Parameter cpu_count is determined by Oracle Software when instance started, But we can set it manually. This parameter can affect lots of hidden initialization parameters,see:
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
SQL> host cat /proc/cpuinfo|grep processor
processor : 0
processor : 1
SQL> show parameter cpu_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cpu_count integer 2
create table cpu_2_parameters as
SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
FROM SYS.x$ksppi x, SYS.x$ksppcv y
WHERE x.inst_id = USERENV ('Instance')
AND y.inst_id = USERENV ('Instance')
AND x.indx = y.indx;
SQL> alter system set cpu_count=128 scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area 1702887424 bytes
Fixed Size 2093192 bytes
Variable Size 486543224 bytes
Database Buffers 1073741824 bytes
Redo Buffers 140509184 bytes
Database mounted.
Database opened.
SQL> show parameter cpu_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cpu_count integer 128
SQL> create table cpu_128_parameters as
2 SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
3 FROM SYS.x$ksppi x, SYS.x$ksppcv y
4 WHERE x.inst_id = USERENV ('Instance')
5 AND y.inst_id = USERENV ('Instance')
6 AND x.indx = y.indx;
Table created.
SQL> col name for a35
SQL> set linesize 200 pagesize 2000;
SQL> col "while_cpu_count= 2" for a20
SQL> col "while_cpu_count= 128" for a02
SQL> col "while_cpu_count= 128" for a20
SQL> SELECT a.name,
2 a.value "while_cpu_count= 2",
3 b.value "while_cpu_count= 128",
4 a.describ
5 FROM cpu_2_parameters a,
6 cpu_128_parameters b
7 WHERE a.name=b.name
8 AND a.value!=b.value
9 ORDER BY 1;
NAME while_cpu_count= 2 while_cpu_count= 128 DESCRIB
----------------------------------- -------------------- -------------------- --------------------------------------------------------------------------------
__db_cache_size 33554432 536870912 Actual size of DEFAULT buffer pool for standard block size buffers
_cursor_db_buffers_pinned 445 841 additional number of buffers a cursor can pin at once
_db_block_buffers 67184 126464 Number of database blocks cached in memory: hidden parameter
_db_block_lru_latches 8 512 number of lru latches
_enqueue_hash_chain_latches 2 128 enqueue hash chain latches
_enqueue_locks 2300 2490 locks for managed enqueues
_flashback_generation_buffer_size 8388608 536870912 flashback generation buffer size
_log_parallelism_max 2 8 Maximum number of log buffer strands
_log_simultaneous_copies 4 256 number of simultaneous copies into redo buffer(# of copy latches)
_num_longop_child_latches 2 128 number of child latches for long op array
_parallel_min_message_pool 903840 1076000 minimum size of shared pool memory to reserve for pq servers
_small_table_threshold 1343 2529 threshold level of table size for direct reads
cpu_count 2 128 number of CPUs for this instance
db_cache_size 33554432 536870912 Size of DEFAULT buffer pool for standard block size buffers
db_writer_processes 1 16 number of background database writer processes to start
log_buffer 6104064 136249344 redo circular buffer size
parallel_max_servers 40 135 maximum parallel query servers per instance
sga_max_size 1065353216 1702887424 max total SGA size
SQL> show parameter sga_max_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
sga_max_size big integer 1016M
It looks like sga may grow while the instance has more processors. Oracle Parallel Servers can has more differ with cpu_count . Sometimes this parameter make performance issues.
