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 23:08:21 -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

My desk top has 8 cores, including hyperthreading. Thus deleting files
would lock up one core but that should not affect GC threads if core
lock-up is an issue? Would # journal records be proportional to #
blocks deleted. And thus deleting N blocks, one block at a time would
create N times more journal records than deleting all N blocks in "one
shot"?

--Cuong

On Wed, Sep 11, 2013 at 11:02 PM, Sidorov, Andrei
<Andrei.Sidorov@...isi.com> wrote:
> It would lock-up one core whichever jdb/sdaX runs on. This will usually
> happen upon commit that runs every x seconds, 5 by default (see “commit”
> mount option for ext4). I.e. deleting 5 files one by one with 1 second
> interval in between is basically the same as deleting all of them “at once”.
>
> Yes, fallocated files are the same wrt releasing blocks.
>
> Regards,
> Andrei.
>
> On 12.09.2013 01:45, Cuong Tran wrote:
>> 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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