util: delay creation of debug context

[ofrobots]

Now that it is really simple to take profiles with node (thanks to #2090), I was looking a profile I got from a trivial JSON.stringify benchmark.

I was really surprised by the number of ticks I got in RunInDebugContext. The full profile is here but here's an excerpt:

38    0.7%    0.7%  node::ContextifyContext::RunInDebugContext(v8::FunctionCallbackInfo<v8::Value> const&)

I looked into this. It seems that #1471 introduced a change which now creates a Debug context by default in the util module. This is needed to be able to inspect promises. I think this is fine, but we should not be doing the really expensive operation of creating a Debug context on default startup of node. This patch fixes that.

Based on my measurements, this saves ~24ms in node startup time measured as follows:

#!/usr/bin/bash
# Note: this requires GNU date
start=$(date +%s%N)
for c in {0..60}
do
  iojs -e " "
done
new=$(date +%s%N)
elapsed=$(((new - start)/60))
echo $elapsed

This reduces startup time from ~90ms → ~66ms on my machine. I think it is important that node startup is as fast as possible. I don't like deferred initialization as I am proposing, but I think it is a much lesser evil than spending 24ms on each node startup.

R=@bnoordhuis, @monsanto

[mscdex]

LGTM

[targos]

LGTM, the difference is huge !

[bnoordhuis]

LGTM. Obvious in hindsight.

[Fishrock123]

So does this mean that the first time you console.log a promise it'l take like 24ms?

[monsanto]

LGTM

[rvagg]

https://jenkins-iojs.nodesource.com/job/iojs+any-pr+multi/194/

[ofrobots]

@Fishrock123 Yes, it is going to be expensive for the first promise that is inspected.

[cjihrig]

Definitely worth the tradeoff IMO. LGTM.

[ofrobots]

The CI looks decent to me, ignoring the flaky failures. I will land this later today.

[ofrobots]

Landed in ab47965.