mmap vs sbrk, performance comparison

问题:

Which of these calls is faster on average? I've heard that mmap is faster for smaller allocations but I haven't heard a comparison of either. Any information on performance for these would be nice.

回答1:

You should tag this with a particular implementation (like linux) since the answer surely varies by implementation. For now I'll assume Linux since it's the most popular.

With that said, brk is in theory more optimizable, and in practice it runs about 10% faster on my machine. Allocating one page, these are the times I get:

• brk: min 2550 cycles, typical 2650 cycles
• mmap: min 2700 cycles, typical 2800 cycles

I remember hearing something along the lines of brk being able to skip locking the mmap semaphore, which would explain the discrepancy.

Note: I updated these times after adjusting my test to make a dummy calls prior to timing, to ensure that the code would all be in the cache.