I am interested in forcing a CPU cache flush in Windows (for benchmarking reasons, I want to emulate starting with no data in CPU cache), preferably a basic C implementation or Win32 call.
Is there a known way to do this with a system call or even something as sneaky as doing say a large memcpy?
Intel i686 platform (P4 and up is okay as well).
The x86 instruction WBINVD writes back and invalidates all caches. It is described as:
Writes back all modified cache lines in the processor’s internal cache to main memory and invalidates (flushes) the internal caches. The instruction then issues a special-function bus cycle that directs external caches to also write back modified data and another bus cycle to indicate that the external caches should be invalidated.
Importantly, the instruction can only be executed in ring0, i.e. the operating system. So your userland programs can't simply use it. On Linux, you can write a kernel module that can execute that instruction on demand. Actually, someone already wrote such a kernel module: https://github.com/batmac/wbinvd
Luckily, the kernel module's code is really tiny, so you can actually check it before loading code from strangers on the internet into your kernel. You can use that module (and trigger executing the WBINVD instruction) by reading /proc/wbinvd, for example via cat /proc/wbinvd.
However, I found that this instruction (or at least this kernel module) is really slow. On my i7-6700HQ I measured it to take 750µs! This number seems really high to me, so I might have made a mistake measuring this -- please keep that in mind! Explanation of that instruction just say:
The amount of time or cycles for WBINVD to complete will vary due to size and other factors of different cache hierarchies.
