Minimizing ITT API Overhead
The ITT API overhead and its impact on the overall application
performance depends on the amount of instrumentation code added to the
application. When instrumenting an application with ITT API, you should balance
between application performance and the amount of performance data that you
need to collect, in order to minimize API overhead while collecting sufficient
performance data.
Follow these guidelines to achieve good balance between overall
performance of the instrumented application and instrumentation detail:
- Instrument only those paths within your application that are important for analysis.
- Create ITT domains and string handles outside the critical paths.
- Filter data collection by different aspects of your application that can be analyzed separately. The overhead for a disabled API call (thus filtering out the associated call) is always less than 10 clock ticks, regardless of the API.
Conditional Compilation
For best performance in the release version of your code, use
conditional compilation to turn off annotations. Define the macro
INTEL_NO_ITTNOTIFY_API
before you include
ittnotify.h
during compilation to eliminate
all __itt_*
functions from your code.
You can also remove the static library from the linking stage by
defining this macro.
Usage Example: Using Domains and String Handles
The ITT APIs include a subset of functions which create domains and
string handles. These functions always return identical handles for the same
domain names and strings. This requires these functions to perform string
comparisons and table lookups, which can incur serious performance penalties.
In addition, the performance of these functions is proportional to the log of
the number of created domains or string handles. It is best to create domains
and string handles at global scope, or during application startup.
The following code section creates two domains in the global scope.
You can use these domains to control the level of detail that is written to the
trace file.
__itt_domain* basic = __itt_domain_create(L"MyFunction.Basic"); __itt_domain* detailed = __itt_domain_create(L"MyFunction.Detailed"); // Create string handles at global scope. __itt_string_handle* h_my_funcion = __itt_string_handle_create(L"MyFunction"); void MyFunction(int arg) { __itt_task_begin(basic, __itt_null, __itt_null, h_my_function); Foo(arg); FooEx(); __itt_task_end(basic); } __itt_string_handle* h_foo = __itt_string_handle_create(L"Foo"); void Foo(int arg) { // Skip tracing detailed data if the detailed domain is disabled. __itt_task_begin(detailed, __itt_null, __itt_null, h_foo); // Do some work here... __itt_task_end(detailed); } __itt_string_handle* h_foo_ex = __itt_string_handle_create(L"FooEx"); void FooEx() { // Skip tracing detailed data if the detailed domain is disabled. __itt_task_begin(detailed, __itt_null, __itt_null, h_foo_ex); // Do some work here... __itt_task_end(detailed); } // This is my entry point. int main(int argc, char** argv) { if(argc < 2) //Disable detailed domain if we do not need tracing from that in this //application run detailed ->flags = 0; MyFunction(atoi(argv[1])); }