Use a slice instead of a persistent list for temporary accumulation of lists

[bboreham]

Reduce garbage created in Merge() and Add() from O(n) to O(1) by using a slice with enough capacity to hold the temporary head-portion of the list accumulated while walking the list to insert element(s).

(EDIT: except in the case where all merged data is newer, Merge() discards at least one of the input lists, so in that sense still creates O(n) garbage. I've reduced the internally-created garbage)

[tomwilkie]

Nice! I can't see any problems with this.

[2opremio]

Contributes to #1010

Thanks @bboreham !

I have measured the impact this would have when using Scope to monitor our service (6 probes) but, interestingly, it doesn't look very high. Here are the results of an -alloc_objects -focus 'Metric' heap profile of the:

• Probe: pprof.localhost:4041.alloc_objects.alloc_space.001.pb.gz

• App: pprof.localhost.alloc_objects.alloc_space.001.pb.gz

In what scenario did this make a considerable difference? Do you happen to have allocation profiles?

[2opremio]

I've just realized that we are running Scope on the service with --probe.processes=false which will probably explain the difference. I will try again without this.

(No wonder I was surprised by the low CPU consumption).

If I get a similar profile, it will tell us that most of the allocations come from the metrics of the process topology.

[2opremio]

After re-enabling the process topology and connecting the browser I get a closer behavior but not as close as I expected:

Here's the App's -alloc_objects heap profile (with 4 connected probes in the dev environment):

And here's the same profile with focusing on revCons (-focus 'revCons'):

So, this would reduce object allocation in ~5%.

Anyways, the change LGTM. @bboreham mentioned offline that this change was done after profiling the object allocation https://github.com/weaveworks/weavedemo , I will take a look at that next.

[bboreham]

The expected reduction is about half the allocations seen through revCons. We save the accumulator list, but still have to build a new list.