Broken thread scheduling in indexed loop (missing safepoint?)

[Vitaly Davidovich]

Hi Volker,

Just curious - what happens if a STW GC needs to occur right as this type
of loop is entered? Does the VM just stall until the loop exits? What if
this loop does a fast path allocation on some iteration? Do all allocations
check for safepoints internally?

Thanks

Sent from my phone
On Nov 30, 2012 1:41 PM, "Volker Simonis" <volker.simonis at gmail.com> wrote:

> Hi,
>
> This is a long standing problem of HotSpot (compared for example to
> J9). It doesn't put Safepoints into counted int loops (because it
> assumes they will terminate just "fast enough" which is not the case
> in your example). You can see another example for this behavior in
> these slides "http://www.progdoc.de/papers/Jax2012/jax2012.html#%288%29"
> together with the generated assembler code.
>
> You can easily solve the problem by making your loop variable a "long"
> instead of an "int". In that case, HotSpot will be more cautious and
> place a safepoint into the loop.
>
> Regards,
> Volker
>
> On Fri, Nov 30, 2012 at 2:05 PM, Alexey Goncharuk
> <agoncharuk at gridgain.com> wrote:
> > Hi,
> >
> > We faced some weird issue with thread scheduling. At a first glance it
> > looked like it relates to
> > http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7160161 but not
> exactly.
> >
> > This is the code we ran:
> >
> > public static void main(String[] args) throws Exception { Thread worker =
> > new Thread() { @Override public void run() { double d = 0; for (int j =
> 1; j
> > < 2000000000; j++) d += Math.log(Math.E * j); System.out.println(d); } };
> > Thread reporter = new Thread() { @Override public void run() { try {
> while
> > (true) { Thread.sleep(1000); System.out.println("Running: " +
> > System.currentTimeMillis()); } } catch (InterruptedException ignored) {
> > Thread.currentThread().interrupt(); } } }; reporter.start();
> worker.start();
> > worker.join(); reporter.interrupt(); }
> >
> > One can expect that printing thread would output messages during all the
> > calculation time, however it hangs after 3-4 iterations. Setting
> > -XX:FreqInlineSize=0 as described in original bug report does not help in
> > this case, but if I extract loop body into a separate method, setting
> this
> > option works. Example passes with -Xint option as well. (Tested with
> > 1.6.0_33, 1.6.0_37, 1.7.0_07 on Windows and 1.6.0_33 on Linux)
> >
> > I saw #7160161 marked as resolved, so I just wanted to confirm if
> behavior
> > we see really relates to this issue and it was fixed (bug report covers
> > non-Counted loop only).
> >
> > Also, is there any other workarounds rather then extracting the method
> and
> > specifying FreqInlineSize=0?
> >
> > Thanks,
> > Alexey Goncharuk
> >
>
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