Question: I hear that Oracle large page support
can be important to I/O but I don't understand about large pages in
Oracle. How do large pages work? Can I enable large pages in
Answer: Oracle large pages are
functionally equivalent to huge pages in Linux. Large pages are an
operating system construct similar to RAM "fencing" which pins the
RAM, making it ineligible for swapping. Oracle large pages locks-
out any defined portion of system memory for large items, and Oracle
large pages are commonly used for the Oracle SGA RAM region. Moving
the SGA exclusively into large page system memory, ensures that
there are no swaps (vmstat page-out or page-in operations) from SGA
operations, and Oracle large pages removes a significant amount of
pressure from the SQL cache (shared_pool_size, db_cache_size, &c).
With the SGA entirely pinned into system memory with Oracle
large pages, we are also saving resources on cache swaps required
for the data buffer and library cache areas.
Oracle large pages support began in Oracle 10g Release 1.
Because of new operating system support, Oracle 10g Release 1 or
later can make use of processor memory addressing resources for more
efficient page usage. When large page support is enabled, the
Oracle Database buffers will be more quickly accessible by the CPU.
This is because instead of loading the buffers in the
default 4 kb internal OS buffer page size, the pages are loaded in
increments of 2 MB (or larger, depending on the chipset and total
system memory available). This Oracle large page size reduces the
total number of data pages required to allocate the SGA at startup
time and reduces the CPU time needed to handle the virtual memory
translations because the SGA structures and guaranteed to be in RAM.
Oracle recommends specifying enough large pages to contain the
entire SGA region (as defined by sga_max_size). The Oracle
instance attempts to allocate large pages when the lock_sga
parameter is set to true. If the SGA size exceeds the size of memory
available for pinned or large pages, then the portion of the SGA
exceeding these sizes is allocated to ordinary shared memory and are
eligible for swap-in operations, which can serious degrade
Large Pages on Linux:
To enable large pages in Linux, follow these steps.
- You will need to know the default large page size, which can
be accomplished with the command : # grep
- You need to know the size of the largest SGA * 1024.
- Set the number of large pages by setting the
vm.nr_hugepages kernel parameter to specify the number of
large pages you want to reserve. This number must be at least
large enough to hold the entire SGA.
For example, if /proc/meminfo tells you the large page size is 4
MB and the total SGA size is 512 MB, then the value for
vm.nr_hugepages must be 128.
Large Pages on Solaris:
Under Solaris, Oracle uses large pages by default, and will be
4MB or larger, depending on the specific hardware configuration.
If you want to verify the size of the pages, you can do so from
within the Oracle server process by running pmap -x*s*
A word of caution, if you are using pageable large pages within
your Solaris system, Oracle will need to reserve enough swap space
the size of your entire SGA or the instance will fail to start.
Large Pages on HP/UX:
On HP-UX, Oracle Database uses the largest virtual memory page
size setting available for process-private memory (defined by L).
When the virtual memory page size is set to L, HP-UX allocates the
available memory to pages of 1MB, 4MB, 16MB, etc, until it reaches
the 1BG limit, or until it reaches the total amount of allocated
memory. If you allocate enough memory to Oracle for the operating
system to be able to allocate memory in larger data page size units,
then the operating system will allocate the maximum page size at
Large Pages on Windows:
To enable large pages in Windows, make sure that the
Windows registry keys ORA_LPENABLE and ORA_LPSIZE are defined as a
string (REG_SZ) datatype. Here is the Oracle10g Windows registry
entry for ora_lpenable to enable large page support in Windows:
"ORA_LPENABLE" = "1"
Large Pages on AIX:
For AIX Oracle large page support, give the Oracle user ID
the CAP_BYPASS_RAC_VMM and CAP_PROPAGATE capabilities by following
- First check the current capabilities: #lsuser –a
Note: only the root user can display the
- Add the CAP_BYPASS_RAC_VMM and CAP_PROPAGATE
capabilities to the list of capabilities already assigned to
this user ID, if any:
- Configure the AIX large page pool by calculating the
number of large pages required for the SGA:
- Configure the number and size of large pages:
#vmo -p -o lgpg_regions=num_of_large_pages
For more information on Hugepages and Large Pages, please check
out the following pages:
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