - T0,
+    couch_stats:update_histogram([fsync, time], DtUSec / 1000),
     couch_stats:increment_counter([fsync, count]),
     Res.