# Garbage Collection Pauses & Non-interruptable System Calls

Paul Hohensee Paul.Hohensee at Sun.COM
Tue Apr 14 15:06:22 PDT 2009

The url for "Hotspot Runtime Overview of JNI" didn't come through for me.

In any case, as Ramki noted, Hotspot lets native code called from Java
run free
during GC pauses, unless said native code calls back into the jvm for
some reason,
including just returning back to Java from native.  Hotspot does _not_
try to
pause threads executing native code, nor does Hotspot use signal mechanisms
to block threads executing Java code so GC can happen.  Hotspot uses a
polling
mechanism instead because signal delivery mechanisms on Unix and Linux are
unreliable.  I doubt you'll be able to reproduce the JRockit issue with
Hotspot.
You might have other problems, but not that one.

I suggest forwarding your questions to the JRockit team at Oracle.
Though I suspect
some of them are on this list too. :)

Please try Hotspot before you give up on Java.  From your description,
you should
performance info
accessible from

http://java.sun.com/performance

and Jon Masamitsu's blog

http://blogs.sun.com/jonthecollector/

Paul

Mark R Maxey wrote:
>
> After reading the last paragraph of HotSpot Runtime Overview of JNI
> more closely, I understand more.  I think we're almost on the same
> page.  The problem seems to be that all threads are suspended until
> the Java thread returns from the native call.
>
>
> Mark Maxey
> Raytheon, Garland
> 580/2/P22-1
> (972)205-5760
> Mark_R_Maxey at Raytheon.com
>
>
> *Mark R Maxey/US/Raytheon*
>
> 04/14/2009 12:37 PM
>
>
> To
> 	"Y. Srinivas Ramakrishna" <Y.S.Ramakrishna at Sun.COM>
> cc
> 	hotspot-gc-dev at openjdk.java.net, Y.S.Ramakrishna at Sun.COM, Andrew M
> Dungan/US/Raytheon at MAIL, David A Lilly/RCS/Raytheon/US at MAIL, Mark R
> Maxey/US/Raytheon at MAIL
> Subject
> 	Re: Garbage Collection Pauses & Non-interruptable System CallsLink
> <Notes://MK2-MSG05/86256EF3005F851B/38D46BF5E8F08834852564B500129B2C/E1F59319D41C5C4886257598005422BD>
>
>
>
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>
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>
>
> encouraging.
>
> Let me confess something I should have done up-front.  The behavior
> we're seeing is using JDK 5 via JRockit R27.6.  We're in the process
> of reproducing these problems under HotSpot JDK 6 Update 12, though
> it'll be a few days before we can do so.  The reason I'm pinging this
> forum is to research in advance what differences we might expect
> between the two JVMs.
>
> Let me describe exactly what we're seeing as provided by doing an
> strace on the process:
>
>    1. A Java thread calls a native C code that ultimately calls a
>       pwrite().  We suspect that the device driver ultimately makes a
>       non-interruptable system call to transfer the data directly from
>       our mem-aligned 128 MB buffer to disk.
>    3. The GC thread waits on mutex #1 (presumably waiting on all the
>       threads to signal it that it can begin GC)
>    4. The Java thread wakes mutex #1 (presumably signaling the GC it
>    5. The Java thread waits on mutex #2 (presumably waiting on GC to
>       finish)
>    6. The GC thread wakes mutex #2 (presumably telling the Java thread
>       it can resume processing)
>
>
> We're seeing times between #3 & #4 that are proportional to the amount
> of time spent in the pwrite().  We also see some overhead between #5
> &#6 that is proportional to the number of Java threads we have
> (currently between 30 & 40 that we've created not counting the JVMs).
>
> Unfortunately, the JRockit logging only reveals the actual time GC
> takes (#4 - #5).  Hopefully, HotSpot's logging includes the total time
> (#2 - #6).
>
> I'm pursuing these questions with Oracle/BEA.  Again, I'm just trying
> get a feel for HotSpot's behavior in comparison.  While we're using
> JRockit today, HotSpot will be our ultimate platform.
>
>
> One alternate solution that has been suggested is infrequently calling
> GC explicitly within our code during special times when we know we can
> afford to take the hit.  We would even accept a greater hit than
> normal if we could avoid being impacted during critical times.
> Everything I've ever read says to not do this, but I'm curious why in
> this case this is a bad idea.  Note that we're using the concurrent
