Understanding Linux Hugepages

Hugepages can be extremely useful for systems having bigger RAM.

$ grep Huge /proc/meminfo

HugePages_Total:         0

HugePages_Free:          0

HugePages_Rsvd:          0

HugePages_Surp:          0

Hugepagesize:         2048 kB

Hugepagesize is the size of one Hugepage, on most x86 or x86_64 machines,  it's set to 2048 kB.
HugePages_Total is the number of Hugepages configured on the machine. In Redhat Linux, we can change this value by updating /etc/sysctl.conf

echo "vm.nr_hugepages = 200" >> /etc/sysctl.conf

 After updating, /sbin/sysctl -p may make the setting take effect, but it's better to restart the machine.

To confirm Hugepages is configured after rebooting,

$ grep Huge /proc/meminfo

HugePages_Total:       200

HugePages_Free:        200

HugePages_Rsvd:          0

HugePages_Surp:          0

Hugepagesize:         2048 kB

By setting Hugepages to 200, we are telling linux to reserve 200 Hugepages. So the memory associated with Hugepages is: HugePages_Total x Huagepagesize = 200 x 2048kB = 400M.
The memory associated with Hugepages is always allocated, and it cannot be swapped out.

Before setting Hugepages,

$ grep Mem /proc/meminfo

MemTotal:                1030940 kB

MemFree:                  861852 kB

After setting Hugepages, check memory usage again:

$ grep Mem /proc/meminfo

MemTotal:                1030940 kB

MemFree:                  452252 kB

We can see that the free memory dropped from 841M to 441M, after setting HugePages_Total to 200. The 400M memory is the associated to Hugepages, and it will always be allocated.

We can use Hugepages to lock some memory for oracle SGA, so the SGA will never be paged out, thus improving the database performance.