SGA bigger than than the amount of HugePages configured (Linux –

Tanel Poder


I just learned something new yesterday when demoing large page use on Linux during my AOT seminar.

I had 512 x 2MB hugepages configured in Linux ( 1024 MB ). So I set the USE_LARGE_PAGES = TRUE (it actually is the default anyway in This allows the use of large pages (it doesn’t force, the ONLY option would force the use of hugepages, otherwise the instance wouldn’t start up). Anyway, the previous behavior with hugepages was, that if Oracle was not able to allocate the entire SGA from the hugepages area, it would silently allocate _the entire SGA _from small pages. It was all or nothing. But to my surprise, when I set my SGA_MAX_SIZE bigger than the amount of allocated hugepages in my testing, the instance started up _and _the hugepages got allocated too!

It’s just that the remaining part was allocated as small pages, as mentioned in the alert log entry below (and in the latest documentation too – see the link above):

Thu Oct 24 20:58:47 2013
Thu Oct 24 20:58:54 2013
Shutting down instance (abort)
License high water mark = 19
USER (ospid: 18166): terminating the instance
Instance terminated by USER, pid = 18166
Thu Oct 24 20:58:55 2013
Instance shutdown complete
Thu Oct 24 20:59:52 2013
Adjusting the default value of parameter parallel_max_servers
from 160 to 135 due to the value of parameter processes (150)
Starting ORACLE instance (normal)
****************** Large Pages Information *****************

Total Shared Global Region in Large Pages = 1024 MB (85%)

Large Pages used by this instance: 512 (1024 MB)
Large Pages unused system wide = 0 (0 KB) (alloc incr 16 MB)
Large Pages configured system wide = 512 (1024 MB)
Large Page size = 2048 KB

  Total Shared Global Region size is 1202 MB. For optimal performance,
  prior to the next instance restart increase the number
  of unused Large Pages by atleast 89 2048 KB Large Pages (178 MB)
  system wide to get 100% of the Shared
  Global Region allocated with Large pages

The ipcs -m command confirmed this (multiple separate shared memory segments had been created).

Note that despite what documentation says, there’s a 4th option, called AUTO for the USE_LARGE_PAGES parameter too (in I think), which can now ask the OS to increase the number of hugepages when instance starts up – but I would always try to pre-allocate the right number of hugepages from start (ideally right after the OS reboot – via sysctl.conf), to reduce any overhead potential kernel CPU usage spikes due to search and defragmentation effort for “building” large consecutive pages.

Marting Bach has written about the AUTO option here.