JFR thread sampling mechanism

[Kirk Pepperdine]

Hi Gil,

I would support an improvement in sampling as there is an obvious bias which allows me to write benchmarks where JFR completely misses things it should find. That said, I’m not sure that waking a thread up every 10ms is a great idea as it is very disruptive to Linux thread scheduling. I’d very much like to experiment with lower sampling rates.

Kind regards,
Kirk


> On Jun 30, 2019, at 9:20 AM, Gil Tene <gil at azul.com> wrote:
> 
> 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?
>