Issue w/ Recursive Virtual Thread Spawning

[Ryan Schmukler]

Thanks for the quick response! Is this limitation something that is planned
to be changed in the future? If so, is there any estimate as to when? ie.
this year, next year, some day, etc.

Cheers again!

On Wed, Apr 19, 2023 at 9:34 AM Alan Bateman <Alan.Bateman at oracle.com>
wrote:

> On 19/04/2023 15:38, Ryan Schmukler wrote:
> > Hello,
> >
> > I am not sure if this is the appropriate venue or if I should be
> > reporting bugs via the Oracle bug page. Apologies if this is
> > incorrect; this is also my first mailing list.
> >
> > I am the author of a library that brings Loom related functionality
> > into Clojure: https://github.com/teknql/tapestry
> >
> > I recently stumbled upon an interesting behavior where recursively
> > spawning fibers can cause the program to block infinitely. The
> > (clojure) code that can reproduce this can be seen at this gist:
> > https://gist.github.com/rschmukler/314b786206246c906caa6a2f99d731a7
> >
> > Basically if you have 4 virtual threads that recursively spawn 4
> > virtual threads, which recursively spawn 4  virtual threads, which
> > recursively spawn 4  virtual threads, you will hit some sort of
> > blocking issue and your code will freeze. 4^4 = 256  virtual threads
> > which I think should be very doable for Loom.
> >
>
> There is currently a scalability issue / limitation when parking while
> holding a monitor. The limitation is that parking the virtual thread
> doesn't release the underlying carrier thread to do other work. There is
> a diagnostic option and a JFR event to help identify these issues. You
> can read more about this issue and the diagnostic options in JEP 444. I
> did a quick run of your test with -Djdk.tracePinnedThreads=full and it
> produces the output beyond. Looking for "<== monitors" in the output as
> it seems the lazy-seq methods are synchronized.
>
> Thread[#51,ForkJoinPool-1-worker-10,5,CarrierThreads]
>
> java.base/java.lang.VirtualThread$VThreadContinuation.onPinned(VirtualThread.java:185)
> java.base/jdk.internal.vm.Continuation.onPinned0(Continuation.java:393)
>      java.base/java.lang.VirtualThread.park(VirtualThread.java:595)
>      java.base/java.lang.System$2.parkVirtualThread(System.java:2620)
> java.base/jdk.internal.misc.VirtualThreads.park(VirtualThreads.java:54)
> java.base/java.util.concurrent.locks.LockSupport.park(LockSupport.java:219)
>
> java.base/java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1864)
>
> java.base/java.util.concurrent.ForkJoinPool.unmanagedBlock(ForkJoinPool.java:3780)
>
> java.base/java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3725)
>
> java.base/java.util.concurrent.CompletableFuture.waitingGet(CompletableFuture.java:1898)
>
> java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2072)
>      clojure.core$deref_future.invokeStatic(core.clj:2304)
>      clojure.core$deref.invokeStatic(core.clj:2324)
>      clojure.core$deref.invoke(core.clj:2310)
>      clojure.core$map$fn__5884.invoke(core.clj:2757)
>      clojure.lang.LazySeq.sval(LazySeq.java:42) <== monitors:1
>      clojure.lang.LazySeq.seq(LazySeq.java:51) <== monitors:1
>      clojure.lang.RT.seq(RT.java:535)
>      clojure.core$seq__5419.invokeStatic(core.clj:139)
>      clojure.core$apply.invokeStatic(core.clj:662)
>      clojure.core$apply.invoke(core.clj:662)
>      user$eval141$fn__142$fn__143.invoke(NO_SOURCE_FILE:10)
>      user$ppmap$fn__136$fn__137.invoke(NO_SOURCE_FILE:10)
>      clojure.lang.AFn.applyToHelper(AFn.java:152)
>      clojure.lang.AFn.applyTo(AFn.java:144)
>      clojure.core$apply.invokeStatic(core.clj:667)
>      clojure.core$with_bindings_STAR_.invokeStatic(core.clj:1977)
>      clojure.core$with_bindings_STAR_.doInvoke(core.clj:1977)
>      clojure.lang.RestFn.invoke(RestFn.java:425)
>      clojure.lang.AFn.applyToHelper(AFn.java:156)
>      clojure.lang.RestFn.applyTo(RestFn.java:132)
>      clojure.core$apply.invokeStatic(core.clj:671)
>      clojure.core$bound_fn_STAR_$fn__5767.doInvoke(core.clj:2007)
>      clojure.lang.RestFn.invoke(RestFn.java:397)
>      clojure.lang.AFn.run(AFn.java:22)
>      java.base/java.lang.VirtualThread.run(VirtualThread.java:314)
>
> The only way to workaround this right now is to change this code to use
> j.u.concurrent locks.
>
> -Alan
>
>
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