Thanks, Erik.
Is there any chance you could point me toward the documentation for the
jdk.Safepoint* events? It's difficult to tell at a glance if the sum of the
differences between the safepoint begin and end times is equivalent to
hotspotRuntimeManagementBean.getTotalSafepointTime(). Does that include the
sync time (which appears to have its own SafepointStateSynchronization event)?
I don't understand what beginSafepointEvent.getDuration() means in this
context, is sync time included only in the safepoint sync event, duration of
the SafepointBegin event, or time between begin and end event end times?
In logging framework code we've found that converting between time objects and
numeric values doesn't always (on some jres ever?) optimize away Instant
allocations. The default JFR configuration appears to set time thresholds for
safepoint events which I'd expect reduces overhead, but would prevent us from
accumulating data when there are frequent, quick safepoints. Otherwise the cost
is likely to be much greater than the old approach.
In a similar vein, is there any way for us to accumulate this data without
dedicating an entire OS thread to the RecordingStream?
Sorry for the barrage of questions, I appreciate your help!
Carter Kozak
On Wed, Jun 16, 2021, at 15:44, Erik Gahlin wrote:
> It's possible to access safepoint information using JFR, for example:
>
> try (RecordingStream r = new RecordingStream()) {
> r.enable("jdk.SafepointBegin");
> r.enable("jdk.SafepointEnd");
> r.onEvent("jdk.SafepointBegin", e -> System.out.println("begin: " +
> e.getEndTime()));
> r.onEvent("jdk.SafepointEnd", e -> System.out.println("end: " +
> e.getEndTime()));
> r.start();
> }
>
> Erik
>
> On 2021-06-16, 21:12, "serviceability-dev on behalf of Carter Kozak"
> <serviceability-dev-r...@openjdk.java.net
> <mailto:serviceability-dev-retn%40openjdk.java.net> on behalf of
> cko...@ckozak.net <mailto:ckozak%40ckozak.net>> wrote:
>
> As java 16 and beyond lock down access to internal components by default,
> it can be difficult to produce Prometheus-style metrics describing
> application safepoints. I’ve been monitoring these metrics so that I can be
> alerted when an application spends more than ~10% of time in safepoints for
> some duration, because it means that something has gone wrong: Most often GC
> spirals, however excessive thread dumps, deadlock checks, etc can contribute.
> This has been one of the most meaningful tools in my arsenal to detect
> general JVM badness, however there doesn’t seem to be a way to access the
> data in newer JREs without allowing access to internal components.
>
> Previously I was able to use something along these lines, which required
> legacy sun.management component access.
>
> sun.management.HotspotRuntimeMBean hotspotRuntimeManagementBean =
> sun.management.ManagementFactoryHelper.getHotspotRuntimeMBean();
> long totalSafepointTimeMillis =
> hotspotRuntimeManagementBean.getTotalSafepointTime();
>
> Before I get ahead of myself, I’d like to confirm that I haven’t missed a
> supported pathway to access safepoint time. If my read is correct and there’s
> no way to access this information from inside the running JVM, would it be a
> a reasonable addition to the public API?
>
> Thanks,
> Carter Kozak
>
>
>
