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 _______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
