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Oracle cpu_count and number of cores

Oracle Database Tips by Donald BurlesonJune 16, 2016

Question: What do I do to know the number of cores on my Oracle server?

Answer:  To know the number of cores you need only look at the cpu_count parameter.  At install time, Oracle knows if the CPU has core architectures and sets cpu_count to the number of cores.

SQL> show parameter cpu

The oracle kernel will determine the number of CPUs on a system during startup.  However, some new processors (i.e. Intel) have dual cores, where we have two CPU's on one chip, and they appear as two processors when they only occupy one slot, while this is good for software performance it can be hazardous to your license, Oracle has stated that they will charge a CPU license fee for the extra cores in multiple core CPUs. 

Intel Hyper-threading chips are an example of a single chip that appears to be two CPU's to the Oracle kernel.  On Intel-based Oracle servers running Linux or Windows, the cpu_count is set to double the number of physical CPUs when using the hyper-threading setting of the OS or BIOS.

Why is cpu_count important?

The Oracle cpu_count is determined when you start Oracle.  The cpu_count affects the Oracle cost-based optimizer through many calculated parameters that use cpu_count as their basis and are considered every time that Oracle creates an execution plan for a SQL statement.  A doubled setting for cpu_count can result in changes to your explain plans when moving from a single to multiple CPUs or when you increase the number of CPUs in your system. 

A single-core hyperthreading CPU counts as one CPU for licensing, a dual-core CPU counts as 2.

If you are licensed for diagnostic and tuning packs, you can display the number of sockets, cores and CPU's with this script:

-- ******************************************************
-- CPU data from dba_hist_osstat
-- ******************************************************
col c1 heading '#|CPUs'        format 999
col c2 heading '#|CPU|Cores'   format 999
col c3 heading '#|CPU|Sockets' format 999
   (select max(value) from dba_hist_osstat 
    where stat_name = 'NUM_CPUS')        c1,
   (select max(value) from dba_hist_osstat 
    where stat_name = 'NUM_CPU_CORES')   c2,
   (select max(value) from dba_hist_osstat 
    where stat_name = 'NUM_CPU_SOCKETS') c3
from dual;


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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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