runtime: use MADV_FREE on linux as well

[CAFxX]

On linux currently sysUnused uses madvise(MADV_DONTNEED) to signal the kernel that a range of allocated memory contains unneeded data. After a successful call, the range (but not the data it contained before the call to madvise) is still available but the first access to that range will unconditionally incur a page fault (needed to 0-fill the range).

A potentially faster alternative is MADV_FREE, available since linux 4.5 (and already used for bsd). The mechanism is very similar, but the page fault will only be incurred if the kernel, between the call to madvise and the first access, decides to reuse that memory for something else.

Off the top of my head, there are two ways to support both pre-4.5 and 4.5+ kernels:

• Use a fallback approach (i.e. try MADV_FREE first and MADV_DONTNEED if MADV_FREE fails): this is e.g. what node does
• Detect at runtime startup whether the running kernel supports MADV_FREE (e.g. by attempting to call madvise(MADV_FREE) on an allocated but unused range)

[bradfitz]

/cc @aclements

[gopherbot]

Change https://golang.org/cl/135395 mentions this issue: runtime: use MADV_FREE on Linux if available