[cfe-dev] Accuracy of -ftime-report

[Argyrios Kyrtzidis]

On 5 Aug 2010, at 14:37, Amaury Pouly wrote:

> 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.

Yes but the difference between the 'time' utility and the internal timers is not constant, with '-fsyntax-only' the internal timers report lower numbers but with '-emit-llvm-bc' they report higher ones.

In general is 'time' utility the more reliable one ?

-Argiris

> 
> 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
> 
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> 
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20100805/9f8f6437/attachment.html>