swapinfo -mat

[John Guster]

how is the value of the avail(MB) in memory line calculated in swapinfo -mat? Thanks.

[Steven E. Protter]

Shalom,

Total memory is RAM plus swap.

Available memory is the above total minus memory in use.

Memory in use includes swap that is reserved even if the system is not actively pageing.

SEP

[John Guster]

thought "total" is the sum of avail "device" and avail "memory"; avail "device" is what we configure using disks(asuming use disk only); only avail "memory" is not clear how it is defined in swapinfo -mat output.
Thanks.

[Steven E. Protter]

Shalom,

Does not matter whether its device swap or filesystem swap, its memory as far as swapinfo goes.

SEP

[spex · Accepted solution]

Hi John,

This guide is excellent:
ftp://hprc.external.hp.com/memory.htm

Here's part of the section on swap:

* dev line(s):

o are the actual physical swap device(s)
o show if swapping has actually occurred. In other words, the PCT USED column in the dev lines represents the value last attained during a previous period of swapping. This is analogous to the high-water mark that a flood leaves.
o to check to see if swapping is currently occuring, use 'vmstat -v 5 5' to see if the 'po' (page outs) is sustained above 0.

* reserve line(s)

o indicate how much of the swap device(s) has(have) been set aside for memory should it need to be swapped.

* memory line:

o indicative of how much of pseudo-swap has been reserved
o when present, indicates pseudo-swap is enabled (i.e. swapmem_on kernel paraemter is set to 1, which is the default.) The size of pseudoswap is calculated to be 75% of the size of RAM (a.k.a. memory.) In other words, it does not refer to acual physical memory use!!!! Pseudo-swap was designed specifically for large memory systems for which acutal swapping is never (or rarely) expected to occur, so thereâ s less need to use actual physical disk space for swap.

* total line:

o the PCT USED value shown in the total line indicates how much swap space has been actually reseved for swap. When this percentage gets near 100%, processes will not start up (unable to fork process) and new shared memory segments can not be created.

PCS

[John Guster]

so it appears there is no definite cutoff line for avail memory value,it is between 70-80% of RAM depending on the size of the RAM. Thanks for doc.

[John Guster]

thanks.