[cfe-dev] Accuracy of -ftime-report

[Amaury Pouly]

That can be due to lots of things. Just consider this difference:
- Wall time: preception of time form the user. That is, if you run your
program at time T and it ends at time T+D, then the wall time is D normally
- Processor time: time allocated on the processor, which can even be greater
than the wall time if the program uses several processors (if you have two
processors, you can in theory use 2*D processor time for a wall time of D).

I don't know how the times are computed but it could be related to these
points. And perhaps the time needed to do all the initialisation and loading
of the executable is counted somewhere.

Amaury Pouly


2010/8/5 Argyrios Kyrtzidis <kyrtzidis at apple.com>

> Hi,
>
> When I try '-ftime-report' the numbers differ from 'time', either higher or
> lower, e.g:
>
> > time clang -cc1 lib/VMCore/Dominators.cpp -Iinclude -Ilib/VMCore -D_DEBUG
> -D_GNU_SOURCE -D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -ftime-report
> -emit-llvm-bc -o t.o
>
> [LLVM pass timers]
>
>
> ===-------------------------------------------------------------------------===
>                          Miscellaneous Ungrouped Timers
>
> ===-------------------------------------------------------------------------===
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
>  --- Name ---
>    0.8070 ( 87.2%)   0.0550 ( 73.7%)   0.8620 ( 86.2%)   0.8620 ( 86.2%)
>  Clang front-end timer
>    0.0747 (  8.1%)   0.0090 ( 12.0%)   0.0837 (  8.4%)   0.0840 (  8.4%)
>  LLVM IR Generation Time
>    0.0435 (  4.7%)   0.0106 ( 14.3%)   0.0541 (  5.4%)   0.0541 (  5.4%)
>  Code Generation Time
>    0.9251 (100.0%)   0.0746 (100.0%)   0.9998 (100.0%)   1.0001 (100.0%)
>  Total
>
>
> real 0m0.903s
> user 0m0.827s
> sys 0m0.074s
>
>
>
> > time clang -cc1 lib/VMCore/Dominators.cpp -Iinclude -Ilib/VMCore -D_DEBUG
> -D_GNU_SOURCE -D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -ftime-report
> -fsyntax-only
>
>
> ===-------------------------------------------------------------------------===
>                          Miscellaneous Ungrouped Timers
>
> ===-------------------------------------------------------------------------===
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
>  --- Name ---
>    0.6823 (100.0%)   0.0317 (100.0%)   0.7139 (100.0%)   0.7139 (100.0%)
>  Clang front-end timer
>    0.6823 (100.0%)   0.0317 (100.0%)   0.7139 (100.0%)   0.7139 (100.0%)
>  Total
>
>
> real 0m0.736s
> user 0m0.690s
> sys 0m0.043s
>
>
>
> Why the difference, am I doing something wrong ?
>
> -Argiris
>
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