c10mbench - raw scalability benchmarks

This project measures raw scalability benchmarks, namely memory speeds and synchronization speeds. Just type make to compile it, and make run to run it.

Explanation of Benchmarks

Memory Latency

Memory access is slow. CPUs have small, high-speed caches to store copies of frequently used day. At scale, there is to much data to fit within the cache. Therefore, memory access speed becomes a limiting factor.

The benchmarks measure latency by randomly accessing memory. Results vary between about 57 nanosecons and 80 nanoseconds.

Huge Pages

User-mode uses virtual-memory. Page tables translate virtual addresses to physical ones. At scale, these pages tables no longer fit in cache. 16-gigabytes worth of data requires 32-megabytes worth of pages tables, where a cache is around 10-megabytes.

Therefore, every memory access becomes two, doubling memory latency.

One way to mitigate this is through the use of "huge pages" of 2-megabytes instead of 4-kilobytes. This reduces the page tables to 64-kilobytes, which can fit with the cache.

This program runs the memory benchmarks twice, once with huge pages enabled, an once with them disabled.

Ring Buffer

The ring-buffer is an extremely efficient way to pass messages between threads. This program benchmarks this. It also benchmarks the alternative of using pipes, passing messages through the operating system. It shows that it's 10 to 50 times faster using the ring-buffer in user-mode using shared-memory than passing messages through pipes.

Cache Bouncing

The biggest concern for multicore synchronization is avoiding data sharing. This benchmark demonstrates this by repeatedly adding the same memory value from multiple threads.