Conas a fheidhmíonn cuimhne JVM agus bailiú dramhaíola?

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Bainistríonn an JVM **cuimhne go huathoibríoch** — ag sannadh oibiachtanna agus ag saor a dhéanamh ar na cinn nach féidir a bhaint amach trí **bailiú dramhaíola (GC)** — mar sin níor cheart duit cuimhne a shaoradh de láimh. Is ríthábhachtach tuiscint a fháil ar na réigiúin cuimhne agus ar iompar an GC chun feidhmíochta a bhreisiú agus fadhbanna cuimhne a dhiagnóisiú. ## Réigiúin cuimhne an JVM ```text HEAP — where all OBJECTS live (shared across threads), GC operates here ├── Young Generation — new objects (most die young) │ ├── Eden — new allocations go here │ └── Survivor — objects that survived a young GC └── Old (Tenured) Gen — long-lived objects (promoted from young) STACK — per-thread; method frames, local variables, primitive locals, references Metaspace — class metadata (replaced PermGen in Java 8+) ``` Maíonn oibiachtanna ar an **heap** ; maíonn athróga áitiúla agus tagairtí ar gach **stack** gach snáithe. Díríonn an GC ar an heap. ## An hipitéis giniúnach ```text Observation: MOST objects die young (short-lived). → GC splits the heap into generations and collects the young gen FREQUENTLY (cheap, fast) and the old gen RARELY (expensive). ``` ## Conas a fheidhmíonn GC: marcáil agus geal ```text 1. MARK — starting from GC roots (stack refs, static fields), mark all REACHABLE objects 2. SWEEP — reclaim memory of unmarked (unreachable) objects 3. COMPACT— (often) move survivors together to reduce fragmentation ``` Is dramhaíola an t-oibiacht nuair nach bhfuil sé **inrochtana** ó aon GC root. Ní gá duit a shaorú — aimsíonn an GC agus aisghabhann sé é. ## Minor vs Major GC ```text Minor GC — collects the Young Generation; frequent, fast, short pause Major/Full GC — collects the Old Generation (and more); rarer, longer pause → long Full GC pauses ("stop-the-world") are a common performance concern ``` ## Halgartaim GC (roghnaigh de réir ualach oibre) ```text G1 GC (default, Java 9+) — balanced, predictable pause times, good general choice Parallel GC — max throughput (batch jobs), longer pauses acceptable ZGC / Shenandoah — very low pause times for large heaps (latency-sensitive apps) ``` ## Leatadar cuimhne fós ann (in ainneoin GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List