ShaharMintz at 14:32, 31 March 2009

2009-03-31T14:32:22Z

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To utilize Larges Pages in the host to back your KVM guest one needs to use hugetlbfs. A little bit of setup is needed to get this going.

= Host Kernel Config =
Your host kernel config needs to have:

CONFIG_HUGETLBFS=y
CONFIG_HUGETLB_PAGE=y

= Host /etc/fstab =
hugetlbfs is indeed a pseudo-filesystem, like tmpfs, so you need to mount it somewhere and add an entry to fstab if you want to automatically have it mounted on boot.

% sudo mkdir /hugetlbfs

Add an entry to fstab like this:

hugetlbfs /hugetlbfs hugetlbfs defaults 0 0

= Calculate amount number of pages to reserve =
On x86, large pages are 2M, so determine the max size of the guest, and with some fudge factor (about 30 pages, completely arbitrary value); we have something like:

HPAGES = $(($GUEST_SIZE_IN_MB / 2) + 30)

For example, a 2G guest would yield 1054 2MB pages.

= Reserve large pages after boot =

Take your calculated value and echo that into proc quickly after a fresh boot of the machine to ensure you haven't fragmented your memory such that you can't reserve enough large pages.

echo $HPAGES > /proc/sys/vm/nr_hugepages

= Check your reservation =
You can check whether or not your reservation was successful by looking at /proc/meminfo

% tail -n 5 /proc/meminfo
HugePages_Total: 1054
HugePages_Free: 1054
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB

= Using large page reservation =

To use your large page reservation you need to add an option to your guest command line. '-mem-path <path/to/hugetlbfs/mount/point>' If you followed the above commands, then you would add '-mem-path /hugetlbfs'

= Checking large page usage =

After launching your guest with large pages, you can check that your guest is using the large pages by sampling the HugePages_Free value from /proc/meminfo. This value should go down as your guest pages in memory.

UsingLargePages - Revision history

ShaharMintz at 14:32, 31 March 2009