Optimize Memory Allocation

Optimize JVM Memory Allocation

The MCP fetches the data — allocation rankings, allocation flamegraphs, host metrics; you analyze: name the hot allocation sites, identify the pattern, recommend and apply the fix.

Java/JVM only. zymtrace memory-allocation profiling is supported only for the JVM — it reads the allocation profile, stacks weighted by bytes allocated (and object count), not on-CPU time. A non-Java workload has no allocation profile: stay in optimize-cpu-workloads for its on-CPU hotspots. For GPU/device memory use optimize-gpu-workloads. A service that's crashing/OOMKilled rather than churning is a profiler/backend problem — route to troubleshoot-zymtrace-profiler.

The common discipline — data-source policy, pre-flight, rank-first vs. drill-down, scope-to-own-code/ROI, the always-recommend-and-apply-the-fix rule, the output-template skeleton, severity sizing, and security — lives in shared/analysis-conventions.md. Read it. This skill adds the allocation-specific protocol and call-tree rendering on top.

Connection setup lives in configure-zymtrace-mcp; this skill assumes the MCP is connected.

Enable allocation profiling first — it's opt-in

JVM allocation profiling is off by default. Before analyzing, confirm the Java service actually has an allocation profile (try the allocation ranking / flamegraph for it). If it's empty, it isn't enabled yet — turn it on, wait for profiles to arrive, then analyze. Don't fabricate an analysis when there's no allocation data; enable and wait. Full reference: https://docs.zymtrace.com/allocation-profiling.

Prerequisite: the target runs a supported JVM — OpenJDK, Azul Zulu, or Azul Zing.

To enable, ask the user for these inputs, then apply:

• Target — a CEL expression scoping which workloads to profile (by container / deployment / host / project). Get the scope from the user; see the docs for CEL syntax.
• Sampling interval — bytes between allocation samples. Default 16mib (production); 512kib for fine-grained at higher overhead.
• JVMTI backend — use JVMTI as the sampling backend? Default on where supported.

Apply it one of two ways (per the docs):

1. Agent config — no redeploy (preferred). Set an agent rule via the agent-config MCP tool if your client exposes one, else the gateway API POST <gateway-url>/public/api/v1/agent/config/set — read the exact request body from <gateway-url>/api-docs/openapi.json (don't guess field names). This is also the Settings → Agent Config / Allocation Profiles page in the UI.
2. Profiler startup flag — needs a profiler restart. -alloc-profile=<interval> (env ZYMTRACE_ALLOC_PROFILE), e.g. -alloc-profile=16mib, plus optional -use-jvmti (env ZYMTRACE_USE_JVMTI). See install-zymtrace-profiler. Use this when editing the profiler launch is easier than a live rule.

Why allocation matters

Allocation is rarely free: high allocation rate drives GC frequency and pause time, and the allocation + collection work burns CPU that never shows up as your own hot function. So an allocation analysis pays off two ways — fewer/shorter GC pauses (latency) and reclaimed CPU (throughput / cost). When you've quantified the CPU recovered, dollarize it with the cost method in optimize-cpu-workloads § Cost: annualize the cores.

The allocation analysis protocol

The MCP pulls the data; you do the analysis. Establish a data path first (pre-flight, in the shared doc), and confirm allocation profiling is enabled (above) — if the Java service has no allocation profile yet, enable it and wait for data before continuing.

1. Rank first if the request is rank-shaped ("what's allocating the most", "which code creates the most garbage") — use topentities / topfunctions on the allocation dimension (concise rankings), then drill into the top site with hot_traces. Rank by bytes allocated over the window (show the figure per entry); include host and container names where the data has them. Mark unmodifiable runtime/library allocations (JIT, classloading, framework internals you can't change) non-actionable and drill into the highest user-owned site. See scope-to-own-code/ROI in the shared doc.

2. Pull allocation-related metrics first, for context. Allocation rate (bytes/sec), GC time and frequency, heap used / churn — to establish whether GC pressure is actually a problem or allocation is cheap here. Carry these into the recap; they tell you if the fix is worth it.

3. Pull the allocation call tree at the named scope (executable / container / pod / host / time range) on the allocation dimension — use the hot_traces MCP tool when available (zymtrace 26.5.1+), else fall back to flamegraph (see the data-source policy in the shared doc). Read which allocation sites dominate by bytes.

