RFC: G1 as default collector (for real this time)

[Thomas Schatzl]

Hi all,


   there have been some recent discussions around making G1 the default 
for all use-cases, both internally at Oracle and at the OpenJDK 
Committers Workshop. With this e-mail I want to bring this subject to a 
wider audience to gather feedback around potential problems with such a 
move.


As you all may know, G1 is the default collector in the Hotspot VM. 
However in some situations, some say in (too) many situations, the VM 
selects Serial GC "by default" anyway. :)

 From what I understand there are the following reasons to keep Serial 
GC _as default option_ in the context of "small" environments:

   * throughput: G1's large write barrier makes an argument about 
throughput being too far off and noticeable. Ongoing efforts ([1] which 
we plan for JDK 25) show that the difference is going to be much smaller 
if it ever was.

Further, as soon as Serial GC is running for longer this advantage 
diminishes a lot due to full collections and can result in G1 actually 
performing better.

   * (native) memory footprint: G1 has made great strides in native 
memory usage.

In the past particularly remembered sets were of concern, but their 
memory usage has been reduced significantly over the past few years.
E.g. with above change the entire young gen remembered set is also 
managed on the card table exactly like in Serial GC.

[I would also like to state that I would be surprised if remembered 
sets, with a recent JDK and G1, are ever an issue with heaps Serial GC 
targets]

Heap management tends to be worse with Serial GC, mostly due to its 
strict generational boundaries. G1's region based layout avoids wasting 
memory.

   * latency: if this has ever been a disadvantage, Serial GC's full 
collections are worse compared to G1's incremental collections.

   * startup: the time to start up the VM is not that different between 
these two collectors. Other components are much more relevant here.

   * historical inertia: at the time there was need to select a default, 
there has been nothing but Serial and Parallel GC. JDK 9 simply replaced 
Parallel GC as default for "server class" machines, probably as path of 
lesser resistance and at the time known shortcomings in some of the 
above areas.

Some initial testing showed that Serial GC much better when constraining 
it to the same environment (single thread, heaps < 1.7g) than G1.

At the same time, looking at the current situation from an end users 
point of view, it is very much confusing for them, getting a different 
garbage collector depending on environment, based on some somewhat 
arguable criteria?

This change would also make the expectations ("g1 is default since jdk 
9") match the actual behavior.

I am looking forward to hear about your opinion about making G1 
unconditionally default.

Thanks,
   Thomas

[1] https://bugs.openjdk.org/browse/JDK-8340827