C++ cache aware programming

c++ optimization caching cpu-cache
Mat picture Mat · Dec 17, 2009 · Viewed 32.9k times · Source

is there a way in C++ to determine the CPU's cache size? i have an algorithm that processes a lot of data and i'd like to break this data down into chunks such that they fit into the cache. Is this possible? Can you give me any other hints on programming with cache-size in mind (especially in regard to multithreaded/multicore data processing)?

Thanks!

Answer

Robert S. Barnes picture Robert S. Barnes · Dec 24, 2009

According to "What every programmer should know about memory", by Ulrich Drepper you can do the following on Linux:

Once we have a formula for the memory requirement we can compare it with the cache size. As mentioned before, the cache might be shared with multiple other cores. Currently {There definitely will sometime soon be a better way!} the only way to get correct information without hardcoding knowledge is through the /sys filesystem. In Table 5.2 we have seen the what the kernel publishes about the hardware. A program has to find the directory:

/sys/devices/system/cpu/cpu*/cache

This is listed in Section 6: What Programmers Can Do.

He also describes a short test right under Figure 6.5 which can be used to determine L1D cache size if you can't get it from the OS.

There is one more thing I ran across in his paper: sysconf(_SC_LEVEL2_CACHE_SIZE) is a system call on Linux which is supposed to return the L2 cache size although it doesn't seem to be well documented.

Related questions

How do you interpret cachegrind output for caching misses?

Out of curiosity I ran coded up several different versions of matrix Multiplication and ran cachegrind against it. In my results below, I was wondering which parts were L1,L2,L3 misses and references and what it all really means? …

Read More
In C++, should I bother to cache variables, or let the compiler do the optimization? (Aliasing)

Consider the following code (p is of type unsigned char* and bitmap->width is of some integer type, exactly which is unknown and depends on which version of some external library we're using): for (unsigned x = 0; x < static_…

Read More
Why is processing a sorted array faster than processing an unsorted array?

Here is a piece of C++ code that shows some very peculiar behavior. For some strange reason, sorting the data miraculously makes the code almost six times faster: #include <algorithm> #include <ctime> #include <iostream> …

Read More