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Hugepages are Not used by Database Buffer Cache (Doc ID 829850.1)

Last updated on FEBRUARY 03, 2019

Applies to:

Oracle Database - Enterprise Edition - Version and later
Linux x86
***Checked for relevance on 27-Apr-2012***


Following the steps in <Note 748637.1> you have configured your Linux for Very Large Memory with HugePages.
As described on <Note 749851.1> you have MEMORY_TARGET & MEMORY_MAX_TARGET initialization parameters disabled as the use of 11g Automatic Memory Management (AMM) is incompatible with HugePages.
Your SGA size is less then the size of the HugePages_Total (HugePages_Total*Hugepagesize)
But despite this the HugePages are not used by the database buffer cache.

Here I would like to show you an example of a possible scenario:

Suppose to have a RDBMS instance ASM enabled:
*.shared_pool_size 67108864 --> 64MB
*.db_cache_size 25165824 --> 24MB
*.log_buffer 4194304 --> 4MB

Checking on /proc/meminfo you can verify how the HugePages are configured on your system before and after.


- Before ASM is running:

$ grep -i huge /proc/meminfo
HugePages_Total: 5120
HugePages_Free: 5120
HugePages_Rsvd: 0
Hugepagesize: 2048 kB

HugePages_Total is the size of the pool of hugepages.
HugePages_Free is the number of hugepages in the pool that are not yet allocated.
HugePages_Rsvd is short for "reserved," and is the number of hugepages
for which a commitment to allocate from the pool has been made, but no



- After ASM is running:

$ grep -i huge /proc/meminfo

HugePages_Total:  5120

HugePages_Free:   5078

HugePages_Rsvd:     11

Hugepagesize:     2048 kB

5120 - 5078 = 42 * 2MB = 84MB

             11 * 2MB = 22MB


RDBMS instance:
*.db_block_size 8192 --> 3GB
*.shared_pool_size 318767104 --> 304MB
*.log_buffer 1048576 -->  10MB

$ grep -i huge /proc/meminfo
HugePages_Total: 5120
HugePages_Free: 4970
HugePages_Rsvd: 80
Hugepagesize: 2048 kB

--> 5078 - 4970 = 108 * 2MB = 216MB
                   80 * 2MB = 160MB


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