On 2023-04-06 15:56, Benjamin Saint-Cyr via lttng-dev wrote:
Dear LTTng Developers,
I am currently conducting performance research on Kubernetes for my master's
thesis and am using LTTng for data collection and Trace Compass for analysis.
I have a couple of questions and concerns, and I would appreciate your
guidance to ensure I am using your tools effectively.
1. In the Control Flow view, I can see the CPU throttling due to the
quota defined in the Cgroup as the pod runs and then waits for the CPU
periodically. However, it is unclear whether the pod is waiting due to a full
CPU or because of timeshares. I can easily find this information in
/sys/fs/cgroup/cpu,cpuacct/kubepods/<burstable or
besteffort>/pod<UID>/cpu.stat if the pod is being throttled. Is there a way to
extract this information using LTTng or Trace Compass?
2. I am attempting to determine which PID is running a specific
Container/Pod UID. Currently, I can deduce this by observing the process named
runc[*:INIT], as it creates the cgroup and clones the process for the
container. However, this approach is fragile and only works due to an
implementation detail. It also fails if tracing starts after deployment. This
information is easily available in
/sys/fs/cgroup/cpu,cpuacct/kubepods/<burstable or
besteffort>/pod<UID>/<container ID>/cgroup.procs. Is there an existing method
within LTTng to obtain this information that I may not be aware of?
The PID namespace context is probably the closest thing to what you are
looking for, see
"https://lttng.org/man/3/lttng-ust/v2.13/#doc-_context_information";. The
hierachy of the namespaces can be recorded in a statedump.
I have been informed that it might be possible to read the information in
cpu.stat or cgroup.procs using a state dump at the beginning or end, but the
process remains unclear. Alternatively, I could instrument the kernel or the
process handling the accounting, but this would be quite challenging. Another
option would be to use something similar to cAdvisor, but its efficiency is
questionable since it dedicates an entire process to reading a few files.
I don't think we have anything cgroup related in the statedump at the moment,
not that it's impossible but it was never implemented.
I would greatly appreciate any guidance or suggestions you may have on
improving my approach and effectively utilizing LTTng for my research.
Thank you for your time and assistance.
Best regards,
Benjamin Saint-Cyr
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