Garbage Collection Pauses & Non-interruptable System Calls

[Y. Srinivas Ramakrishna]

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 threads. 
>  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 
> filesystem goes bad.
>
> 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 threads. 
> We thought about using pthreads, but were concerned about its signal 
> 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
>
>