[cfe-dev] Accuracy of -ftime-report

[Daniel Dunbar]

On Thu, Aug 5, 2010 at 6:43 AM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> 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 ?

The individual timers are just as reliable as 'time', for tracking
user time, but you have to know what they mean (i.e., exactly what
they are counting).

What is happening here is that some of the timers overlap, I suspect.
The Clang frontend timer probably is ending up including the LLVM IR
generation timer.

 - Daniel

> -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
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>
>
>
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