Bug 930

Created attachment 515 [details]
Add MeasureCacheSizes unsupported module.

Eigen is able to use CPUID to query the cache structure on x86. That's great and the next question is how to do the same on ARM; unfortunately, the equivalent thing on ARM is generally not accessible from userspace. See:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0344f/Chdebced.html

On ARM architectures, it is up to the operating system to expose this information to userspace; unfortunately, as of 2015, Android kernels don't.

So for now, if we want to run optimally fast on Android, we probably have to resort to empirically measuring cache sizes by running some kind of benchmark.

The proposed patch here adds an unsupported module doing that.

It offers only one function, measure_cache_sizes, returning all the cache sizes in an array of integers. It's not too bad; in practice, it runs in no more than 1 second and successfully detects the cache structure of various x86 and ARM CPUs that I've tested it on.

This module intentionally doesn't try to offer functions to teach Eigen itself about this cache structure. Instead, we intentionally leave that up to the application. Some applications may only want to directly pass that to Eigen, while other applications may want to cache the resulting values on disk... it doesn't feel like Eigen should try to help with that.

Here is a sample program using this function to dump the cache structure:

#include <unsupported/Eigen/CXX11/MeasureCacheSizes>
#include <iostream>

int main()
{
  std::vector<size_t> cache_sizes;
  Eigen::measure_cache_sizes(cache_sizes);
  for (size_t i = 0; i < cache_sizes.size(); i++) {
    std::cout << "L" << (i+1) << " cache size: "
              << cache_sizes[i] << std::endl;
  }
}

Output on a Core i7:

L1 cache size: 32768
L2 cache size: 262144
L3 cache size: 8388608

real	0m0.989s
user	0m0.987s
sys	0m0.000s


On a Nexus 4 phone:

L1 cache size: 16384
L2 cache size: 1048576

    0m0.83s real     0m0.70s user     0m0.09s system

So in both cases it finds the right values in about 1 second.