map: use batch lookups in Map.Iterate() about ebpf

[tommyp1ckles]

Stumbled upon this, I did some work recently in Cilium that dealt with the issue of default batch sizes that is likely related.

There's been work done to significantly improve the batch lookup API, but it would be nice to start looking at maybe providing a iterator pattern that would satisfy the Go rangefunc experimental feature.

[lmb]

I would love it if Map.Iterate transparently used batch lookups if possible. (Less of a fan of a new API, we might have to break compat when / if Go gets iterators.) The devil is in the details though:

• How many keys and values do we fetch in one go? There is not really a number that is correct for all cases. I'd probably start with a heuristic of using a page of memory for keys and values each, something like os.Getpagesize() / Sizeof(T). Allow the user to override that somehow. MapIterator.Items or something like that?
• How do we deal with differences in semantics? For example, right now concurrent iteration and delete is undefined behaviour. I don't expect that to change with batch lookup (can't get more undefined) but is there a difference in practice to the reset behaviour we get for single key lookups?

[lmb]

I would love that. However, your work plus my recent experience with #1485 makes me think that the current API is basically unusable in a generic fashion. We don't know anything about the map in question except the max entries really. In the worst case, we could have a single bucket with max_entries in it. How would we deal with that? Fall back to regular iteration at some point?