Messages in this thread |  | | Date | Thu, 17 Oct 2002 14:26:35 +1000 | | From | Con Kolivas <> | | Subject | Re: Pathological case identified from contest |
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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?
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