gc logging basic question

[Tao Feng]

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 .
>
> 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
>
>
>
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