Messages in this thread | | | Date | Thu, 17 Oct 2002 17:16:46 +1000 | From | Con Kolivas <> | Subject | Re: Pathological case identified from contest |
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Quoting Con Kolivas <conman@kolivas.net>:
> Quoting Andrew Morton <akpm@digeo.com>: > > > Con Kolivas wrote: > > > > > > I found a pathological case in 2.5 while running contest with > process_load > > > recently after checking the results which showed a bad result for > > 2.5.43-mm1: > > > > > > 2.5.43-mm1 101.38 72% 42 31% > > > 2.5.43-mm1 102.90 75% 34 28% > > > 2.5.43-mm1 504.12 14% 603 85% > > > 2.5.43-mm1 96.73 77% 34 26% > > > > > > This was very strange so I looked into it further > > > > > > The default for process_load is this command: > > > > > > process_load --processes $nproc --recordsize 8192 --injections 2 > > > > > > where $nproc=4*num_cpus > > > > > > When I changed recordsize to 16384, many of the 2.5 kernels started > > exhibiting > > > the same behaviour. While the machine was apparently still alive and > would > > > respond to my request to abort, the kernel compile would all but stop > > while > > > process_load just continued without allowing anything to happen from > > kernel > > > compilation for up to 5 minutes at a time. This doesnt happen with any > 2.4 > > kernels. > > > > > > > Well it doesn't happen on my test machine (UP or SMP). I tried > > various recordsizes. It's probably related to HZ, memory bandwidth > > and the precise timing at which things happen. > > > > The test describes itself thusly: > > > > * This test generates a load which simulates a process-loaded system. > > * > > * The test creates a ring of processes, each connected to its > predecessor > > * and successor by a pipe. After the ring is created, the parent > process > > * injects some dummy data records into the ring and then joins. The > > * processes pass the data records around the ring until they are killed. > > * > > > > It'll be starvation in the CPU scheduler I expect. For some reason > > the ring of piping processes is just never giving a timeslice to > > anything else. Or maybe something to do with the exceptional > > wakeup strategy which pipes use. > > > > Don't now, sorry. One for the kernel/*.c guys. > > Ok well I've done some profiling as suggested by wli and it shows pretty > much > what I find in the results - it gets stuck while doing process_load and > never > moves on. > > recordsize 8192 kern profile: > c01223ac 76997 4.48583 do_anonymous_page > c0188694 135835 7.91373 __generic_copy_from_user > c0188610 345071 20.1038 __generic_copy_to_user > c0105298 801429 46.6911 poll_idle > sysprofile: > 00000000 160258 5.03854 (no symbol) > /lib/i686/libc-2.2.5.so > c0188610 345071 10.8491 __generic_copy_to_user > /home/con/kernel/linux-2.5.43/vmlinux > c0105298 801429 25.1971 poll_idle > /home/con/kernel/linux-2.5.43/vmlinux > 00000000 1132668 35.6113 (no symbol) > /usr/lib/gcc-lib/i686-pc-linux-gnu/2.95.3/cc1 > > Normal run consistent with doing kernel compilation most of the time. > > recordsize 16384 kernprofile: > c0111ef4 403545 4.3407 do_schedule > c0105298 558704 6.00965 poll_idle > c0188694 2571995 27.6655 __generic_copy_from_user > c0188610 4489796 48.2941 __generic_copy_to_user > sysprofile: > c0111ef4 403545 4.24896 do_schedule > /home/con/kernel/linux-2.5.43/vmlinux > c0105298 558704 5.88264 poll_idle > /home/con/kernel/linux-2.5.43/vmlinux > c0188694 2571995 27.0807 __generic_copy_from_user > /home/con/kernel/linux-2.5.43/vmlinux > c0188610 4489796 47.2734 __generic_copy_to_user > /home/con/kernel/linux-2.5.43/vmlinux > > I had to abort the run with recordsize 16384 but you can see it's just stuck > in > process_load copying data between forked processes. > > Can someone else on lkml decipher why it gets stuck here?
Well this has become more common with 2.5.43-mm2. I had to abort the process_load run 3 times when benchmarking it. Going back to other kernels and trying them it didnt happen so I dont think its my hardware failing or something like that.
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