Re: [gentoo-user] Strive for zero swap usage?

[Michael Mol <mikemol@gmail.com>]

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On Friday, October 07, 2016 04:33:27 AM Grant wrote:
> >>> Swap usage on Linux always seems a little tricky to me.  Should my
> >>> goal on a web server be zero swap usage, meaning the attached graph
> >>> should show no green lines at all if I'm doing it right?
> >> 
> >> No.  You want things that aren't in use to be swapped, like memory
> >> leaks and such.  You don't want things that will be used to be
> >> swapped.
> > 
> > Does this look OK?  It looks to me like heavy swapping in and out with
> > plenty of free memory (minus buffers/cache).
> 
> Or put another way, how do I know when swapping is a problem?  I'm
> running munin so I can look over graphs of my system's characteristics
> but I'm not sure what to look for to determine if I'm swapping
> excessively.

"Swapping excessively" is inherently a use-case-specific problem, but it comes 
down to two questions:

* Do you notice your system spending time in iowait swapping data in while 
you're waiting on it? 
* Do you notice your system spending time in iowait swapping data out while 
you're waiting on it? (I.e. as it tries to make room for new memory 
allocations)

If the answer to those questions is yes, then you're swapping excessively. If 
not, you're not.

There are ways other than swap to find yourself in iowait, though. I wonder 
what might a good metric of combining iowait numbers with swap event counts. 
Swap events without iowait are likely imperceptible.

But it does all come down to perception and how you want to manage it. I have 
some nodes that swap a *lot*, but I don't care as long as they don't fall 
behind in their workload. And I have some nodes that I don't permit to swap at 
all, as that causes latency spikes that are difficult to nail down, or can cause 
snowballing cascade events across several nodes and processes that interact 
with each other.

-- 
:wq

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