Optimizing JVM Settings for 1M Daily Logins on an 8GB Server

Step 1: Capacity Planning for a New System

Estimate object allocation per second, calculate young generation size, and predict GC frequency to decide how many instances and what memory each needs.

Compute memory used by objects created each second.

Estimate young generation size and Minor GC interval.

Adjust machine count and JVM heap to keep GC pauses acceptable.

Example: a login system with 1 M requests per day, peak 100 req/s, three 4C8G servers, each with 4 GB heap and 2 GB young generation can handle the load.

Step 2: Choosing a Garbage Collector

Decide between throughput‑oriented and latency‑oriented GC.

Throughput = CPU time for application / (CPU time for application + CPU time for GC)

Response time = average GC pause time

Large heap favors throughput; smaller heap reduces pause time. The trade‑off must be balanced.

GC Design Considerations

GC must stop the world (STW) to move objects.

STW pauses block request processing.

Young generation uses copying algorithms; old generation prefers non‑copying.

Goal: fewer, shorter GC pauses.

CMS vs. G1

Common combos: ParNew + CMS for low‑latency services, G1 for larger heaps (≥8 GB) and throughput‑oriented workloads.

Step 3: Partition Size Planning

Set heap size with -Xms and -Xmx (usually half of physical memory). Choose young generation size with -Xmn or -XX:NewRatio; typical values are 3/8 of heap for stateless services or up to 3/4 for lightweight services.

Set thread stack size with -Xss (usually 512 KB–1 MB).

Step 4: Stack Memory Size

Typical -Xss is 512 KB–1 MB; many threads can consume hundreds of megabytes.

Step 5: Object Tenuring Threshold

Adjust -XX:MaxTenuringThreshold (e.g., 5) so objects survive a few Minor GCs before promotion.

-Xms3072M -Xmx3072M -Xmn2048M -Xss1M -XX:MetaspaceSize=256M -XX:MaxMetaspaceSize=256M -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=5

Step 6: Direct Allocation to Old Generation

Use -XX:PretenureSizeThreshold=1M for large objects.

-Xms3072M -Xmx3072M -Xmn2048M -Xss1M -XX:MetaspaceSize=256M -XX:MaxMetaspaceSize=256M -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=5 -XX:PretenureSizeThreshold=1M

Step 7: CMS Old‑Generation Tuning

Typical flags:

-XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=70 -XX:+UseCMSInitiatingOccupancyOnly -XX:+AlwaysPreTouch

Step 8: OOM Dump and GC Logging

-XX:+HeapDumpOnOutOfMemoryError

-XX:HeapDumpPath=${LOGDIR}/

-Xloggc:/dev/xxx/gc.log -XX:+PrintGCDateStamps -XX:+PrintGCDetails

General JVM Parameter Template (ParNew+CMS)

-Xms4g -Xmx4g -Xmn2g -Xss1m -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=10 -XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=70 -XX:+UseCMSInitiatingOccupancyOnly -XX:+AlwaysPreTouch -XX:+HeapDumpOnOutOfMemoryError -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -Xloggc:gc.log

G1 Template for Large Heaps

-Xms8g -Xmx8g -Xss1m -XX:+UseG1GC -XX:MaxGCPauseMillis=150 -XX:InitiatingHeapOccupancyPercent=40 -XX:+HeapDumpOnOutOfMemoryError -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -Xloggc:gc.log

GC Selection Guidance

Small heap (<100 MB): Serial GC.

Single‑core: Serial GC.

Throughput‑oriented: Parallel GC.

Low‑latency: G1, ZGC, or CMS.

Metaspace vs. Permanent Generation

Metaspace resides in native memory, removing the fixed 64 MB limit of PermGen and avoiding OOM caused by class metadata.

-XX:MetaspaceSize=256M -XX:MaxMetaspaceSize=256M

Stop‑The‑World, OopMap, and Safepoints

GC pauses require all threads to stop at safepoints; OopMap records object reference locations for correct relocation.