JEP 189: Shenandoah: An Ultra-Low-Pause-Time Garbage Collector

[Andrew Dinn]

On 20/01/14 20:53, Krystal Mok wrote:
> Thanks for your answers. The point on using more threads to slow down
> mutators / speed up collectors is interesting.

Implementing a suitable scheduling algorithm (based on whatever
heuristics) is going to be a very 'interesting' challenge but it does
promise a gradual decrease in mutator throughput rather than simply
dropping off the edge of a cliff.

Azul's collector uses a heuristic to predict when to start its GC cycle
but -- as of last time I checked with Gil Tene -- that only extends to
decreasing or increasing the amount of time spent in mutation+collection
relative to the amount of time spent in mutation alone i.e. collection
is started in the latter N% of mutation time where N% is predicted to be
enough time to complete the cycle given the current burn rate, head room
and expected collection time as monitored over the last few GCs.

Obviously that only helps up until the point where mutation+collection
time = 100% of mutation time, at which point you have to hope the GC can
keep up -- that or just add more memory (Gil's suggestion and one which
has a lot to recommend it). However, if this limit case does need to be
dealt with then heuristics which either i) vary the collector thread
count or ii) modify the timeslice or frequency of collector threads look
like a possible remedy.

> What I was asking was mostly from a completeness perspective on the
> algorithm, not that the stop-the-world fallback will really happen in
> normal scenarios. It seems to me that, the way Shenandoah does
> concurrent evacuation through Brooks-style barrier will lead to it
> needing more headroom, and when there's not enough headroom it'll need
> to make sure it can handle all cases, right?

I believe Christine has already answered this in the affirmative.

regards,


Andrew Dinn
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