Hi, Large file deletions are likely to lock cpu for seconds if you're running non-preemptible kernel < 3.10. Make sure you have this change: http://patchwork.ozlabs.org/patch/232172/ (available in 3.10 if I remember it right). Turning on preemption may be a good idea as well. Regards, Andrei. On 12.09.2013 00:18, Cuong Tran wrote: > 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-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
