Post-Install Database Tasks - Oracle Exadata Recipes: A Problem-Solution Approach - page 595

Check your database instance alert log for information for Large Pages Information , as
displayed in the next code:

10.

■ the Large Pages Information alert log messages are new in oracle 11.2.0.3. prior to 11.2.0.2, you will see
HugePages Information sections in your alert log at startup.

Note

****************** Large Pages Information *****************

Total Shared Global Region in Large Pages = 16 GB (100%)

Large Pages used by this instance: 8193 (16 GB)
Large Pages unused system wide = 16404 (32 GB) (alloc incr 32 MB)
Large Pages configured system wide = 25226 (49 GB)
Large Page size = 2048 KB
***********************************************************

My Oracle Support note 1392497.1 provides a good description of how to correlate the information in the alert log
with the /proc/meminfo HugePages output:
•

Large Pages configured system wide should equal HugePages_Total from /proc/meminfo ,
which is the value you specified when you modified your kernel parameters above.

•

Large Pages used by this instance = the number of pages locked now, plus pages
reserved for future usage.

•

Large Pages unused system wide = the number of available HugePages , which is
vm.nr_hugepages minus any allocated or reserved pages at the time the instance started.
If you compare each instance's alert log after the startup, you'll see different numbers for
each instance, depending on which one was started first, second, third, and so on.

•

HugePages reservation was introduced in Linux kernel 2.6.17 to postpone allocating all an
instance's required pages at startup for performance reasons. On recent Exadata patch levels,
you should have a kernel version that supports this.

How It Works
Using HugePages for Oracle databases running Linux is a way to improve memory management and performance
that typically helps with OLTP workloads on databases with large memory requirements. For Oracle Exadata
configurations running Oracle Linux, it is a means to increase the memory page size from 4 KB to 2 MB. Some of the
benefits of using HugePages include the following:

HugePages are not swappable, so there is no page-in/page-out overhead. •

HugePages relieves pressure on Translation Lookaside Buffer (TLB) functions. A TLB is a
buffer on a processor that contains part of the system's page table, used to do virtual memory
address translation. With HugePages , fewer translations will be required, TLB entries will cover
a larger part of the overall address space, and fewer TLB misses will occur.

• Using

• With large page sizes, there is potentially much less page table overhead.

HugePages pages are not subject for replacement.

• Page table lookup overhead is reduced since

HugePages , bottlenecks for page table access are avoided. • Since fewer pages are required with