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

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Date:	Wed, 11 Sep 2013 22:45:10 -0700
From:	Cuong Tran <cuonghuutran@...il.com>
To:	"Sidorov, Andrei" <Andrei.Sidorov@...isi.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

Awesome fix and thanks for very speedy response. I have some
questions. We delete files one at a time, and thus that would lock up
one core or all cores?

And in our test, we use falloc w/o writing to file. That would still
cause freeing block-by-block, correct?
--Cuong

On Wed, Sep 11, 2013 at 10:32 PM, Sidorov, Andrei
<Andrei.Sidorov@...isi.com> wrote:
> 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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>>
>
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