(Splitting this part of the telemetry out from the timing stuff.) How often does a CC force a GC? This would have a nasty impact on responsiveness if it is common. It happens when there's an overflow while ungraying.

Hmm. _This_ may be what's been biting me on m-c. I was certainly seeing a slew of CCs and GCs all in a row. Could we also add this info the to the CC/GC logging we do to the console?

Ideally, this won't happen much, so if it shows up in about:telemetry that would be a sign that something is wrong. Though those overflows seemed to happen a lot in Thunderbird. But yes, that's a good idea. Another approach would be to enrich the GC's interface a bit, so you could pass in a string that tells it why you are invoking it, and it could print that out in the reason.

(In reply to Andrew McCreight [:mccr8] from comment #2) > Ideally, this won't happen much, so if it shows up in about:telemetry that > would be a sign that something is wrong. Though those overflows seemed to > happen a lot in Thunderbird. But yes, that's a good idea. Another approach > would be to enrich the GC's interface a bit, so you could pass in a string > that tells it why you are invoking it, and it could print that out in the > reason. That sounds like a great idea, but I vote for an enum.

(In reply to Taras Glek (:taras) from comment #3) > That sounds like a great idea, but I vote for an enum. The only reason I was suggesting a string is that having a type defined that works in both JS land and non-JS land can be a pain, but I guess I can just put it wherever the signature for the external JS hook lives and it shouldn't be a problem. I'll look into filing a new bug for that.

Filed bug 706227 for adding an interface for API users to communicate a reason for the GC.

The control flow here is a little weird. I added NS_LIKELY mostly as a way to indicate to the reader what the hot path is. We only want to ping telemetry on non-shutdown CCs, as those are always going to force a GC, so we don't want to skew the stats one way or the other by pinging in those cases.

Comment on attachment 579107 [details] [diff] [review] add telemtry for GC forcing CC Review of attachment 579107 [details] [diff] [review]: ----------------------------------------------------------------- ::: xpcom/base/nsCycleCollector.cpp @@ +2685,5 @@ > > nsCycleCollectionJSRuntime* rt = > static_cast<nsCycleCollectionJSRuntime*> > (mRuntimes[nsIProgrammingLanguage::JAVASCRIPT]); > + if (NS_LIKELY(!aForceGC)) { These NS_LIKELY are probably not really important... I think evidence that they help is scant.

Okay, I just removed the NS_LIKELYs. The comment is probably enough to figure out what is happening. Thanks for the review. https://hg.mozilla.org/integration/mozilla-inbound/rev/ad9fce0aed78

Taras pointed out to me that this measure has gone up 257.71% recently. Of course, this is hit so rarely that could just mean that 3 people are hitting it instead of 1...