10-14 October 2016
San Francisco Marriott Marquis
America/Los_Angeles timezone

Optimizing ROOT's Performance Using C++ Modules

10 Oct 2016, 14:30
Sierra A (San Francisco Mariott Marquis)

Sierra A

San Francisco Mariott Marquis

Oral Track 5: Software Development Track 5: Software Development


Philippe Canal (Fermi National Accelerator Lab. (US)) Vasil Georgiev Vasilev (Fermi National Accelerator Lab. (US))


ROOT version 6 comes with a C++ compliant interpreter cling. Cling needs to know everything about the code in libraries to be able to interact with them.
This translates into increased memory usage with respect to previous versions of

During the runtime automatic library loading process, ROOT6 re-parses a
set of header files, which describe the library; and enters "recursive" parsing.
The former has a noticeable effect on CPU and memory performance, whereas the
latter is fragile and can introduce correctness issues. An elegant solution to
the shortcoming is to feed the necessary information only when required and in a
non-recursive way.

The LLVM community has started working on a powerful tool for reducing build
times and peak memory usage of the clang compiler called "C++ Modules".
The feature matured and it is on its way to the C++ standard. C++ Modules are
a flexible concept, which can be employed to match CMS and other experiments'
requirement for ROOT: to optimize both runtime memory usage and performance.

The implementation of the missing concepts in cling and its underlying LLVM
libraries and adopting the changes in ROOT is a complex endeavor. I describe the
scope of the work and I present a few techniques used to lower ROOT's runtime
memory footprint. I discuss the status of the C++ Modules in the context of ROOT
and show some preliminary performance results.

Primary Keyword (Mandatory) Analysis tools and techniques

Primary author

Vasil Georgiev Vasilev (Fermi National Accelerator Lab. (US))


Philippe Canal (Fermi National Accelerator Lab. (US))

Presentation Materials

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