[ltt-dev] UST Instrumenting function entries and exits

[Matthew Khouzam]

On 11-05-13 10:36 AM, Bernd Hufmann wrote:
> Hi Matthew
>
> This is an interesting feature. This could be used to create call trees 
> which could be visualized in different ways.
>
> I have two questions around this:
>
> 1) Performance
> What is the performance of such an instrumentation? I think it adds a 
> constant overhead to each function call and exit. This overhead will be 
> smaller if the compiler can use inline methods instead of plain function 
> calls. Moreover,  these instrumentation function calls (inline or not) 
> will be done every time, even if the ust_makers are disabled. Ideally, 
> what we want is to have only the ust_makers at entry and exit of the 
> functions. So, we would benefit from the performance of UST.
preliminary tests using very fast functions ( take an integer, increment
return) show about a 2x slowdown.
More results will follow shortly.

> 2) Exceptions
> In C++ the software designer could use exceptions to exit methods in for 
> example an error case. Does this instrumentation cover this, too?

As Michel said, it should, I will test it though, thanks for the input.

<unrelated> Another visualization option would be to visualize the call
stack vs time. Like a histogram. </unrelated>
> Thanks
> Bernd
>
> On 05/12/2011 01:32 PM, Matthew Khouzam wrote:
>> Hello world,
>> I just made a little program that I'm testing out and want some opinions
>> now that Mathieu D and Nils are not able to read their emails. ;)
>>
>> This is a shared object (or code injected straight into the source) that
>> will allow ust calls to be hooked onto the function entries and exits.
>>
>> Here is the basic code.
>> <code>
>> #include<execinfo.h>
>> #include<ust/marker.h>
>>
>> // will add a cache for the function names later.
>>
>> void __cyg_profile_func_enter (void *this_fn,
>>                                           void *call_site)
>> {
>>      char *funcname = backtrace_symbols(&this_fn, 1)[0];
>>      ust_marker(entry, "func_entry %s", funcname);
>>      // memory leak
>> }
>>
>> void __cyg_profile_func_exit  (void *this_fn,
>>                                           void *call_site)
>> {
>>      char *funcname = backtrace_symbols(&this_fn, 1)[0];
>>      ust_marker(exit, "func_exit %s", funcname);
>>      // memory leak
>> }
>> </code>
>> When you make the main program, you must include in the cflags
>> -finstrument-code.
>>
>> I would like some comments on this and to get the idea rolling to
>> include something similar but less ugly in ust's mainline.
>>
>> The first thing I will say : backtrace_symbols does a malloc, we are
>> therefore doing two mallocs per function calls and no frees.
>> I think we do need function names and not offsets though, the user
>> experience is key in this feature since it is not adding a new
>> functionality, just making people have an easier time.
>> I hope that something like this would be able to make us generate UST
>> "sequence diagram" like viewers.
>>
>> Thanks world,
>> Matthew
>>
>>
>> _______________________________________________
>> ltt-dev mailing list
>> ltt-dev at lists.casi.polymtl.ca
>> http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev