Collecting metrics from processes is expensive, especially on platforms like
Windows. The executor code has a 5s cache of stats to ensure that we don't
thrash syscalls on nodes running many allocations. But the timestamp used to
calculate TTL of this cache was never being set, so we were always treating it
as expired. This causes excess CPU utilization on client nodes.
Ensure that when we fill the cache, we set the timestamp. In testing on Windows,
this reduces exector CPU overhead by roughly 75%.
This changeset includes two other related items:
* The `telemetry.publish_allocation_metrics` field correctly prevents a node
from publishing metrics, but the stats hook on the taskrunner still collects
the metrics, which can be expensive. Thread the configuration value into the
stats hook so that we don't collect if `telemetry.publish_allocation_metrics =
false`.
* The `linuxProcStats` type in the executor's `procstats` package is misnamed as
a result of a couple rounds of refactoring. It's used by all task executors,
not just Linux. Rename this and move a comment about how Windows processes are
listed so that the comment is closer to where the logic is implemented.
Fixes: https://github.com/hashicorp/nomad/issues/23323
Fixes: https://hashicorp.atlassian.net/browse/NMD-455
* windows: revert process listing logic to that of v1.6.10
In Nomad 1.7 much of the process management code was refactored, including
a rewrite of how the process tree of an executor was determined on Windows
machines. Unfortunately that rewrite has been cursed with performance issues
and bugs. Instead, revert to the logic used in v1.6.10.
* changelog
In #20619 we overhauled how we were gathering stats for Windows
processes. Unlike in Linux where we can ask for processes in a cgroup, on
Windows we have to make a single expensive syscall to get all the processes and
then build the tree ourselves. Our algorithm to do so is recursive and quadratic
in both steps and space with the number of processes on the host. For busy hosts
this hits the stack limit and panics the Nomad client.
We already build a map of parent PID to PID, so modify this to be a map of
parent PID to slice of children and then traverse that tree only from the root
we care about (the executor PID). This moves the allocations to the heap but
makes the stats gathering linear in steps and space required.
This changeset also moves as much of this code as possible into an area
not conditionally-compiled by OS, as the tagged test file was not being run in CI.
Fixes: https://github.com/hashicorp/nomad/issues/23984
