AMM under Linux uses POSIX shared memory that relies on /dev/shm. By default, /dev/shm is limited to 50 % of physcial RAM and uses 4 KB memory pages.
How much physical RAM does the server have installed and how large is your Oracle SGA? However, using POSIX shared memroy on servers with more than 16 GB of RAM installed is not recommended. If your Oracle SGA is using that much memory you are wasting a lot of memory just for managing the amount of memory pages and cannot take advantage of the TLB cache to improve virtual address translation.
I suggest you look into setting up Kernel Hugepages, which use 2 MB pages under Linux and configure Oracle SGA using ASMM. You should also see a drastic increase in Oracle server performance. http://docs.oracle.com/cd/B28359_01/server.111/b32009/appi_vlm.htm.