Add system.cpu.count metric

[trask]

I wasn't sure if this should be a resource attribute or metric, but I tested that cpu count can change at runtime, e.g. docker update --cpuset-cpus, and nproc and Java Runtime.availableProcessors() pick up the change at runtime, so seems like it should be a metric.

Changes

Adds a new metric system.cpu.count to capture the number of CPUs available.

[tigrannajaryan]

I wasn't sure if this should be a resource attribute or metric, but I tested that cpu count can change at runtime, e.g. docker update --cpuset-cpus, and nproc and Java Runtime.availableProcessors() pick up the change at runtime, so seems like it should be a metric.

• I think there was recent discussion that said the set of attributes can't change at the runtime (I can't find the link).
• CPU number is a dimension of system.cpu.* metrics.

Does this mean system.cpu.* metrics are broken since one of the attributes has a varying number of possible values over time?

[bogdandrutu]

Per the definition of the javadoc, https://www.tutorialspoint.com/java/lang/runtime_availableprocessors.htm the availableProcessors does not represent the number of CPU on the host/container.

[bogdandrutu]

I believe this should be a "process.cpu.available" or something like that metric. Since this is just the number of cores available to that specific instance of the JVM. May even be a jvm specific metric.

[bogdandrutu]

I think this is a Gauge, since it does not sum "anything".

[trask]

it counts sums cores(?)

[bogdandrutu]

@trask here is what I found https://www.oracle.com/java/technologies/javase/8u191-relnotes.html#JDK-8146115 which means that the value is ... definitely not the number of total CPUs in the environment. Or it can or cannot be to be more exact :)

[trask]

I believe this should be a "process.cpu.available" or something like that metric. Since this is just the number of cores available to that specific instance of the JVM.

this is a great point

I did a test:

taskset --cpu 0-1 java ...

and Runtime.availableProcessors() reported just the two cpus, not the full system cpu count.

I'll open a new PR for process.cpu.available.

I don't have any specific need for this PR anymore, but happy to move it forward if there's interest.

[tigrannajaryan]

I don't have any specific need for this PR anymore, but happy to move it forward if there's interest.

I am in favour of closing unless there is good evidence that the CPU count at the system level can actually change at runtime and there is a use case that needs this as a metric.

[trask]

No objection to closing