Basic Support for Swap

[tbartelmess]

This PR adds basic support to make Swap space a schedulable resource. While especially in cloud environments swap usage can be tricky since the performance can vary a lot. However, for some cases that process very ununiform data (especially batch workload), swap is often needed to not over-provision Memory to a great extent that then never ends up being used. At @Marketcircle, we are using a fork of Nomad with those changes.

There are no changes to the documentation yet, and also the metric collection is not finalized.

The main change is that "Swap" is a schedulable resource and can be defined in the resources of a task.

      resources = {
        memory = 2048
        cpu    = 1750
        swap   = 2048
}

Nodes do get fingerprinted for available swap space and the docker and lxc schedulers have been updated to enforce the limit.

[jippi]

go fmt

[jippi]

go fmt

[jippi]

go fmt

[jippi]

go fmt

[dadgar]

@tbartelmess Great job on this PR but unfortunately this is not something we would like in Nomad. The reason being is that when a subset of allocations on a node are using swap, it can lead to degraded performance of the entire node and thus negatively impact the other allocations. In Nomad 0.9 we will be targeting fingerprinter plugins, custom resources and drivers as plugins. With these newer features you will be able to accomplish your goal of making Nomad swap aware without it being in core. Hope my explanation makes sense and that the path forward to not maintaining a fork is acceptable! Thanks again!

[jippi]

@dadgar my 50c on this is to allow the feature, but as a client configuration knob

there are tons of valid cases for needing this, and hiding it behind a flag would nomad team to say we told you it was a terrible idea if someone complains :)

[shantanugadgil]

I totally agree about letting the user allow swap for dockers as well.
Going through the master CHANGELOG, what is confusing is that Nomad initially (somewhere before v0.4.1 allowed this behavior), which was then changed, not be allowed at all.
Then a feature to allow override the total memory of the client landed recently, which mentions SWAP as one of the possibilities of the feature.

Like @jippi says, I too has imagined that this could be a client/driver option; something like driver.docker.enable_swap, maybe?

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