Process accounting/timing has broken recently
John Baldwin
jhb at freebsd.org
Mon Dec 6 16:18:51 UTC 2010
On Sunday, December 05, 2010 6:18:29 pm Steve Kargl wrote:
> Sometime in the last 7-10 days, some one made a
> change that has broken process accounting/timing.
>
> laptop:kargl[42] foreach i ( 0 1 2 3 4 5 6 7 8 9 )
> foreach? time ./testf
> foreach? end
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 69.55 real 38.39 user 30.94 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 68.82 real 40.95 user 27.60 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 69.14 real 38.90 user 30.02 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 68.79 real 40.59 user 27.99 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 68.93 real 39.76 user 28.96 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 68.71 real 41.21 user 27.29 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 69.05 real 39.68 user 29.15 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 68.99 real 39.98 user 28.80 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 69.02 real 39.64 user 29.16 sys
> Max ULP: 0.501607 for x in [-18.000000:88.709999] with dx = 1.067100e-04
> 69.38 real 37.49 user 31.67 sys
>
> testf is a numerically intensive program that tests the
> accuracy of expf() in a tight loop. User time varies
> by ~3 seconds on my lightly loaded 2 GHz core2 duo processor.
> I'm fairly certain that the code does not suddenly grow/loose
> 6 GFLOP of operations.
The user/sys thing is a hack (and has been). We sample the PC at stathz (~128
hz) to figure out a user vs sys split and use that to divide up the total
runtime (which actually is fairly accurate). All you need is for the clock
ticks to fire just a bit differently between runs to get a swing in user vs
system time.
What I would like is to keep separate raw bintime's for user vs system time in
the raw data instead, but that would involve checking the CPU ticker more
often (e.g. twice for each syscall, interrupt, and trap in addition to the
current once per context switch). So far folks seem to be more worried about
the extra overhead rather than the loss of accuracy.
--
John Baldwin
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