Huge pages¶

Pages, the TLB and huge pages¶

Pages

Physical memory is segmented into a series of contiguous regions called pages. Each page contains a number of bytes, referred to as the page size. The system retrieves memory by accessing entire pages, rather than byte by byte.

Translation Lookaside Buffer (TLB)

A TLB is used to map the virtual addresses of pages to the physical addresses in actual memory. The TLB is a cache and is not limitless, storing only the most recent or frequently accessed pages. During normal operation, processes will sometimes attempt to retrieve pages that are not stored in the cache. This is known as a TLB miss and results in a delay as the processor iterates through the pages themselves to find the missing address mapping.

Huge Pages

The standard page size in x86 systems is 4 kB. This is optimal for general purpose computing but larger page sizes - 2 MB and 1 GB - are also available. These larger page sizes are known as huge pages. Huge pages result in less efficient memory usage as a process will not generally use all memory available in each page. However, use of huge pages will result in fewer overall pages and a reduced risk of TLB misses. For processes that have significant memory requirements or are memory intensive, the benefits of huge pages frequently outweigh the drawbacks.

Persistent Huge Pages

On Linux hosts, persistent huge pages are huge pages that are reserved upfront. The HugeTLB provides for the mechanism for this upfront configuration of huge pages. The HugeTLB allows for the allocation of varying quantities of different huge page sizes. Allocation can be made at boot time or run time. Refer to the Linux hugetlbfs guide for more information.

Transparent Huge Pages (THP)

On Linux hosts, transparent huge pages are huge pages that are automatically provisioned based on process requests. Transparent huge pages are provisioned on a best effort basis, attempting to provision 2 MB huge pages if available but falling back to 4 kB small pages if not. However, no upfront configuration is necessary. Refer to the Linux THP guide for more information.

Enabling huge pages on the host¶

Persistent huge pages are required owing to their guaranteed availability. However, persistent huge pages are not enabled by default in most environments. The steps for enabling huge pages differ from platform to platform and only the steps for Linux hosts are described here. On Linux hosts, the number of persistent huge pages on the host can be queried by checking /proc/meminfo:

$ grep Huge /proc/meminfo
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

In this instance, there are 0 persistent huge pages (HugePages_Total) and 0 transparent huge pages (AnonHugePages) allocated. Huge pages can be allocated at boot time or run time. Huge pages require a contiguous area of memory - memory that gets increasingly fragmented the long a host is running. Identifying contiguous areas of memory is an issue for all huge page sizes, but it is particularly problematic for larger huge page sizes such as 1 GB huge pages. Allocating huge pages at boot time will ensure the correct number of huge pages is always available, while allocating them at run time can fail if memory has become too fragmented.

To allocate huge pages at run time, the kernel boot parameters must be extended to include some huge page-specific parameters. This can be achieved by modifying /etc/default/grub and appending the hugepagesz, hugepages, and transparent_hugepages=never arguments to GRUB_CMDLINE_LINUX. To allocate, for example, 2048 persistent 2 MB huge pages at boot time, run:

# echo 'GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX hugepagesz=2M hugepages=2048 transparent_hugepage=never"' > /etc/default/grub
$ grep GRUB_CMDLINE_LINUX /etc/default/grub
GRUB_CMDLINE_LINUX="..."
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX hugepagesz=2M hugepages=2048 transparent_hugepage=never"

Reboot the host, then validate that huge pages are now available:

$ grep "Huge" /proc/meminfo
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
HugePages_Total:    2048
HugePages_Free:     2048
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

There are now 2048 2 MB huge pages totalling 4 GB of huge pages. These huge pages must be mounted. On most platforms, this happens automatically. To verify that the huge pages are mounted, run:

# mount | grep huge
hugetlbfs on /dev/hugepages type hugetlbfs (rw)

In this instance, the huge pages are mounted at /dev/hugepages. This mount point varies from platform to platform. If the above command did not return anything, the hugepages must be mounted manually. To mount the huge pages at /dev/hugepages, run:

# mkdir -p /dev/hugepages
# mount -t hugetlbfs hugetlbfs /dev/hugepages

There are many more ways to configure huge pages, including allocating huge pages at run time, specifying varying allocations for different huge page sizes, or allocating huge pages from memory affinitized to different NUMA nodes. For more information on configuring huge pages on Linux hosts, refer to the Linux hugetlbfs guide.

Customizing instance huge pages allocations¶

By default, an instance does not use huge pages for its underlying memory. However, huge pages can bring important or required performance improvements for some workloads. Huge pages must be requested explicitly through the use of flavor extra specs or image metadata. To request an instance use huge pages, run:

$ openstack flavor set m1.large --property hw:mem_page_size=large

Different platforms offer different huge page sizes. For example: x86-based platforms offer 2 MB and 1 GB huge page sizes. Specific huge page sizes can be also be requested, with or without a unit suffix. The unit suffix must be one of: Kb(it), Kib(it), Mb(it), Mib(it), Gb(it), Gib(it), Tb(it), Tib(it), KB, KiB, MB, MiB, GB, GiB, TB, TiB. Where a unit suffix is not provided, Kilobytes are assumed. To request an instance to use 2 MB huge pages, run one of:

$ openstack flavor set m1.large --property hw:mem_page_size=2Mb

$ openstack flavor set m1.large --property hw:mem_page_size=2048

Enabling huge pages for an instance can have negative consequences for other instances by consuming limited huge pages resources. To explicitly request an instance use small pages, run:

$ openstack flavor set m1.large --property hw:mem_page_size=small

Finally, to leave the decision of huge or small pages to the compute driver, run:

$ openstack flavor set m1.large --property hw:mem_page_size=any

For more information about the syntax for hw:mem_page_size, refer to the Flavors guide.

Applications are frequently packaged as images. For applications that require the IO performance improvements that huge pages provides, configure image metadata to ensure instances always request the specific page size regardless of flavor. To configure an image to use 1 GB huge pages, run:

$ openstack image set [IMAGE_ID]  --property hw_mem_page_size=1GB

If the flavor specifies a numerical page size or a page size of "small" the image is not allowed to specify a page size and if it does an exception will be raised. If the flavor specifies a page size of any or large then any page size specified in the image will be used. By setting a small page size in the flavor, administrators can prevent users requesting huge pages in flavors and impacting resource utilization. To configure this page size, run:

$ openstack flavor set m1.large --property hw:mem_page_size=small

For more information about image metadata, refer to the Image metadata guide.