G1 Concurrent Marking

---
name: g1-concurrent-marking
description: >
  G1's concurrent marking cycle: SATB and the pre-write barrier, the cycle phases and the
  single mark bitmap with TAMS, adaptive IHOP triggering, mark stack overflow and its
  consequences, humongous allocation and eager reclaim, and mixed-collection candidate
  selection. Use when the log shows "Concurrent Mark Restart for Mark Stack Overflow", when
  "Pause Full" follows incomplete marking cycles, when "Concurrent Mark From Roots" grows
  longer cycle over cycle, when marking starts well away from 45% occupancy, when someone
  proposes -XX:G1HeapOccupancyPercent or -XX:+G1SummarizeConcMark, or when humongous
  allocations are frequent. Does not cover regions, remembered sets and the evacuation pause
  itself (g1-internals), choosing flag values against an SLO (g1-tuning-for-slo), or
  configuring and parsing the GC log (gc-log-analysis).
---

# G1 Concurrent Marking

## Purpose

Decide why a G1 marking cycle is failing to do its job — starting too late for the real
old-generation allocation rate, restarting after object-mark stack overflow,
or never finishing before the
old generation fills — and which of those the evidence in the log actually supports. Marking
provides liveness for mixed-collection candidates. Late or aborted marking can reduce reclamation
headroom, but does not inevitably cause full GC; inspect the actual allocation/fallback cause.

The failure this prevents is treating the visible last event as the cause. A `Pause Full` at
the end of a chain needs its recorded cause and preceding capacity evidence. Explicit collection,
allocation pressure and evacuation failure require different investigations.

## Workflow

The detailed model is HotSpot JDK 25. Reuse the target's vendor/update, effective flags,
complete logs and accepted outcome budgets; preserve an older project's baseline. These
are implementation details, not Java language guarantees.

1. **Confirm which phase names you are reading.** G1's cycle includes `Pause Young (Concurrent
Start)`, `Concurrent Mark From Roots`, `Pause Remark`, `Pause Cleanup` and, on JDK 25,
   `Concurrent Cleanup for Next Mark`. `Pause Mark Start` / `Pause Mark End` are ZGC's —
   confirm the collector and hand off rather than applying G1 flags.
2. **Use a complete cycle for cycle-level conclusions.** A fragment can identify a recorded
   cause or phase, but not establish the missing chronology. If material evidence is absent,
   arrange bounded collection through `gc-log-analysis`; `-Xlog:gc*` includes the info-level
   phases. Read the instrumentation reference for additional overflow or IHOP detail.
3. **Check whether the trigger is adaptive before touching it.**
   Use the target's effective flags and `gc+ihop=debug` predictor state. A fresh
   `java -XX:+PrintFlagsFinal -version` shows that invocation's configuration, not the
   running service's overrides or learned state. With `G1UseAdaptiveIHOP=true`,
   `InitiatingHeapOccupancyPercent` supplies the initial threshold; a start away from 45%
   alone establishes neither a faulty nor a well-calibrated predictor.
4. **Track each trigger with its effective threshold, old-allocation rate, marking duration and
   post-cycle reclaim.** Rising start occupancy alone can reflect a changed old capacity/live set
   or a healthy adaptive threshold. “Too late” requires insufficient headroom for marking plus
   the mixed-collection/reclamation phase, evacuation failure or full-GC evidence.
5. **Classify the failure mode from the log line, not from the full GC.** Mark stack
   overflow, evacuation failure and humongous pressure produce different upstream evidence.
   Use the table in `references/marking-pathologies.md`.
6. **Separate humongous allocated from humongous reclaimed** before calling it pressure —
   eager reclaim returning regions every young GC is healthy behaviour, not a leak.
7. **Re-measure any region-size or IHOP change under the same allocation and promotion
   rate** as the original measurement, and check the effect on mixed collections: region size
   changes the Garbage-First granularity, not just the humongous threshold.

Finish with the supported phase/cause, remaining material uncertainty and the next discriminating
check or justified no-change decision. Healthy reclaim and sufficient headroom within accepted
application budgets do not require tuning.

