My desk top has 8 cores, including hyperthreading. Thus deleting files would lock up one core but that should not affect GC threads if core lock-up is an issue? Would # journal records be proportional to # blocks deleted. And thus deleting N blocks, one block at a time would create N times more journal records than deleting all N blocks in "one shot"? --Cuong On Wed, Sep 11, 2013 at 11:02 PM, Sidorov, Andrei <Andrei.Sidorov@xxxxxxxxxx> wrote: > It would lock-up one core whichever jdb/sdaX runs on. This will usually > happen upon commit that runs every x seconds, 5 by default (see “commit” > mount option for ext4). I.e. deleting 5 files one by one with 1 second > interval in between is basically the same as deleting all of them “at once”. > > Yes, fallocated files are the same wrt releasing blocks. > > Regards, > Andrei. > > On 12.09.2013 01:45, Cuong Tran wrote: >> Awesome fix and thanks for very speedy response. I have some >> questions. We delete files one at a time, and thus that would lock up >> one core or all cores? >> >> And in our test, we use falloc w/o writing to file. That would still >> cause freeing block-by-block, correct? >> --Cuong >> >> On Wed, Sep 11, 2013 at 10:32 PM, Sidorov, Andrei >> <Andrei.Sidorov@xxxxxxxxxx> wrote: >>> 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
