Sysctl -a shows 0 while it is set to 68719476736

Question

Hi All,
I came across an issue while I was installing Oracle RAC 11g on Red Hat Linux.
I set kernel.shmmax=68719476736 in */etc/sysctl.conf* (This value is equal to 64 GB). But, my RAM is only 4 GB. So, while I was executing sysctl -a | grep shm, the value shown was kernel.shmmax=0. But, sysctl -p shows the value specified in /etc/sysctl.conf
Then I tried with value 4294967296 (this value is equal to 4GB, the size of RAM) for kernel.shmmax in /etc/sysctl.conf. Both sysctl -a | grep shm and sysctl -p showed the same value (4294967296 ) set in /etc/sysctl.conf
My question is, why is sysctl -a | grep shm showing kernel.shmmax=0 while the value set was 68719476736?
Help is much appreciated.
Thanks

Answer 1

My question is, why is sysctl -a | grep shm showing kernel.shmmax=0 while the value set was 68719476736?in 32bit Address space can max point to 4GB(2^32) so if you give shmmax more than 4GB in 32bit than linux considers it has 0 bytes.......
Official RedHat note : If you set SHMMAX to 4294967296 bytes (4*1024*1024*1024=4GB) on a 32 bit system, then SHMMAX will essentially bet set to 0 bytes since it wraps around the 4GB value. This means that SHMMAX should not exceed 4294967295 on a 32 bit system.
ref (7.1. Setting SHMMAX Parameter) : https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/5/pdf/Tuning_and_Optimizing_Red_Hat_Enterprise_Linux_for_Oracle_9i_and_10g_Databases/Red_Hat_Enterprise_Linux-5-Tuning_and_Optimizing_Red_Hat_Enterprise_Linux_for_Oracle_9i_and_10g_Databases-en-US.pdf