Re: gprof

[Michael Trimarchi <trimarchi@gandalf.sssup.it>]

Ken Raeburn wrote:

> On Sep 20, 2005, at 06:31, Michael Trimarchi wrote:
>
>> >But also, if you care about the accuracy of the results, you  may  
>> need to modify the C runtime support code for >profiling, which   
>> typically updates the per-function data in a manner that is not   
>> thread-safe.
>> may you explain more precisely this point?
>> Regards
>> Michael
>
>
> With basic profiling, the runtime support code keeps track of how  
> often the CPU program counter is in a given range of values, with  
> fairly fine granularity.  Later this table is dumped out, and (g)prof  
> interprets it in combination with symbol information from the  
> executable.  For example, addresses XXX through YYY correspond to  
> function A, and so many ticks at a certain frequency were counted  
> with the PC in that range, so here's the amount of time the program  
> spent in that function.  But that may be inaccurate in multithreaded  
> programs if the counter is implemented as read counter value N,  
> increment value, another thread runs for a bit and changes the  
> counter, store counter value N+1; you've just lost the change made by  
> the other thread.
>
> For graph profiling, you also need data recorded on entry to a  
> function indicating where the function was called from, and how many  
> times it was called from each call site.  Since you could have  
> arbitrarily many such call sites, this is likely to involve dynamic  
> memory allocation, walking through some data structures, etc.  If  
> it's not done just right, it might even result in crashes in  
> multiprocessor, multithreaded situations, if you're really unlucky  
> with the timing.
>
> I've never gotten around to modifying the support code to try to make  
> it thread-safe.  Using mutex locks would be the obvious approach, but  
> probably kind of expensive compared to some of the atomic operations  
> a few processors have available, or "store if another cpu or thread  
> hasn't stored here, and set condition codes to tell me", etc.
>
> You might just get kind of lucky with the existing support code,  
> though, your program might not crash, and the numbers might even be  
> vaguely accurate...
>
> Ken
>
 For example, to compute the time spent inside a function foo()  
(starting at address X and ending at address Y) the system sample each
 tick looking if the program counter is between X and Y. That may be 
inaccurate in multithreaded programs if the accounting of a
 counter C is implemented as the following set of instructions:
read counter value C
C=C+1
store C
If another thread runs for a bit interrupting the thread between 2 and 3,
 and that thread changes the counter, then the value stored by the 
interrupted
 thread is N+1, losing an increment. Is it ok?

Regards Michael