# TODO — memory / allocation optimizations

Background: the scanner runs millions of virtual threads (one per port probe today).
Per-probe allocation churn, not peak RSS, is the main memory pressure source (GC load).
Hot path: per host -> `ScanHostTask` -> per port -> `ScanPortTask` / `ScanUdpPortTask`.

These items were reviewed and approved in this order. Nothing is implemented yet.
Constraint (multi-scan): Scanner must remain able to run several parallel scans with
different configurations and targets in one JVM. Anything static must be a true
compile-time constant, never per-scan state.

## Phase 2 — approved small changes (do when green-lit, one commit each)

- [ ] `ScanPortTask` / `ScanUdpPortTask`: make `waitInMillis` / `waitInNanos`
      `static final` (they are hardcoded 100 ms constants, not config — safe for
      multi-scan). Saves 16+ bytes on every port task.
- [ ] `PortRangeIterator`: delete dead field `done` (never read).
- [ ] `PortRangeIterator`: remember last `nextSetBit` result so `next()` does not
      rescan; `hasNext()` + `next()` currently both call `nextSetBit`.
- [ ] Parse port range **once per scan** (in `Scanner`/context), not per host:
      remove `new PortRange(scan.getPorts())` from `ScanHostTask`; workers iterate
      the shared immutable BitSet with a plain int cursor. Saves per host: one
      string parse + up to two 8 KB BitSet copies.
- [ ] `CliPrinter`: pre-compile the `stty size` regex as a `static final Pattern`.
      NOTE: the `getColumnWidth()` process spawn runs on purpose (terminal can be
      resized) — do NOT cache the width itself.
- [ ] `CliPrinter.printErrors` / `printPortsResults` / `PortList.getPorts()`:
      replace `String.format` + `stream().boxed().toList()` with direct
      `StringBuilder` appends over the BitSet.

## Phase 2b — deferred (user: not implemented now, revisit later)

- [ ] **`LinuxUdpSocketAPI` allocation churn per probe** (see
      `wrapNativeMethodCall`, `destination`, `sendEmptyDatagram`)
      Today every native call allocates: one errno call-state segment,
      one rebuilt `Object[] argv`, plus the `destination` segment per port
      (4–5 small allocations per probe).
      - Idea: allocate the errno state + argv arrays once per probe (or per
        worker) and pre-size the destination segment.
      - **Hard constraint (user):** the errno call-state must NOT be a single
        segment shared across all virtual threads. Any reuse must stay scoped
        to one worker thread / one probe at a time; otherwise keep status quo.
      - Also review the per-port `Arena.ofConfined()` + `new LinuxUdpSocketAPI`
        pair (confined arenas assume thread affinity which virtual threads
        do not guarantee across park/yield).

## Phase 3 — deferred (measure first)

- [ ] **Replace "1 virtual thread + Callable + CompletionService per port" with a
      fixed worker pool per host**
      - Start N persistent virtual worker threads per host
        (N = min(maxWorkersPerHost, socketLimit), capped), pulling ports from a
        shared cursor and pushing `PortResult`s to a shared queue the consumer
          drains (today: `PortScanResultAccumulator` + `CompletionConsumer`).
      - Removes per port: the vthread, `ScanPortTask`, Integer boxes (×2),
        Runnable wrapper, `CompletableFuture`, `QueueElement`, `PollState`.
      - ~40–50 % fewer bytes per probe + far less GC + no per-port
        thread-create/park cycles.
      - Caveats: touches cancellation / "partial results on Ctrl-C" behavior
        (see `CompletionConsumer` contract). Keep `socketLimit` semaphore as the
        global throttle. Must stay compatible with parallel scanners.
      - Only do if Phase 2 numbers still show GC pressure dominated by per-port
        scaffolding. Measure with `-Xlog:gc*`, JFR `jdk.ObjectAllocationInNewTLAB`,
        wall-time on a fixed target set (e.g. `ips-for-testing.txt` × `1-1024`).

## Phase 4 — optional / measure first

- [ ] **Reduce per-port atomic traffic in `Scan` counters**: `portStart()` /
      `portFinish()` do 3 CAS each per port. `socketCounter` increments in exact
      lockstep with `portCounter` in both tasks — consider dropping it or packing
      counters to cut ~1/3 of per-port CAS work. CPU-only win, profile before.

## Explicitly NOT doing (decisions)

- `CliPrinter.getColumnWidth()`: keep the `stty size` process spawn per call
  (intentional: terminal can be resized).
- `LinuxUdpSocketAPI`: do NOT share the errno call-state segment across
  worker threads (would be a data race). Keep per-call allocation.
- Do NOT remove the default online check to "save" exception stack traces —
  it is a deliberate memory saver (prevents 1024 timed-out throws per offline
  host). Keep `PortResult` exceptions un-retained (accumulator already stores
  only `ExceptionInfo`).