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Re: gprof


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


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