Messages in this thread | | | Date | Thu, 2 Sep 2004 16:56:27 +0200 | From | Paolo Molaro <> | Subject | incorrect time accouting |
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While benchmarking, a user pointed out that time(1) reported incorrect user and system times when running mono. A typical example (running on 2.6.8.1 is):
$ /usr/bin/time mono so-sieve.exe 5000 Count: 1028 0.02user 0.00system 0:01.97elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (1major+1509minor)pagefaults 0swaps
Where so-sieve.exe is a cpu-bound benchmark. On 2.6.8.1 very low user and system times are reported every time, while on both 2.4.19 and 2.2.20 sometimes the correct (or at least sensible) results are reported, while sometimes very low timings are reported as well. top reports high cpu usage and low idle percentages, but with no cpu time accounted to the mono process. This looks like a mild security issue, since it appears there is some way to circumvent the kernel's idea of the cpu resource usage of a process, so the limits set become useless and users could bog down the system.
http://primates.ximian.com/~lupus/time-strace has the result of strace -tt -f time /usr/local/bin/mono so-sieve.exe 5000 2> time-strace The highlights include:
[pid 9630] 19:22:28.609566 execve("/usr/local/bin/mono", ["/usr/local/bin/mono", "so-sieve.exe", "5000"], [/* 33 vars */]) = 0 Pid 9630 is the main process: it creates a thread that will execute the bulk of the cpu-intensive code (pid 9633):
[pid 9630] 19:22:28.839532 clone(Process 9633 attached child_stack=0x40d17b48, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID|CLONE_DETACHED, parent_tidptr=0x40d17bf8, {entry_number:6, base_addr:0x40d17bb0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}, child_tidptr=0x40d17bf8) = 9633 The main loop starts here, after some memory allocation:
[pid 9633] 19:22:28.849138 brk(0x82ae000) = 0x82ae000 And it ends about two seconds later, with the next entry in the trace for the 9633 pid:
[pid 9633] 19:22:30.780451 brk(0) = 0x82ae000 At the end, wait4 is called to collect the times, less than 0.5 sec user+system: [pid 9629] 19:22:30.821596 <... wait4 resumed> [WIFSIGNALED(s) && WTERMSIG(s) == SIGKILL], 0, {ru_utime={0, 14997}, ru_stime={0, 1999}, ...}) = 9630
http://primates.ximian.com/~lupus/mono-1.1.1.tar.gz is the mono source, if you don't have mono installed to reproduce (I tried to reproduce with a simple pthread program what mono is doing, executing cpu-intensive code in a subthread, but times are reported correctly there). http://primates.ximian.com/~lupus/so-sieve.exe is the sample program, but other programs exibited the same behaviour. Let me know if more info is needed to track down the problem.
lupus
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