RFR: 8338977: Parallel: Improve heap resizing heuristics [v22]

[SilenceZheng66]

On Sat, 13 Dec 2025 02:04:27 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/psParallelCompact.cpp line 1070:
>> 
>>> 1068:     size_policy->sample_old_gen_used_bytes(MAX2(pre_gc_values.old_gen_used(), old_gen->used_in_bytes()));
>>> 1069: 
>>> 1070:     if (UseAdaptiveSizePolicy) {
>> 
>> I think this code block could raise heap size don't expand in some specific case, maybe you should optimize it. @albertnetymk
>
> @SilenceZheng66 Could you elaborate the issue, ideally, in a JBS ticket? (A reproducer/gc-log would be nice, but detailed description works as well.)

@albertnetymk Sure.  I will give detailed description here.

GC：PS MarkSweep、PS Scavenge

JVM configuraton:  
-Xmx2g 
-Xms256M 
-XX:MetaspaceSize=256m 
-XX:MaxMetaspaceSize=256m 
-XX:+PrintGCDateStamps 
-XX:+PrintGCDetails 
-XX:-UseAdaptiveSizePolicy 
-XX:+PrintAdaptiveSizePolicy 
-XX:+HeapDumpBeforeFullGC 
-XX:+HeapDumpOnOutOfMemoryError 
-XX:HeapDumpPath=export/Logs/ 
-Xloggc:/export/Logs/gclogs.log 

JVM version：25.191-b12, although it's an old version without maintenance, I find out the problem might still in the latest version.

Problem Description: 
When starting a application(like SpringBoot), objects loading to VM continuously, my CPU usage suddenly began to skyrocket, followed by a full GC lasting over ten minutes with more than 2,700 occurrences. Examining the GC logs revealed that prior to the full GC, several successful young GC events had taken place until the old generation was completely filled. I read the source code and find out when -XX:-UseAdaptiveSizePolicy was set,  VM thread can't expand psOldGen, only GC worker can expand psOldGen's size in that condition. I assume that the transition from the younger generation to the older generation has been successful, so that GC worker's expansion was not reached, that made long term full GC happened.

Check List:
1. Container has enough place for heap expansion
2. Xms and Xmx were setting differently, same configuration in Serial GC can raise heap expansion

GC log sample:
// Full GC Starting
[PSYoungGen: 76276K->10735K(76288K)] 244793K->181638K(251392K), 0.0078092 secs] [Times: user=0.04 sys=0.00, real=0.01 secs] 
2025-12-10T14:11:58.663+0800: 24.604: [Heap Dump (before full gc): , 0.0000482 secs]2025-12-10T14:11:58.663+0800: 24.604: [Full GC (Ergonomics) [PSYoungGen: 10735K->0K(76288K)] [ParOldGen: 170902K->173351K(175104K)] 181638K->173351K(251392K), [Metaspace: 107770K->107770K(1146880K)], 0.5258832 secs] [Times: user=1.82 sys=0.00, real=0.52 secs] 
2025-12-10T14:11:59.268+0800: 25.210: [Heap Dump (before full gc): , 0.0000684 secs]2025-12-10T14:11:59.269+0800: 25.210: [Full GC (Ergonomics) [PSYoungGen: 65536K->0K(76288K)] [ParOldGen: 173351K->174276K(175104K)] 238887K->174276K(251392K), [Metaspace: 109096K->109096K(1148928K)], 0.3031080 secs] [Times: user=1.10 sys=0.01, real=0.30 secs] 
2025-12-10T14:11:59.710+0800: 25.651: [Heap Dump (before full gc): , 0.0000752 secs]2025-12-10T14:11:59.710+0800: 25.651: [Full GC (Ergonomics) [PSYoungGen: 65536K->0K(76288K)] [ParOldGen: 174276K->174980K(175104K)] 239812K->174980K(251392K), [Metaspace: 110351K->110351K(1148928K)], 0.3664816 secs] [Times: user=1.21 sys=0.00, real=0.37 secs]
// Full GC Running
2025-12-10T14:12:52.408+0800: 78.350: [Heap Dump (before full gc): , 0.0000535 secs]2025-12-10T14:12:52.409+0800: 78.350: [Full GC (Ergonomics) [PSYoungGen: 65536K->64255K(76288K)] [ParOldGen: 174893K->174893K(175104K)] 240429K->239149K(251392K), [Metaspace: 148330K->148330K(1183744K)], 0.2442300 secs] [Times: user=0.68 sys=0.00, real=0.25 secs] 
2025-12-10T14:12:52.656+0800: 78.598: [Heap Dump (before full gc): , 0.0000711 secs]2025-12-10T14:12:52.657+0800: 78.598: [Full GC (Ergonomics) [PSYoungGen: 65536K->64333K(76288K)] [ParOldGen: 174893K->174893K(175104K)] 240429K->239227K(251392K), [Metaspace: 148379K->148379K(1183744K)], 0.2102640 secs] [Times: user=0.53 sys=0.02, real=0.21 secs] 
2025-12-10T14:12:52.871+0800: 78.813: [Heap Dump (before full gc): , 0.0000622 secs]2025-12-10T14:12:52.872+0800: 78.813: [Full GC (Ergonomics) [PSYoungGen: 65534K->64252K(76288K)] [ParOldGen: 174893K->174893K(175104K)] 240428K->239146K(251392K), [Metaspace: 148488K->148488K(1183744K)], 0.2125166 secs] [Times: user=0.54 sys=0.00, real=0.22 secs]  
// Full GC Ending
2025-12-10T14:32:47.691+0800: 1273.632: [Heap Dump (before full gc): , 0.0000943 secs]2025-12-10T14:32:47.691+0800: 1273.633: [Full GC (Ergonomics) [PSYoungGen: 65536K->24273K(76288K)] [ParOldGen: 174627K->174626K(175104K)] 240163K->198899K(251392K), [Metaspace: 152567K->152567K(1187840K)], 0.3627505 secs] [Times: user=1.12 sys=0.00, real=0.37 secs] 
2025-12-10T14:35:18.172+0800: 1424.113: [Heap Dump (before full gc): , 0.0001014 secs]2025-12-10T14:35:18.172+0800: 1424.114: [Full GC (Ergonomics) [PSYoungGen: 65536K->9012K(76288K)] [ParOldGen: 174626K->174852K(175104K)] 240162K->183865K(251392K), [Metaspace: 152611K->152611K(1187840K)], 0.2574051 secs] [Times: user=0.63 sys=0.00, real=0.26 secs] 
2025-12-10T14:38:49.044+0800: 1634.986: [Heap Dump (before full gc): , 0.0001049 secs]2025-12-10T14:38:49.045+0800: 1634.987: [Full GC (Ergonomics) [PSYoungGen: 65536K->0K(76288K)] [ParOldGen: 174852K->133113K(175104K)] 240388K->133113K(251392K), [Metaspace: 152680K->152680K(1187840K)], 0.3655573 secs] [Times: user=1.01 sys=0.00, real=0.36 secs] 
2025-12-10T14:42:49.842+0800: 1875.784: [GC (Allocation Failure) AdaptiveSizePolicy::update_averages:  survived: 1671680  promoted: 0  overflow: false

If you need other details just let me know, thx!

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PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2619293977