Memory Management ​

Basic Concepts ​

The memory is an important component of a computer, and is used to temporarily store operation data in the CPU and data exchanged with an external memory such as hardware. In particular, a non-uniform memory access architecture (NUMA) is a memory architecture designed for a multiprocessor computer. The memory access time depends on the location of the memory relative to the processor. In NUMA mode, a processor accesses the local memory faster than the non-local memory (the memory is located in another processor or shared between processors).

Memory Monitoring ​

1. free: displays the system memory status. Example:

   shell

   # Display the system memory status in MB.
   free -m

   The output is as follows:

   shell

   [root@openEuler ~]# free -m
                  total        used        free      shared  buff/cache   available
   Mem:            2633         436         324          23        2072        2196
   Swap:           4043           0        4043

   The fields in the command output are as follows:

   Field	Description
   total	Total memory size.
   used	Used memory.
   free	Free memory.
   shared	Total memory shared by multiple processes.
   buff/cache	Total number of buffers and caches.
   available	Estimated available memory to start a new application without swapping.

2. vmstat: dynamically monitors the system memory and views the system memory usage.

   Example:

   shell

   # Monitor the system memory and display active and inactive memory.
   vmstat -a

   The output is as follows:

   shell

   [root@openEuler ~]# vmstat -a
   procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
   r  b   swpd   free  inact active   si   so    bi    bo   in   cs us sy id wa st
   2  0    520 331980 1584728 470332    0    0     0     2   15   19  0  0 100  0  0

   In the command output, the field related to the memory is as follows:

   Field	Description
   memory	Memory information. -swpd: usage of the virtual memory, in KB. -free: free memory capacity, in KB. -inact: inactive memory capacity, in KB. -active: active memory capacity, in KB.

3. sar: monitors the memory usage of the system.

   Example:

   shell

   # Monitor the memory usage in the sampling period in the system. Collect the statistics every two seconds for three times.
   sar -r 2 3

   The output is as follows:

   shell

   [root@openEuler ~]# sar -r 2 3
   
   04:02:09 PM kbmemfree   kbavail kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kb
   dirty
   04:02:11 PM    332180   2249308    189420      7.02    142172   1764312    787948     11.52    470404   1584924
   36
   04:02:13 PM    332148   2249276    189452      7.03    142172   1764312    787948     11.52    470404   1584924
   36
   04:02:15 PM    332148   2249276    189452      7.03    142172   1764312    787948     11.52    470404   1584924
   36
   Average:       332159   2249287    189441      7.03    142172   1764312    787948     11.52    470404   1584924
   36

   The fields in the command output are described as follows:

   Field	Description
   kbmemfree	Unused memory space.
   kbmemused	Used memory space.
   %memused	Percentage of the used space.
   kbbuffers	Amount of data stored in the buffer.
   kbcached	Data access volume in all domains of the system.

4. numactl: displays the NUMA node configuration and status.

   Example:

   shell

   # Check the current NUMA configuration.
   numactl -H

   The output is as follows:

   shell

   [root@openEuler ~]# numactl -H
   available: 1 nodes (0)
   node 0 cpus: 0 1 2 3
   node 0 size: 2633 MB
   node 0 free: 322 MB
   node distances:
   node   0
   0:  10

   The server contains one NUMA node. The NUMA node that contains four cores and 6 GB memory. The command also displays the distance between NUMA nodes. The further the distance, the higher the latency of cross-node memory accesses, which should be avoided as much as possible.

   numastat: displays NUMA node status.

   shell

   # Check the NUMA node status.
   numastat

   shell

   [root@openEuler ~]# numastat
                           node0
   numa_hit                 5386186
   numa_miss                      0
   numa_foreign                   0
   interleave_hit             17483
   local_node               5386186
   other_node                     0

   The fields in the numastat command output are described as follows:

   Field	Description
   numa_hit	Number of times that the CPU core accesses the local memory on a node.
   numa_miss	Number of times that the core of a node accesses the memory of other nodes.
   numa_foreign	Number of pages that were allocated to the local node but moved to other nodes. Each numa_foreign corresponds to a numa_miss event.
   interleave_hit	Number of pages of the interleave policy that are allocated to this node.
   local_node	Size of memory that was allocated to this node by processes on this node.
   other_node	Size of memory that was allocated to other nodes by processes on this node.