Tuning Global Cache - Expert Oracle RAC Performance Diagnostics and Tuning - page 381

Each instance has its own SGA, several background processes, and each instance runs on a separate machine
in the cluster having its own CPU and physical memory. Keeping the configurations identical across all nodes in the
cluster would be beneficial for easy maintenance and load balancing. In the following sections, we discuss the various
RAC related background processes.

LMSn
The Global cache service' is multithreaded and includes block server processes ( LMS n ) that copy data blocks directly
from the holding instance's buffer cache to the foreground process on the requesting instance. The LMS processes
produce read consistent copies (versions) for any blocks that are being requested for consistent read by the remote
instance. They maintain the global state of each buffer in the global cache. The initial number of LMS n processes is
driven by the parameter GCS_SERVER_PROCESSES .
The number of LMS processes (Table 13-1 ) to allocate to an instance is based on the number of CPUs on the
server and is driven by the following algorithm:

If the server has between 1 to 3 CPUs, Oracle sets this parameter to value 1; and if the server
has between 4 to 15 CPUs, Oracle sets this parameter to value 2 .

Table 13-1. LMS Processes 1

Oracle RAC 10g (10.2.0.4) Oracle RAC 11g and 12c

# of CPUs # of LMS # of CPUs # of LMS

<= 2 1 <= 4 1

<= 12 2 <= 16 2

>= 12 #cpu/4 >= 16 #cpu/4

Additional LMS for every 32 CPUs

However, the method of calculating the value for this parameter changes when the number of CPUs is more
than 16. The value is computed using the formula 2 + (CPUs/32). If the result includes a fraction, then the fraction is
disregarded. That is, if you had 20 CPUs, then 2 + (20/32) would equal 2 LMS processes. A more specific example, on
a ProLiant DL370 G6 server with 16 cores (2 physical × 8 core), the number of LMS processes will be 2 + 16/32 = 2.5.
Because the fraction part of this value is ignored, the value is set to 2.
In an ASM instance (clustered environment), where the Cache Fusion activity is confined to the transfer of
metadata information when disks are added or removed, the value of this parameter is set to 1.
Prior to Oracle Database 11g Release 2, the value of this parameter was also based on the number of CPUs; however,
Oracle used a standard formula of allocating and starting one LMS process for every pair of CPUs. For example, for the
ProLiant DL370 G6 server with 16 cores, Oracle will set the value of this parameter to 4 (4 LMS processes).
Oracle supports up to 36 LMS processes (0-9 and a-z).

LMON
The global enqueue service monitor (LMON) is a background process that monitors the state and health of the
RDBMS instances in the cluster. It communicates with the cluster monitor and initiates reconfiguration when the state
of a node or instance changes. When a node joins or leaves the cluster, it coordinates the reconfiguration of locks and
resources. In particular, LMON performs reconfiguration of GES and GCS resources.

1 Source: Oracle Corporation.