asmhatre/zgc-jdk25-benchmarks
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Add flags: production JVM flag cheat sheet for Spring Boot on JDK 25/27

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MaxRAMPercentage vs a hardcoded -Xmx, -XX:-UseContainerSupport, the
ActiveProcessorCount lie, and which diagnostic flags are worth leaving
on in prod, verified in real Docker containers on JDK 25 and JDK 27.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
This commit is contained in:
asmhatre
2026-10-01 04:34:04 +00:00
co-authored by Claude Sonnet 5
parent e83c9949d8
commit 2f88734f5d
22 changed files with 832 additions and 0 deletions
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flags/docs/output/03-activeprocessorcount-lie.txt
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$ docker run --rm --cpus=1 --memory=1g --entrypoint java flags-demo:jdk27 -XX:+PrintFlagsFinal -version | grep -E "ParallelGCThreads|ConcGCThreads|ActiveProcessorCount"
$ docker run --rm --cpus=1 --memory=1g --entrypoint java flags-demo:jdk27 -XX:ActiveProcessorCount=8 -XX:+PrintFlagsFinal -version | grep -E "ParallelGCThreads|ConcGCThreads|ActiveProcessorCount"
=== ActiveProcessorCount lying to the JVM: 1 real CPU, told there are 8 ===
--- JDK27, --cpus=1, no override (correct) ---
int ActiveProcessorCount = -1 {product} {default}
uint ConcGCThreads = 1 {product} {ergonomic}
uint ParallelGCThreads = 1 {product} {default}
--- JDK27, --cpus=1, -XX:ActiveProcessorCount=8 (lying) ---
int ActiveProcessorCount = 8 {product} {command line}
uint ConcGCThreads = 2 {product} {ergonomic}
uint ParallelGCThreads = 8 {product} {default}