> GC, so I'm not even sure if System.gc() supports this.
>
>
> Thanks again!
>
>
> Mark Maxey
> Raytheon, Garland
> 580/2/P22-1
> (972)205-5760
> Mark_R_Maxey at Raytheon.com
>
>
> *"Y. Srinivas Ramakrishna" <Y.S.Ramakrishna at Sun.COM>*
> Sent by: Y.S.Ramakrishna at Sun.COM
>
> 04/14/2009 10:19 AM
>
>
> To
> 	Mark R Maxey <Mark_R_Maxey at raytheon.com>
> cc
> 	hotspot-gc-dev at openjdk.java.net
> Subject
> 	Re: Garbage Collection Pauses & Non-interruptable System Calls
>
>
>
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>
> Hello Mark --
>
> I am assuming your threads doing DMA are actually executing native
> code (or
> waiting for signals in native code).  Threads in native code do not
> need to
> synchronize \in any manner with GC while they are executing native code.
> It is only the transitions to and from native mode (from Java code) that
> require
> synchronization. Roughly speaking, the JVM fences off those native
> threads so that, in the event that they need to re-enter the JVM or
> access the Java heap, they will be suspended until a GC/safepoint that
> is in progress is completed.
>
> Thus, I do not believe you need to fear that a long-running DMA call would
> cause GC's to be delayed (which I understand is your  main concern below).
>
> Have you actually seen cases where this is happening? If so, what
> version of the JDK
> are you running?
>
> thanks.
> -- ramki
>
> Mark R Maxey wrote:
> > Hello,
> >
> > I have a problem I was hoping with which I need some advice.
> >
> > We wrote a custom JNI library for file I/O that sits underneath the
> Java
> > NIO FileChannel.  One of our driving requirements is highly performant
> > file I/O.  We achieved this by doing DMA I/O from large direct memory
> > aligned buffers.  The JNI is very trivial - it just takes a buffer and
> > performs the appropriate system call based on the parameters given
> to it.
> > 100% of the logic for calculating offsets, buffer management, etc.
> is all
> > in our implementation of java.nio.FileChannel.
> >
> > Here's our problem:  We have requirements to respond to some
> messages in
> > as little as 250 ms.  During this time, we're doing file writes of
> 128 MB
> > that take around 200 ms.  When GC kicks in, it tries to pause all
> >  Because the DMA write is non-interruptable, GC waits for the I/O to
> > complete before being able to pause the thread & run.  That means
> that GC
> > can take well over 200 ms putting us in grave danger of missing our
> > timelines.  Worse, there is always the chance the write will hang
> due to a
> > bad filesystem.   We've seen this cause the JVM to hang indefinitely
> > forcing us to cycle the process.
> >
> > Unless we find a solution that allows GC to continue while doing
> this I/O,
> > we will convert all the code to C++.  While that might solve our
> timeline
> > for that particular process, we have many less performance critical
> > processes that use our JNI FileChannel libraries that would hang if a
> >
> > We've tweaked the file system device timeouts down to a minimum, but
> they
> > are still very high (on the order of several seconds to minutes).  It
> > would be nice if the JVM had a similar timeout for pausing threads,
> i.e.,
> > where the pause times out after X number of milliseconds.  We'd be
> willing
> > to sacrifice a larger heap size and postpone GC in the hopes that
> the next
> > time it ran GC, we wouldn't be in the middle of a non-interruptable
> system
> > call.
> >
> > The only solution being batted around here is pushing the system
> calls out
> > of Java threads and into native threads.  The JNI call would push
> the info
> > for the I/O call onto a native C++ queue where a small number of native
> > threads (3?) would pull the data off the queue and perform the actual
> > system call.   The trick is finding an implementation where the Java
> > thread blocked waiting on a response from the native thread is
> > interruptible.  All this assumes GC doesn't try to pause native
> > interaction with the JVM.  So, we're leaning towards using pipes to
> push
> > data from one thread to another.
> >
> > If you have any suggestions or advice, we are desperate for your wisdom.
> >
> > Thanks!
> >
> >
> > Mark Maxey
> > Raytheon, Garland
> > 580/2/P22-1
> > (972)205-5760
> > Mark_R_Maxey at Raytheon.com
> >
> >
>
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