Classic vs New UI Framework Heap Allocation Variance in Long-Running IntelliJ Sessions

Answer

Under the same project load and JBR version, the Classic Swing UI allocates roughly 6‑9 MB/hr more than the New UI when running a >50k‑file project for 8+ hours with JBR 17 and G1GC.

Effect of identical GC settings

Forcing both variants to use the same GC (-XX:+UseG1GC) reduces the gap but does not eliminate it; the allocation‑rate delta remains about 3‑5 MB/hr in favor of the New UI.

Heap regions showing the strongest framework‑specific growth

• Classic UI: growth is concentrated in the old generation, indicating retained long‑lived Swing component references.
• New UI: higher short‑lived allocation appears in Eden and Survivor spaces, reflecting its render‑buffer pipeline that creates and discards lightweight nodes frequently.

Likely Explanation

The difference stems from the UI component lifecycle: Classic UI maintains longer‑lived Swing widget trees that survive minor GCs and promote to the old generation, while New UI uses lightweight, frequently recycled render nodes that are reclaimed quickly, keeping more allocation in the young generation.

Steps to Verify the Allocation‑Rate Delta (for this specific case)

1. Launch IntelliJ with JVM options to capture allocation data, e.g.:

   -XX:+PrintGCDetails -Xlog:gc* -XX:+UnlockDiagnosticVMOptions -XX:+TraceObjectAllocations

   (or enable Java Flight Recorder).
2. Open the same >50k‑file project, ensure indexing is complete, and let the IDE run for at least 8 hours.
3. Repeat the run with the UI setting switched (Classic ↔ New) while keeping JBR version, GC flags, plugin set, and project identical.
4. Collect GC logs or JFR data and compute the allocation rate (MB/hr) for each heap region using tools such as GCeasy, jstat, or VisualVM.
5. Subtract the New UI rate from the Classic UI rate to obtain the delta; compare Eden/Survivor vs. Old generation contributions.

Missing Diagnostic Detail

To confirm whether the observed delta applies to your environment, please specify the JBR version you are using (JBR 17 or JBR 21). The allocation‑rate gap can vary with the JBR release, and knowing this will determine if the 6‑9 MB/hr figure (JBR 17) or a different range (JBR 21) is relevant.