If concurrent GC degenerates during marking, the degen GC fails to make progress because the evacuation budget fails to include memory that had been reserved for the Collector.

Reproducer:

echo Run OpenJDKTip GenShen GC with memory size 54g with 6s customer period
>&2 echo Run OpenJDKTip GenShen GC with memory size 54g with 6s customer period
~/github/shenandoah.experiments.9-11-2023/build/linux-x86_64-server-release/jdk/bin/java \
  -XX:+UnlockExperimentalVMOptions \
  -XX:-ShenandoahPacing \
  -XX:+AlwaysPreTouch -XX:+DisableExplicitGC -Xms54g -Xmx54g \
  -XX:+UseShenandoahGC \
  -XX:ShenandoahGCMode=generational \
  -Xlog:"gc*=info,ergo" \
  -XX:+UnlockDiagnosticVMOptions \
  -jar ~/github/heapothesys/Extremem/target/extremem-1.0-SNAPSHOT.jar \
  -dInitializationDelay=45s -dDictionarySize=16000000 -dNumCustomers=28000000 \
  -dNumProducts=64000 -dCustomerThreads=2000 -dCustomerPeriod=6s -dCustomerThinkTime=1s \
  -dKeywordSearchCount=4 -dServerThreads=5 -dServerPeriod=5s -dProductNameLength=10 \
  -dBrowsingHistoryQueueCount=5 \
  -dSalesTransactionQueueCount=5 \
  -dProductDescriptionLength=64 -dProductReplacementPeriod=25s -dProductReplacementCount=5 \
  -dCustomerReplacementPeriod=30s -dCustomerReplacementCount=1000 -dBrowsingExpiration=1m \
  -dPhasedUpdates=true \
  -dPhasedUpdateInterval=60s \
  -dSimulationDuration=20m -dResponseTimeMeasurements=100000

Build:
commit 69aa97198c4e71051492f368e37aa50fc75834ee (HEAD -> experiments-9-11, origin/master, origin/HEAD, master)
Author: William Kemper <wkemper@openjdk.org>
Date: Fri Sep 8 17:40:46 2023 +0000