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Diffstat (limited to 'Documentation/vm/numa_memory_policy.txt')
| -rw-r--r-- | Documentation/vm/numa_memory_policy.txt | 33 |
1 files changed, 16 insertions, 17 deletions
diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt index 8242f52d0f22..dd4986497996 100644 --- a/Documentation/vm/numa_memory_policy.txt +++ b/Documentation/vm/numa_memory_policy.txt @@ -302,31 +302,30 @@ MEMORY POLICIES AND CPUSETS Memory policies work within cpusets as described above. For memory policies that require a node or set of nodes, the nodes are restricted to the set of -nodes whose memories are allowed by the cpuset constraints. If the -intersection of the set of nodes specified for the policy and the set of nodes -allowed by the cpuset is the empty set, the policy is considered invalid and -cannot be installed. +nodes whose memories are allowed by the cpuset constraints. If the nodemask +specified for the policy contains nodes that are not allowed by the cpuset, or +the intersection of the set of nodes specified for the policy and the set of +nodes with memory is the empty set, the policy is considered invalid +and cannot be installed. The interaction of memory policies and cpusets can be problematic for a couple of reasons: -1) the memory policy APIs take physical node id's as arguments. However, the - memory policy APIs do not provide a way to determine what nodes are valid - in the context where the application is running. An application MAY consult - the cpuset file system [directly or via an out of tree, and not generally - available, libcpuset API] to obtain this information, but then the - application must be aware that it is running in a cpuset and use what are - intended primarily as administrative APIs. - - However, as long as the policy specifies at least one node that is valid - in the controlling cpuset, the policy can be used. +1) the memory policy APIs take physical node id's as arguments. As mentioned + above, it is illegal to specify nodes that are not allowed in the cpuset. + The application must query the allowed nodes using the get_mempolicy() + API with the MPOL_F_MEMS_ALLOWED flag to determine the allowed nodes and + restrict itself to those nodes. However, the resources available to a + cpuset can be changed by the system administrator, or a workload manager + application, at any time. So, a task may still get errors attempting to + specify policy nodes, and must query the allowed memories again. 2) when tasks in two cpusets share access to a memory region, such as shared memory segments created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and any of the tasks install shared policy on the region, only nodes whose memories are allowed in both cpusets may be used in the - policies. Again, obtaining this information requires "stepping outside" - the memory policy APIs, as well as knowing in what cpusets other task might - be attaching to the shared region, to use the cpuset information. + policies. Obtaining this information requires "stepping outside" the + memory policy APIs to use the cpuset information and requires that one + know in what cpusets other task might be attaching to the shared region. Furthermore, if the cpusets' allowed memory sets are disjoint, "local" allocation is the only valid policy. |
