RFR: 8234312: ZGC: Adjust warmup criteria

[Stefan Karlsson]

Looks good.

StefanK

On 2019-11-18 11:15, Per Liden wrote:
> JDK-8232001 introduced logic which ignores "Metastace GC Threshold" 
> collections until the GC has warmed up. While this works in principle, 
> it also causes some intermittent metaspace test failures. These tests 
> use a small MaxMetaspaceSize, and aggressively loads and unloads 
> classes. It's not completely obvious why these tests started to fail 
> after JDK-8232001. However, we will be collecting metaspace a little 
> bit later now, which could mean we have more fragmentation and are not 
> able to free as much memory when the GC eventually happens.
>
> This patch reverts to the old behavior (we no longer ignore "Metspace 
> GC Threshold" collections until the GC is warm), but instead we only 
> consider the GC to be warm once we've done three "Warmup" GCs.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8234312
> Webrev: http://cr.openjdk.java.net/~pliden/8234312/webrev.0
>
> Testing: Tier1-7 on ZGC. Multiple manual runs of 
> vmTestbase/gc/gctests/LoadUnloadGC (the test that intermittently fails 
> in our CI).
>
> /Per