RFR (M): 8212657: Implementation of JDK-8204089 Timely Reduce Unused Committed Memory

[Aleksey Shipilev]

On 12/18/18 11:39 AM, Thomas Schatzl wrote:
>> One more minor suggestion - G1PeriodicGCInterval should be defined in
>> milliseconds at the moment which seems impractical to me. Not clear
>> why somebody would need to trigger GC less than in minutes, so people
>> will always write a lot of 000. Can we update it to seconds at least?
> 
> You can use SI prefixes like "k" or "M" (probably do not need "G" :) )
> to reduce the number of zeros.
> 
> Not really hung up on the scale, I am going to ask for more opinions.
> We need to come to a conclusion relatively quickly though as we are
> late in the release process already.
> 
> I think the reason for choosing ms is because so far all times were in
> ms, and Shenandoah also uses that scale. Otoh J9 uses seconds too.

Shenandoah also chose "ms" for consistency reasons.

In practical terms, sometimes there is a compelling need to do _concurrent_ GC more frequently than
once a second. Think about tiny heap and GC taking just a few milliseconds. Then, to keep memory
footprint under control, it would make sense to do GC every, say, 300 milliseconds. This is probably
a bad idea for the STW GC, but still can be tolerable if GC cycle is in milliseconds range.

Unfortunately, we don't have support for SI prefixes going downwards from 1 (e.g. milli-, micro-),
right? Keeping the units in milliseconds, and slapping the SI prefix would be much more confusing
with 1K meaning "1 second"...

-Aleksey

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