## Rules

- `-XX:G1HeapOccupancyPercent` does not exist. The flag is
  `-XX:InitiatingHeapOccupancyPercent`, deprecated from JDK 27 and aliased to `-XX:G1IHOP`.
  Confirm every externally sourced flag with `-XX:+PrintFlagsFinal` before it reaches a
  production script.
- `-XX:+G1SummarizeConcMark` was removed by the unified logging work (JEP 158, JDK 9). Use
  `-Xlog:gc+marking=debug`. `-XX:+G1SummarizeRSetStats` went the same way and is already
  `Unrecognized VM option` on JDK 11 — the JVM refuses to start and suggests the flag that did
  survive. For remembered-set cost use `-XX:G1SummarizeRSetStatsPeriod=<n>` with
  `-Xlog:gc+remset=trace`.
- With `G1UseAdaptiveIHOP=true` (default), `InitiatingHeapOccupancyPercent` supplies the initial
  threshold until enough observations exist; the adaptive controller then derives the effective
  threshold. To impose a fixed value, disable adaptive IHOP explicitly. That can make a persistently
  regime-switching workload more predictable, but forfeits adaptation to live-set, allocation-rate
  and cycle-time changes; validate full cycles and headroom before/after.
- `Pause Cleanup` (STW) and `Concurrent Cleanup` (concurrent) are two distinct phases sharing
  a word. Reporting `Concurrent Cleanup` as a short STW pause is self-contradictory.
- The SATB barrier enqueues the **old** value of the field, never the new one. It guarantees
  snapshot reachability together with root processing, tracing and allocation liveness; the time
  a reference was assigned does not determine when its referent was allocated.
- SATB can conservatively over-retain, but must not under-retain. Floating garbage may survive
  until a later reclamation opportunity; a
  lost live object would be heap corruption. Never "optimise" the barrier away from that
  asymmetry.
- The humongous threshold is **strict**: `size > G1HeapRegionSize / 2`. An object of exactly
  half the region size is not humongous. Size payloads with clear margin below the threshold,
  never at it — a worst case that just grazes it makes part of the traffic humongous and part
  of it not, under the same nominal load.
- Since JDK 20 (JDK-8210708) there is a **single** `G1CMBitMap`, not a prev/next pair; TAMS
  distinguishes pre-snapshot from post-snapshot allocation, per region. Native bitmap
  overhead dropped from roughly 3% to 1.5% of the heap, and a concurrent
  "Rebuild Remembered Sets and Scrub Regions" phase appeared between `Pause Remark` and
  `Pause Cleanup`.
- Objects allocated at or above a region's TAMS are implicitly live and are never marked in
  the bitmap. That is what lets promotion continue during a cycle without forcing re-marking.
- Eager reclaim frees eligible humongous regions inside an ordinary STW pause. Eligibility is
  release-specific and considers marking/allocation timing, object kind/pointers, remembered-set
  evidence and pinning; it is not equivalent to “RSet size is zero”. Read `reclaim candidate` and
  `remset/marked/pinned` fields on the target build. An ineligible object may become eligible in a
  later pause or require a complete liveness cycle. **There is no boolean to turn this off from JDK 20.**
  `-XX:+G1EagerReclaimHumongousObjects` — experimental, default `true` — was accepted on 11
  through 19 and is `Unrecognized VM option` from 20 onward, so the JVM refuses to start on it
  (executed on Temurin 11, 17, 18, 19, 20, 21, 24 and 25). What remains is
  `-XX:G1EagerReclaimRemSetThreshold`, the eligibility cut-off itself: experimental and
  **ergonomic**, measured at 16 on JDK 17–24 and 32 on 25, so read it off the runtime rather
  than quoting a value.
- Full GC in G1 has been parallel since JDK 10 (JEP 307). It is still the most expensive
  operation the collector performs — because it processes the whole heap rather than a
  selected subset of regions, not because it is serial. Repeating "serial and therefore slow"
  leads people to read "no longer serial" as "no longer expensive".
- Separate object-graph marking, remembered-set rebuilding and evacuation heap-root scanning.
  A large RSet alone does not show that `Concurrent Mark From Roots` scans that RSet.
- Validate barrier/RSet overhead with phase-aligned CPU evidence and controlled workload trials.
  JMH `-prof gc` reports allocation/GC metrics; it does not isolate barrier instruction cost.

## Mark-stack overflow versus SATB pressure

The mark stack holds objects discovered but not yet scanned while tracing the graph. SATB buffers
hold old references from mutator stores and become additional marking roots. Heavy mutation can
increase marking work, but `Concurrent Mark Restart for Mark Stack Overflow` identifies an
object-mark stack overflow, not an SATB queue overflow or proof that `MarkStackSizeMax` was
reached. JDK 25 can restart before expanding the current capacity; allocation failure and the
maximum also limit growth. Correlate graph breadth/live-set shape, mark-stack expansion, concurrent CPU starvation,
SATB processing and cycle duration. Changing `G1SATBBufferSize` without evidence targets a
different structure; increasing `MarkStackSizeMax` trades native memory for headroom and may only
mask a workload/capacity problem.

## Production handling

Treat flag defaults and log strings as `(vendor, JDK feature, update, build)` facts. Capture the
effective command line and `-Xlog:gc+init`; canary any collector/region/IHOP change under the same
load, and validate tail latency, application throughput/CPU, allocation/promotion, marking CPU,
fallbacks and post-GC live occupancy. GC logs can expose class-loader and path metadata; restrict
and expire them like other production telemetry.

## References

- [The cycle, its log and its flags](references/marking-cycle-log-and-flags.md) — the phase
  sequence with correct names, an annotated log of a complete cycle, the logging and
  diagnostic flags with their defaults, JFR events, and the HotSpot source paths. Read when
  configuring marking instrumentation or reading a marking log for the first time.
- [Marking pathologies](references/marking-pathologies.md) — symptom-to-hypothesis-to-
  instrument table, the SATB buffer and overflow mechanism, evacuation-failure reconciliation,
  and the humongous threshold and eager-reclaim decision. Read when a cycle
  restarts, does not finish, or is followed by a full GC.