gc logging basic question

[Thomas Schatzl]

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