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A system is
memory bound
or has a memory
bottleneck
if memory on the system is insufficient to keep the pages
of all runnable or sleeping processes in physical memory.
To determine if your system
is memory bound run sar -p
(or mpsar -p for SMP)
and look at the value of rclm/s.
If this column is consistently zero, the system does
not have memory problems.
If the number of free pages shown by freemem
(using sar -r) is consistently near or below the value
defined for the GPGSHI kernel parameter, then the
page daemon is probably active.
Confirm this by examining the TIME usage reported by
the ps -el -p 2 command for the page handling daemon,
vhand; if this value is increasing, vhand is
running.
If the value of freemem is consistently low,
this indicates that most of memory is regularly in use.
If this activity is accompanied by swapping then the system
has memory problems.
A more severe indication of insufficient memory is swapping activity.
To check swapping activity use the sar -w command.
If bswot/s shows
values that are consistently non-zero,
this indicates that the system has a memory problem and is
swapping.
Similarly, if the swap queue shows activity, then there are
processes being swapped out to make memory available.
If the entries for swpq-sz and %swpocc
when running sar -q remain blank then
no processes are being swapped and memory is sufficient.
If swpq-sz is greater than zero, then the system
has experienced swapping, and there are runnable processes
on swap.
In some cases, it may do no harm for a few infrequently-used processes to be swapped out, such as the getty processes that are used for the console multiscreens. It does indicate, however, that your system is near to being chronically short of memory if it has to swap whole processes out to free up memory. See ``Paging'' and ``Swapping'' for a description of how the system frees pages of physical memory for use.
Finally, if either vhand or sched are
showing a lot of processing time, indicated by the value of
TIME when running ps -l,
then this would suggest a lot of paging and swapping activity.
The following table is a summary of the commands that can be used to determine if a system is memory bound:
Identifying a memory-bound system
| Command | Field | Description |
|---|---|---|
| [mp]sar -p | rclm/s | pages added to the free list per second |
| [mp]sar -q | %swpocc | percentage of time the swap queue is occupied |
| swpq-sz | number of processes on the swap queue | |
| [mp]sar -r | freemem | available user memory (4KB pages) |
| freeswp | available swap space (512-byte blocks) | |
| [mp]sar -w | bswot/s | average number of pages written to swap per second |
| swap -l | blocks | total size of the swap area (512-byte blocks) |
| free | available swap space (512-byte blocks) | |
| ps -l -p 0 | TIME | CPU time used by the swapper, sched |
| ps -l -p 2 | TIME | CPU time used by the page stealer, vhand |