replication(5)

NAME
     replication - memory replication

DESCRIPTION
     This document describes the memory replication system available in Origin
     Systems.

   Kernel Text Replication
     The code for Irix is replicated during the booting procedure, according
     to the ratio specified by the tunable numa_kernel_replication_ratio in
     /var/sysgen/mtune/numa.  There's always a copy on the master node, and
     for a ratio n, copies are made on every n of the other nodes. 0 means no
     copies. The default value is 1, which causes copies of the kernel text to
     be made on every node.


   User read-only memory replication
     The kernel automatically produces replicas of read-only memory according
     to a dynamic coverage radius algorithm. Every memory object has an
     associated coverage radius that defines a neighborhood of nodes that are
     considered to be close to a physical page associated with the memory
     object. During the page fault handling procedure, Irix first looks for a
     page containing the needed data in the memory object; if such a page is
     not found, the data is read from the backing device. Otherwise, if such
     page exists, the fault handler uses the coverage radius to decide if a
     replica is needed. If the distance between the page home node and the
     faulting node is less than the coverage radius, the same page is used;
     else, if the distance is greater than the coverage radius, a replica is
     made and used.


     Replication happens for read-only pages only. If a read-only page becomes
     writable, Irix shoots down all the replicas.

     This kind of replication can be enabled or disabled using the tunable
     numa_page_replication_enable.

     Replication can also be controlled in order to avoid excessive use of
     resources. Control can be enabled or disabled using
     numa_repl_control_enabled.

     There are two kinds of Replication Control

     *    Traffic Control. When traffic across the links needed to do the copy
          are being over-utilized, replication is not done. The tunable
          numa_repl_traffic_highmark_percentage defines the point at which a
          link is considered over-utilized. This control is currently
          experimental an is disabled for production systems.

     *    numa_repl_mem_lowmark. Replication is not done if the amount of
          physical memory falls below a threshold, defined by
          numa_repl_mem_lowmark, and expressed in terms of number of pages.
SEE ALSO
     numa(5), migration(5), mtune(4), /var/sysgen/mtune/numa, refcnt(5),
     mmci(5), nstats(1), sn(1).