linux-ext4 - Re: Java Stop-the-World GC stall induced by FS flush or many large file deletions

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Date:	Thu, 12 Sep 2013 15:46:18 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Cuong Tran <cuonghuutran@...il.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Java Stop-the-World GC stall induced by FS flush or many large
 file deletions

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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