Hugetlb Page Migration

updated Apr 21 2005

Contents


Motivation

Linux 2.6 has the feature which allows processes to use large pages, which is often called hugetlb pages. This feature will be widely used to gain good performance in high performance computing or others. Hugetlb pages are not exceptions to be migrated in case of memory hot-removal.

What should be done?

Is it possible to migrate hugetlb pages like normal pages? Yes, if some lacked functions are added.

Linux kernel 2.6 manages hugetlb pages very similar to that for normal pages. The pages are placed in the page_tree of the corresponding inode in hugetlbfs as if they're in the page-cache.

Linux VM doesn't support some functionality for hugetlb pages, which are related to demand paging mechanism. This means that it can't handle the page-fault nor it can't reclaim these pages, either. It is strictly required to implement them, on which the memory migration infrastructure depends. With these functions, hugetlb pages can be migrated as normal pages can.

There is another difficulty that hugetlb pages can hardly be allocated as a replacement on demand at this moment, though the memory migration requires it. Physically contiguous memory allocation has been an issue to be solved for a long time. Fortunately, this will become possible with the other related works in the new future, which are "Memory defragmentation" and "Free area splitting within zones." Memory defragmentation can make free contiguous memory regions, sweeping out the normal pages in it. The memory migration mechanism is used to sweep out them.

After all, hugetlb pages can be moved to another place with these implementation.


Download

The sample implementation is down-loadable, which is against linux-2.6.10-rc2-mm4. This patch set is supposed to be applied over memory hotplug code, which already includes the memory migration code for normal pages.


References


Hirokazu Takahashi (a.k.a. taka @ valinux japan)