Re: Process accounting/timing has broken recently

John Baldwin wrote:
> 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.
>
>   
Adding any instruction into global syscall path should be cautioned, it
is worse then before, thinking about a threaded application, a userland
thread may have locked a mutex and calls a system call, the overhead
added to system call path can directly affect a threaded application's
performance now, because the time window the mutex is held
is longer than before, I have seen some people likes to fiddle with
system call path, it should be cautioned.

Regards,
David Xu