[PATCH] D58718: [Memory] Add basic support for large/huge memory pages

Wed Feb 27 08:00:18 PST 2019

aganea created this revision.
aganea added reviewers: rnk, zturner, lhames.
Herald added a subscriber: kristina.
Herald added a project: LLVM.

This adds `Memory::MF_HUGE_HINT` which indicates that `allocateMappedMemory()` shall return a pointer to a large memory pages. However the flag is merly a //hint// because we're not guaranteed in any way that we will get back a large memory page. There are several restrictions:

- Large/huge memory pages aren't enabled by default on modern OSes (Windows 10 and Linux at least), and should be manually enabled/reserved.
- Once enabled, it should be kept in mind that large pages are physical only, they can't be swapped.
- Memory fragmentation can affect the availability of large pages, especially after running the OS for a long time and/or running along many other applications.

`allocateMappedMemory()` will fallback to 4KB pages if it can't allocate 2MB large pages (when `Memory::MF_HUGE_HINT` is provided)

Currently, this patch implements `Memory::MF_HUGE_HINT` only on Windows. The hint will be ignored on Linux, and 4KB pages will always be returned. Unfortunately I don't have a Linux box at hand, and WSL does not seem to support <https://github.com/Microsoft/WSL/issues/3796> huge pages at the moment.

Also, testing on the build system is a bit tricky. I've added a unit test to exercise the codepath, although I can't ensure the OS will return a large/huge memory page. The test would be too fragile otherwise and could fail occasionally.

Repository:
  rL LLVM

https://reviews.llvm.org/D58718

Files:
  include/llvm/Support/Memory.h
  lib/Support/Windows/Memory.inc
  unittests/Support/MemoryTest.cpp

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