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 05:32:45 +0000
From:	"Sidorov, Andrei" <Andrei.Sidorov@...isi.com>
To:	Cuong Tran <cuonghuutran@...il.com>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...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

Hi,

Large file deletions are likely to lock cpu for seconds if you're
running non-preemptible kernel < 3.10.
Make sure you have this change:
http://patchwork.ozlabs.org/patch/232172/ (available in 3.10 if I
remember it right).
Turning on preemption may be a good idea as well.

Regards,
Andrei.

On 12.09.2013 00:18, 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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