optimize: preserving needed fields for memory efficiency

[Colvin-Y]

fix #1737
In production scenarios, the fields of pods are usually quite complex. Testing has shown that in our case, the descheduler's memory usage has been reduced by 2/5.

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[k8s-ci-robot]

Hi @Colvin-Y. Thanks for your PR.

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[googs1025]

/ok-to-test

[googs1025]

Using SetTransform seems to be very helpful for memory optimization, especially in large clusters. We can delete the unused pod node field. 🤔

[googs1025]

/lgtm

cc @ingvagabund

[a7i]

should this move here instead?

[Colvin-Y]

should this move here instead?

ditto

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[a7i]

Nice change @Colvin-Y - do you have any before/after results on memory footprint you can share?

@ingvagabund How does a global change like this impact a plugin? For example, if I write my custom Descheduler plugin which relies on some of the omitted fields, can I override this behavior?

[Colvin-Y]

不错的改变@Colvin-Y - 您能否分享关于内存占用的前后结果？

hi @a7i ,
In our prod env, save 47% memory on avg

[Colvin-Y]

If I write my custom Descheduler plugin which relies on some of the omitted fields, can I override this behavior?

hi @a7i

Before cache start, you can override it. But maybe best practice is to modify func preserveNeeded because there are many plugins but using same cache.

[ingvagabund]

Each plugin might tell which fields are necessary. I.e. through a new method in the similar fashion as scheduler's EventsToRegister. E.g. FieldsToPreserve or similar:

func (d *PodLifeTime) FieldsToPreserve() ([]fwk.ObjectFields, error) {
	return []fwk.ObjectFields{
		{Resource: fwk.Pod, Fields: {...}},
		{Resource: fwk.Node, Fields: {...}},
	}
}

This might be tricky to implement as each plugin will need different fields. So a simple clearing functions as in this PR will not suffice.

[Colvin-Y]

/retest-required

[ingvagabund]

/retest-required

[ingvagabund]

A naive solution would be to enumerate all fields. E.g.:

c.Command = nil
c.Args = nil
c.WorkingDir = ""
c.Ports = nil
...

into

const (
  POD_COMMAND = iota
  POD_ARGS
  POD_WORKING_DIR
  POD_PORTS
  ...

The same for nodes. Then, have each plugin to define a list of fields to preserve:

func func (pl *Plugin) PreservedField() [] uint {
  return []uint {POD_COMMAND, POD_ARGS, ...}
}

Then iterate through all plugins, make a union of all PreservedField() lists and use the list to tell which fields can be cleared:

if _, in := preservedFieldsMap[POD_COMMAND]; !in {
  c.Command = nil
}
if _, in := preservedFieldsMap[POD_ARGS]; !in {
  c.Args = nil
}
...

[Colvin-Y]

A naive solution would be to enumerate all fields. E.g.:

c.Command = nil
c.Args = nil
c.WorkingDir = ""
c.Ports = nil
...

into

const (
  POD_COMMAND = iota
  POD_ARGS
  POD_WORKING_DIR
  POD_PORTS
  ...

The same for nodes. Then, have each plugin to define a list of fields to preserve:

func func (pl *Plugin) PreservedField() [] uint {
  return []uint {POD_COMMAND, POD_ARGS, ...}
}

Then iterate through all plugins, make a union of all PreservedField() lists and use the list to tell which fields can be cleared:

if _, in := preservedFieldsMap[POD_COMMAND]; !in {
  c.Command = nil
}
if _, in := preservedFieldsMap[POD_ARGS]; !in {
  c.Args = nil
}
...

ditto