Oracle 11G Startup failed after altering the number of system processes When I alter the number of system processes to 100000 and restart the database, I get the error below:

ORA-27154: post/wait create failed 
ORA-27300: OS system dependent operation:semids_per_proc failed with status: 0
ORA-27301: OS failure message: Error 0
ORA-27302: failure occurred at: sskgpwcr2
ORA-27303: additional information: semids = 524, maxprocs = 100000


How can I return it to the normal processes?
First you can try:

STARTUP NOMOUNT
ALTER SYSTEM SET processes=400;
SHUTDOWN IMMEDIATE
STARTUP


If Oracle cannot start even in NOMOUNT mode then you can fetch configuration information from spfile:

strings $ORACLE_HOME/dbs/spfile$SID.ora &gt; /tmp/pfile


Then edit processes in /tmp/pfile cleanup some mess made by strings start Oracle with pfile and recreate spfile:

STARTUP pfile=/tmp/pfile
CREATE spfile FROM pfile='/tmp/pfile';