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
