JFR thread sampling mechanism

[Gil Tene]

I would like to discuss a potential improvement to the JFR thread
sampling mechanism, and would like to see if the change we'd
propose has already been considered in the past.

I believe that the current thread sampling mechanism (mostly via
hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp) can be
summarized as: A control thread wakes up periodically (e.g. 100
times per second) and in each period chooses a number (e.g. 5)
threads to sample (by rotating through the overall list of threads)
only if they are "in java", and a number (e.g. 1) threads (by
separately rotating through the overall list of threads) to sample
"only if it is in native". For each thread targeted to sample, the
control thread suspends the target thread (e.g. for linux this is
done by preparing a suspend request a sending a SIGUSR2 to
make the thread deal with it), takes a stacktrace of the suspended
thread, adds the stacktrace to JfrStackTraceRepository, and
resumes the thread (e.g. on linux resumption involves setting up
a resume request and again sending a SIGUSR2 to the thread to
get it to handle it and resume).

We've been contemplating a change to make thread sampling use
Posix timers instead, such that each thread would use a separate
timer, and threads would receive signals based on their CPU
consumption (the timer, e.g. created with timer_create(2), would
be clocked by the thread CPU time of their associated threads,
and signal their threads when that CPU time reaches a level
[of e.g. 10 msec]). The signal handler will then perform the
stacktrace sampling locally on the  thread, and deliver the
stacktrace to JfrStackTraceRepository (either directly or by
enqueing through an intermediary).

There are multiple potential benefits that may arise from switching
to such a scheme, including significant reduction of sampling cost,
improvement of density and focus of samples (fewer lost samples,
ensuring that enough activity in a given thread will end up leading
to a sample for that thread, etc.), and, potentially, an ability to
(with additional  changes) better account for time spent "outside
of java" in e.g. native and runtime code.

Has this (using thread-cpu-time-based posix timer sampling) been
considered before?

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