4. Name the pattern. The MCP returns raw allocation sites + bytes; naming the dominant pattern is your job. Common JVM patterns:

   • Autoboxing churn — Integer.valueOf, Long.valueOf, boxed types in collections / streams (Map<Integer,…>, Collectors.toList() on a boxed stream).
   • String churn — String concatenation in loops, String.format, substring, getBytes, repeated toString(); StringBuilder growth.
   • Collection resizing — ArrayList/HashMap grown from default capacity (Arrays.copyOf, HashMap.resize) because no initial size was given.
   • Buffer / array churn — new byte[] / char[] per request for I/O, serialization, or codecs that could be pooled or reused.
   • Transient object graphs — per-request DTOs and JSON (de)serialization (Jackson/Gson) allocating wrappers, nodes, and temp maps.
   • Lambda / stream / iterator allocations — boxed streams, capturing lambdas, iterator objects in hot loops.

5. Write the recap using the output template (shared doc) with the allocation call-tree rendering below.

6. Apply the fix (shared doc — "Always recommend a fix — then apply it"). The recap is the midpoint, not the finish line. Typical allocation fixes:

   • Presize collections — new ArrayList<>(n), new HashMap<>(expected, 1f).
   • Kill boxing — primitive arrays or a primitive-collection lib (Eclipse Collections, fastutil), IntStream over Stream<Integer>.
   • Reuse buffers — pool byte[]/ByteBuffer, reuse StringBuilder, stream instead of materializing.
   • Trim the hot path — hoist invariant allocations out of loops, replace String.format with StringBuilder/append, gate debug-log string building behind level checks.

Allocation call-tree rendering (Observed Call Tree section)

The output-template skeleton is in the shared doc. The allocation Observed Call Tree block renders like this:

**Observed Call Tree — allocation profile** (<process / container / host / time range>)

<top-level frame>  (<bytes / % of allocated>)
├── <child frame>  (<bytes / %>)
│   └── <leaf alloc site>  (<bytes / %>)  ← allocating type/call, e.g. `Integer.valueOf`, `new byte[]`, `HashMap.resize`
└── <child frame>  (<bytes / %>)  ← hot allocation path

Call-tree conventions:

• The tree is the allocation profile — frames weighted by bytes allocated (with object count when the MCP returns it), not on-CPU time.
• Use ├── and └── for the hierarchy (matches what the MCP returns).
• Annotate leaves with the allocating type or call site and its byte share; mark the dominant path with ← hot allocation path.
• Keep frame / type names exactly as the MCP returns them; don't paraphrase.

Done (allocation-specific, on top of the shared checklist)

• Allocation profiling confirmed enabled for the Java service; if it wasn't, enabled it (target CEL + sampling interval + JVMTI asked of the user, applied via agent config / agent/config/set or profiler flag) and waited for data before analyzing.
• Ranked first when rank-shaped, on the allocation dimension by bytes; runtime/library allocations marked non-actionable, the top user-owned site drilled into.
• Allocation metrics pulled first (allocation rate, GC time/frequency, heap churn) and carried into the recap — so the fix's worth is established.
• Allocation call tree pulled at the named scope; the dominant pattern named (boxing, String churn, collection resize, buffer churn, transient graphs).
• Impact framed as GC pause + CPU reclaimed; CPU portion dollarized via the CPU skill's cost method when quantified.

(Plus the common Done checklist in the shared doc — template, every 🔴/🟡 has a Fix:, fix applied, follow-up question.)

Common pitfalls

• Analyzing before it's enabled. Empty allocation data means profiling is off (it's opt-in), not that the service doesn't allocate. Enable it (agent config / profiler flag), wait for data, then analyze — never invent an analysis from no data.
• Reading bytes as if they were CPU time. The allocation profile weights by bytes allocated, not on-CPU samples — a heavy allocator may be cheap if GC keeps up. Pull the GC/allocation-rate metrics (step 2) before calling it a problem.
• Chasing allocations the user can't change. JIT, classloading, and framework internals show up but aren't action items — mark them non-actionable and lead with the top user-owned site.
• Confusing allocation churn with a leak. This profile shows allocation rate, not retained/leaked memory. A steady-state churner isn't a leak; for OOM/leak-and-crash, route to troubleshoot.
• Expecting the MCP to name the pattern. It returns sites + bytes; naming boxing/String-churn/resize and the fix is your job.

Security constraints

• Common rules (ground in returned data, never analyze local profile files, never query the DB directly) are in shared/analysis-conventions.md.
• Always ground the recommendation in the data the MCP returned (allocation sites, bytes, percentages) — don't fabricate signals the data doesn't show.