Aligning to cache line and knowing the cache line size

Question

To prevent false sharing, I want to align each element of an array to a cache line. So first I need to know the size of a cache line, so I assign each element that amount of bytes. Secondly I want the start of the array to be aligned to a cache line.

I am using Linux and 8-core x86 platform. First how do I find the cache line size. Secondly, how do I align to a cache line in C. I am using the gcc compiler.

So the structure would be following for example, assuming a cache line size of 64.

element[0] occupies bytes 0-63 element[1] occupies bytes 64-127 element[2] occupies bytes 128-191 

and so on, assuming of-course that 0-63 is aligned to a cache line.

Answer 1

I am using Linux and 8-core x86 platform. First how do I find the cache line size.

$ getconf LEVEL1_DCACHE_LINESIZE 64 

Pass the value as a macro definition to the compiler.

$ gcc -DLEVEL1_DCACHE_LINESIZE=`getconf LEVEL1_DCACHE_LINESIZE` ... 

At run-time sysconf(_SC_LEVEL1_DCACHE_LINESIZE) can be used to get L1 cache size.

Answer 2

To know the sizes, you need to look it up using the documentation for the processor, afaik there is no programatic way to do it. On the plus side however, most cache lines are of a standard size, based on intels standards. On x86 cache lines are 64 bytes, however, to prevent false sharing, you need to follow the guidelines of the processor you are targeting (intel has some special notes on its netburst based processors), generally you need to align to 64 bytes for this (intel states that you should also avoid crossing 16 byte boundries).

To do this in C or C++ requires that you use the standard aligned_alloc function or one of the compiler specific specifiers such as __attribute__((align(64))) or __declspec(align(64)). To pad between members in a struct to split them onto different cache lines, you need on insert a member big enough to align it to the next 64 byte boundery