gc logging basic question

[Srinivas Ramakrishna]

We've run into similar issues at Twitter, and are readying a patch (re #
(4) of Thomas' mail) for buffering in-memory
and writing to disk/file out-of-line wrt safepoints or loggers. The patch
is under test and review internally at this time,
and we hope to publish it for review on Open JDK once internal test and
review are complete.

Thomas, does unified logging include GC and will it be done outside of
safepoints? In that case, we can use the same
code rather than spinning code specifically for gc logging (as I am doing
in our patch for this). I must confess I haven't
looked at the unified logging jep or its state in the JDK 9 repo.

thanks.
-- ramki

On Wed, Feb 10, 2016 at 9:12 AM, Gustav Åkesson <gustav.r.akesson at gmail.com>
wrote:

> Hi,
>
> Well, AFAIK the loggning IS part of STW cycle. I had troubles with this
> before, which results in GCs with high systime. Since then I always print
> the logs to RAM disk, which solves the problem.
>
> Best Regards,
> Gustav Åkesson
> Den 9 feb 2016 18:46 skrev "Tao Feng" <fengtao04 at gmail.com>:
>
>> Hi Jenny, Thomas,
>>
>> Thanks a lot for your reply. Indeed we observe high real time in gc which
>> we correlate with high write syscall time. This makes us doubt whether GC
>> logging is part of cycle.
>>
>> Thanks,
>> -Tao
>>
>> On Tue, Feb 9, 2016 at 2:11 AM, Thomas Schatzl <thomas.schatzl at oracle.com
>> > wrote:
>>
>>> Hi,
>>>
>>> On Mon, 2016-02-08 at 22:35 -0800, Yu Zhang wrote:
>>> > Tao,
>>> >
>>> > GC logging is not part of GC STW pause. Though we may see application
>>> > being stopped due to gc logging, if there are a lot of I/O activities
>>> > on the system.
>>> > The gc log file is opened by the following
>>> > _fd = open(file_name, O_WRONLY | O_CREAT | O_TRUNC, 0666);
>>> >
>>> >  Thanks,
>>> > Jenny
>>> > On 2/4/2016 11:02 PM, Tao Feng wrote:
>>> > > Hi,
>>> > >
>>> > > I have a basic question out of curiosity: is GC logging part of GC
>>> > > cycle? If yes, is it a blocking IO or non-blocking IO? And it would
>>> > > be good if someone could help point out the code base about the GC
>>> > > logging part as I have difficult to identify the code location.
>>>
>>>   I think any logging enabled by -Xlog opens the file in asynchronous
>>> write mode, output is buffered. There is no deliberate fsync'ing going
>>> on (again, afaik).
>>>
>>> However, lower levels (OS, file system implementations) might impose
>>> additional synchronization. If a different process saturates I/O to the
>>> disk you want to write to at the same time, the behavior might not be
>>> optimal. :)
>>>
>>> See e.g. http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2014-Apr
>>> il/001835.html
>>> <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2014-April/001835.html>
>>> .
>>>
>>> If you noticed a problem because of that, there are the following
>>> options:
>>>
>>> 1) use a separate disk for the logging (throwing the "tmpfs" keyword
>>> into this bucket)
>>>
>>> 2) use JFR. Afaik its events are written asynchronously using separate
>>> threads. That might or might not be a problem if you saturate I/O
>>> bandwidth of your target disk for a long time (go to option 1 in this
>>> case)
>>>
>>> 3) experiment with different FS implementations that do less blocking.
>>>
>>> 4) provide a patch to improve this situation :-)
>>>
>>> Thanks,
>>>   Thomas
>>>
>>>
>>>
>>
>> _______________________________________________
>> hotspot-gc-use mailing list
>> hotspot-gc-use at openjdk.java.net
>> http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>>
>>
> _______________________________________________
> hotspot-gc-use mailing list
> hotspot-gc-use at openjdk.java.net
> http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20160210/bfd92fed/attachment.html>