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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
