Re: Java Stop-the-World GC stall induced by FS flush or many large file deletions

[Zheng Liu <gnehzuil.liu@gmail.com>]

Hello Cuong,

Could you please tell us the kernel version?  Meanwhile it would be
better if you could paste the result of the following commands:
 * sudo tune2fs -l ${DEV}
 * cat /proc/mounts | grep ${DEV}

I want to know which feature is enabled in your file system.  From your
description, I guess delayed allocation is enabled.  So I suggest that
you can try to disable it.  You can disable it using the following
command:
 * sudo mount -t ext4 -o remount,nodelalloc ${DEV} ${MNT}

Regards,
                                                - Zheng

On Wed, Sep 11, 2013 at 09:17:26PM -0700, Cuong Tran wrote:
> We have seen GC stalls that are NOT due to memory usage of applications.
> 
> GC log reports the CPU user and system time of GC threads, which are
> almost 0, and stop-the-world time, which can be multiple seconds. This
> indicates GC threads are waiting for IO but GC threads should be
> CPU-bound in user mode.
> 
> We could reproduce the problems using a simple Java program that just
> appends to a log file via log4j. If the test just runs by itself, it
> does not incur any GC stalls. However, if we run a script that enters
> a loop to create multiple large file via falloc() and then deletes
> them, then GC stall of 1+ seconds can happen fairly predictably.
> 
> We can also reproduce the problem by periodically switch the log and
> gzip the older log. IO device, a single disk drive, is overloaded by
> FS flush when this happens.
> 
> Our guess is GC has to acquiesce its threads and if one of the threads
> is stuck in the kernel (say in non-interruptible mode). Then GC has to
> wait until this thread unblocks. In the mean time, it already stops
> the world.
> 
> Another test that shows similar problem is doing deferred writes to
> append a file. Latency of deferred writes is very fast but once a
> while, it can last more than 1 second.
> 
> We would really appreciate if you could shed some light on possible
> causes? (Threads blocked because of journal check point, delayed
> allocation can't proceed?). We could alleviate the problem by
> configuring expire_centisecs and writeback_centisecs to flush more
> frequently, and thus even-out the workload to the disk drive. But we
> would like to know if there  is a methodology to model the rate of
> flush vs. rate of changes and IO throughput of the drive (SAS, 15K
> RPM).
> 
> Many thanks.
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