why can't I / how do I compute the derivative of an expression?

[JI Ioannidis]

Best to use an example; this is only an example, I'm not looking for workarounds for this particular query!

I am recording the (accumulated) user CPU time (utime, field 14 in /proc/$PID/stat on Linux machines) in a metric I call iproc_utime, which has several dimensions. I can thus plot the total CPU utilization for all processes named "foo"  with the query:

sum(rate(iproc_utime{cmd="foo"} [5m])))

So far so good. This works fine in the built-in Prometheus grapher, and in Grafana. 

Now, I'm observing that the cpu utilization is sometimes climbing, and sometimes falling, so I want to compute its derivative. If I try 

deriv(sum(rate(iproc_utime{cmd="foo"} [5m]))))

I get: Error executing query: parse error at char 52: expected type range vector in call to function "deriv", got instant vector

How do I compute the derivative? Why can't the derivative of an instant vector be computed?

Thanks,

/ji

[Ben Kochie]

The problem is that sum always returns an instant vector, and it's not possible to do nested sub queries.

However, you can record a new metric and take the deriv of that.  For example:

In a rules file:

cmd:iproc_utime:rate5m = sum by (cmd)(rate(iproc_utime[5m]))

Then you can query it later:

deriv(cmd:iproc_utime:rate5m[5m])

Notice how we're now applying two range-vector specification.  The query engine only allows direct metric names to get range-vector [5m] specifications.  This is to avoid needing to recursive sub-query evaluations.

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