On Sun, 14 Jan 2018 13:52:31 +0000 (UTC), Kiwi User
 wrote:

> The watch points for system performance are:
>
> (1) if the buff/cache figure is small almost all data i/o is going to
>    require physical disk/hdd access which is always noticeably slower
>    than if the required files are in the cache.
>
>    Cache size = total - (used + free) - from the KiB Mem line

Indeed, that's why it is important to have a swap partition or a
swap file, to free up physical memory for use as cache and
buffers.

> (2) if a program is swapping, this adds to the physical i/o load because
>    its continually having the transfer chunks of its memory between RAM
>    and the swap space.
>
>    You can work out which program is swapping because its SHR value
>    tends to exceed its RES value.

Programs that are loaded for a longer time, use swap to get rid of
pages that are only seldomly used from physical memory. The OS
decides on that, the program doesn't have to worry about it.

Swap I/O activity can be observed with
 vmstat 5
, which takes a snapshot every 5 seconds.
Observe the si (swap-in)) and so (swap-out).
Stop the report with ^C.

On a well-configured server that is not overloaded, si and so are
zero most of the time, and not a high burden for an SD card or USB
memory stick.

vmstat has a lot of useful options, it is a nice tool to pinpoint
performance bottlenecks.

--
Regards,
Kees Nuyt

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