rustfs/docs/architecture/scheduler-baseline.md

Scheduler Baseline Inventory

This inventory covers G-011 for rustfs/backlog#675. It is a docs-only snapshot of the current scheduling, backpressure, worker, scanner, heal, and runtime-builder ownership. It does not define new behavior.

Current Owners

Surface	Current owner	Current responsibility	Migration boundary
ConcurrencyManager	rustfs/src/storage/concurrency/manager.rs	Owns the RustFS S3 read-path disk-read semaphore, I/O metrics, priority queue, storage media detection, access-pattern detection, and buffer strategy.	Keep request admission and I/O metrics behavior stable until a controller can consume the same state explicitly.
I/O scheduler core	crates/io-core/src/scheduler.rs	Owns the reusable buffer-size and priority algorithms consumed by the RustFS S3 read path (rustfs/src/storage/concurrency/io_schedule.rs). The former SchedulerManager facade in rustfs-concurrency was removed as zero-caller dead code (backlog#1025).	Treat rustfs-io-core as the reusable algorithm surface; the RustFS S3 read path owns its own scheduling wiring.
RustFS backpressure monitor	rustfs/src/storage/backpressure.rs	Tracks object-pipe watermark state used by RustFS storage backpressure tests and helpers, using the shared PipeBackpressurePolicy from crates/concurrency/src/backpressure.rs. The former BackpressureManager/BackpressurePipe facade in rustfs-concurrency was removed as zero-caller dead code (backlog#1025).	Preserve current state labels and watermark semantics; keep pipe sizing and watermark policy separate from object-read disk semaphore admission.
Workers	crates/concurrency/src/workers.rs	Provides cooperative worker-slot admission with take, give, and wait; current background workflows use it for bounded set workers.	Preserve blocking/wakeup semantics and over-release clamping.
Scanner cycle budget	crates/scanner/src/scanner_budget.rs	Cancels a child token when runtime, object-count, or directory-count budget is reached.	Preserve partial-cycle reason mapping and checkpoint accounting.
Heal admission	crates/heal/src/heal/manager.rs, crates/heal/src/heal/channel.rs, rustfs_common::heal_channel	Owns priority queue admission, duplicate merge/drop/full results, active-task tracking, retry admission, and channel responses.	Preserve low-priority scanner behavior and high-priority escalation gates.
Tokio runtime builder	rustfs/src/server/runtime.rs	Builds the multi-thread runtime from env/defaults, sets thread counts, stack, queue/event intervals, I/O event cap, thread name, and optional dial9 tracing.	Keep runtime defaults and env names stable when later startup phases move ownership.

Current Flow

flowchart TD
    http["HTTP/S3 request"] --> app["app object usecase"]
    app --> guard["ConcurrencyManager::track_request"]
    app --> permit["disk-read semaphore permit"]
    permit --> strategy["I/O queue status and buffer strategy"]
    strategy --> ecstore["ECStore object/read path"]
    ecstore --> setdisks["hashed set disks"]

    scanner["scanner cycle"] --> budget["ScannerCycleBudget"]
    budget --> folder["folder/object scan"]
    folder --> healreq["heal channel request"]
    healreq --> admission["HealManager admission queue"]
    admission --> healworker["heal workers and retries"]

    startup["startup entrypoint"] --> runtime["Tokio runtime builder"]
    runtime --> services["background services"]

Missing State For Later Work

R-015 storage foundation:

• Needs a stable inventory of endpoint publication, local disk prewarm, lock client setup, and per-set readiness state before any scheduler/controller consumes storage topology.
• Must not infer set availability only from request-path I/O metrics.

E-011 extension/runtime consumers:

• Need explicit ownership for runtime admission snapshots before extensions can observe scheduler or backpressure state.
• Must not receive mutable handles to ConcurrencyManager, heal queues, or scanner budget tokens.

C-011 controller work:

• Needs desired/current/status snapshots for request admission, scanner budget, and heal queue pressure before any controller can reconcile them.
• Must keep worker mutation explicit. Read-only status should report None or no-op mutation until a reviewed worker lifecycle PR exists.

Preservation Invariants

• Request reads must keep the same disk-read semaphore admission and active GET accounting.
• I/O queue status and congestion metrics must remain derived from the same permit counts.
• Scanner budget cancellation must keep its reason as runtime, objects, or directories.
• Scanner inline-heal compatibility must continue to use asynchronous heal admission.
• Heal duplicate admission must prefer merge semantics before full-queue rejection.
• High-priority heal admission must still be able to displace lower-priority queued work where the current manager allows it.
• Tokio runtime env names and fallback defaults must remain unchanged.