Add units to all stats

[wohali]

We currently don't expose the units for each stat that we publish under /_node/<node>/_stats. We should.

I bet this is a pretty straightforward update to all of the stats_descriptions.cfg files for someone who knows what all of the stats are.

[tabeth]

Hi @wohali.

I was looking at this issue, and the referenced file. It seems like there are units referenced in the description:

{[couchdb, auth_cache_hits], [
    {type, counter},
    {desc, <<"number of authentication cache hits">>}
]}.

Or were you thinking something like

{[couchdb, auth_cache_hits], [
    {type, counter},
    {unit, Integer},
    {desc, <<"number of authentication cache hits">>}
]}.

[wohali]

Hey @tabeth , thanks for looking at this.

There's a lot of files:

./src/couch/priv/stats_descriptions.cfg
./src/dreyfus/priv/stats_descriptions.cfg
./src/couch_log/priv/stats_descriptions.cfg
./src/mem3/priv/stats_descriptions.cfg
./src/couch_replicator/priv/stats_descriptions.cfg
./src/ddoc_cache/priv/stats_descriptions.cfg
./src/chttpd/priv/stats_descriptions.cfg
./src/rexi/priv/stats_descriptions.cfg
./src/couch_mrview/priv/stats_descriptions.cfg
./src/global_changes/priv/stats_descriptions.cfg
./src/fabric/priv/stats_descriptions.cfg

It's a concern primarily for anything that isn't a counter - for instance, looking at the histograms for length of time, are those in milliseconds? microseconds?

[tabeth]

Ah, I see @wohali

So this is more investigative, where we want (for all of the stats_descriptions.cfg files)

This:

{[dreyfus, rpc, search], [
    {type, histogram},
    {desc, <<"length of a search RPC worker">>}
]}.

To be something more like this:

{[dreyfus, rpc, search], [
    {type, histogram},
    {desc, <<"length (milliseconds) of a search RPC worker ">>}
]}.

So to successfully complete this we need to traverse each file and then figure out what the true values are and update the descriptions. Is that about right?

[wohali]

You got it! Anything that isn't a counter should be reviewed and ensure the description contains the correct unit.

[tabeth]

@wohali OK. I'll take a look at the files and start trying to update the descriptions.

So for example (to make sure I'm doing this right): We have this:

{[dreyfus, index, search], [
    {type, histogram},
    {desc, <<"length of an dreyfus_index search request">>}
]}.

Which seems to be implemented here. dreyfus_util:time/2 seems to call timer:now_diff/2 here, which returns microseconds. However it's divided by 1000, so the answer to the original question posed in the beginning should be milliseconds.

So our original example can be

{[dreyfus, index, search], [
    {type, histogram},
    {desc, <<"length (in milliseconds) of an dreyfus_index search request">>}
]}.

Is that right?

[wohali]

@tabeth Careful. On Windows, erlang:system_time/0 will return time only in ms, not us. You'll need to check the code to be sure this does the right thing on that platform. We may need a source code change here if it does the wrong thing, similar to https://github.com/apache/couchdb/commit/24e2c8bf61a4e82504ea6e6ba2caa779980b9597#diff-7470c79716cb3e7f1b3d572732277d94R349

/cc @rnewson

But yes, that's the right approach.