GlobalDefaultNumberOfThreads should not be greater than GlobalMaximumNumberOfThreads

[N-Dekker]

Description

Users typically call MultiThreaderBase::SetGlobalMaximumNumberOfThreads when they want to reduce (limit) the number of threads used by a process. However, when a user sets the GlobalMaximumNumberOfThreads while leaving the GlobalDefaultNumberOfThreads unspecified, this "default number" may end up becoming larger than the "maximum number"!

Steps to Reproduce

#include <itkMultiThreaderBase.h>
#include <iostream>

int main()
{
  itk::MultiThreaderBase::SetGlobalMaximumNumberOfThreads(3);

  std::cout
    << "Maximum: " << itk::MultiThreaderBase::GetGlobalMaximumNumberOfThreads() << '\n'
    << "Default: " << itk::MultiThreaderBase::GetGlobalDefaultNumberOfThreads() << '\n';
}

Expected behavior

With this example, both Maximum and Default should end up being 3 (at least, if the machine supports having 3 simultaneous threads).

Actual behavior

Output on my (LKEB/LUMC) multi-core machine:

Maximum: 3
Default: 12

Versions

ITK 5.4.0

Environment

Windows

Additional Information

GlobalDefaultNumberOfThreads is being used when constructing the ThreadPool:

ITK/Modules/Core/Common/src/itkThreadPool.cxx

Line 114 in 01e133c

ThreadIdType threadCount = MultiThreaderBase::GetGlobalDefaultNumberOfThreads();

So the number of threads held by the thread pool appears to also occasionally exceed GlobalMaximumNumberOfThreads!

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• Triggered by research by Sebastian (@Svdvoort) (related to ITKElastix issue MultiMetricMultiResolutionRegistration fails when SetNumberOfThreads is used ITKElastix#160) and by Gonzalo Mosquera Rojas and Juancito van Leeuwen.