RFR: 8234312: ZGC: Adjust warmup criteria

[Per Liden]

Thanks Stefan!

/Per

On 11/19/19 8:21 PM, Stefan Karlsson wrote:
> 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
>