Mariusz Witek - L1-HLT Workshop1 STL limitations Simple measurements

Mariusz Witek - L1-HLT Workshop2 STL overhead measurements Usually the difference between static and dynamic solution is negligible in off-line environment but.. In L1 environment the time budget is at the level of miliseconds (all timings in 1 GHz PIII) and STL have to be used with a bit of care. Typical data structure in L1 is an array of locations each containing 5-20 fields Try to measure the difference between STL and “static” solutions using simple example programs containing typical for L1 manipulation of data Measurement made many times, first 10 measurement skipped and outstanding timings rejected.

Mariusz Witek - L1-HLT Workshop3 What should be avoided? The main overhead is due to creation of objects like VELO clusters, 2D VELO tracks for each event (and deleting after end of each event processing). Also filling vector of object is not optimal wrt filling pointers (implies a copy of existing object). Typically dynamic/static = 0.3 ms /0.06 ms Creation of two dimensional STL tables is not optimal. Creation and filling dynamic/static = 0.2 ms / 0.02 ms Sorting of vector of objects means swapping objects in a vector. Sorting of pointers is obviously more optimal. Vector of 500 objects of 6 fields: std::sort objects/ std::sort pointers / customized sort = 0.4 ms/ 0.15 ms / 0.09 ms (sorting might be sensitive to cache memory configuration, may change with time)

Mariusz Witek - L1-HLT Workshop4 Summary Usually STL is satisfactory efficient with respect to static solutions in the L1 environment (creation and manipulation of 1 dim STL vectors) There are some exceptions when we go below the range of 1 ms for the typical L1 data structure (vector of 500 objects of 10 fields each). Static solution can significantly reduce the event initialization phase of an algorithm.