[Perf] Scale or bypass server-side player ray selection in ServerSystemEntitySimulation at high client counts

[Zaldaryon]

Problem

In a Lithos probe capture on Stratum (Vintage Story 1.22.7, 2,185 connected clients, Debian Linux x64), ServerSystemEntitySimulation.OnServerTick accounted for 147,440 total samples (57.1% of the gameplay thread). Within this call tree, GameMain.FindNearestRayIntersectingEntity accounted for 114,383 self samples (44.3% of the entire gameplay thread).

ServerSystemEntitySimulation.cs implements an angle and movement threshold throttle (StratumSelectionMaxSkipTicks = 3) to reduce per-client raytracing. With 2,185 active clients, this threshold still executes at least 728 raytraces on every single server tick. Each trace invokes server.RayTraceForSelection, which calls FindNearestRayIntersectingEntity. That method iterates 8 to 27 chunk volumes in a 3D bounding box around the player and checks entity.IntersectsRay across all entities in those chunks.

On the server, entityPlayer.EntitySelection is not consumed by the tick loop. The entity intersection search is performed solely to check if an entity occludes the player's view of a block, which the server uses only to fire Block.OnBeingLookedAt (in vanilla survival, implemented only by Farmland, FertileGround, and Tapestry for client-side tooltips). Authoritative block breaks, placements, and entity combat interactions are already driven by client-sent interaction packets.

Existing issue #348 covers repeated calls to EntityPlayer.get_IsInteractable during dense entity queries (which accounted for under 1% of samples in this capture). The primary bottleneck under massive concurrency is the volume of chunk traversals and ray-intersection tests executed per tick across thousands of players.

Proposed change

1. Scale StratumSelectionMaxSkipTicks dynamically based on online player count, or expose it as a configurable setting under Performance.EntitySimulation.
2. Skip the FindNearestRayIntersectingEntity occlusion pass when the server only needs BlockSelection for OnBeingLookedAt, or fast-reject chunks that contain no interactable entities.
3. Provide a configuration toggle to disable ambient server-side OnBeingLookedAt raytracing under high-concurrency environments.

Acceptance criteria

• Gameplay-thread CPU time spent in FindNearestRayIntersectingEntity is reduced substantially under high player counts.
• Block breaking, placing, and entity combat continue to function accurately through client interaction packets.
• Server scenarios and smoke tests pass with zero regressions.

Scope

• Server-side change. No custom client required.

[Zaldaryon]

Industry precedents and architectural alternatives

Per-player continuous line-of-sight and ray selection is a well-studied bottleneck in authoritative multiplayer engines:

1. Event-driven validation over continuous polling (Source, Unreal, Minecraft and Paper):
   Dedicated servers generally do not perform ambient per-tick line-of-sight or gaze raycasting for all connected players. Instead, continuous raycasting is delegated to the client. The client submits intended action packets (such as interacting with an entity or breaking a block), and the server validates the line of sight point-in-time on demand. Ambient hover events (like OnBeingLookedAt) are pushed to the client or evaluated on a lazy, low-priority schedule.

2. Dynamic Activity Banding (DAB, Pufferfish, PaperMC):
   In high-density Minecraft forks (like Pufferfish and Purpur), line-of-sight checks and entity tracking frequency are throttled non-linearly using distance banding and server load feedback. When player density increases, expensive geometric tests drop in frequency.

3. Broadphase spatial pruning (PhysX, Chaos, Havok):
   Game physics engines avoid testing individual colliders without a hierarchical broadphase test (ray vs bounding volume hierarchy or chunk AABB). Traversing entities across every chunk in a full 3D bounding box without a ray-chunk box intersection test evaluates many non-intersecting chunks unnecessarily.

Alternatives evaluated

• Option A: Decouple ambient hover and skip server ray selection (Recommended)
  Since authoritative block placement, block destruction, and entity interaction are already driven by incoming client packets (Packet_ClientBlockPlaceOrBreak and Packet_ClientEntityInteraction), server-side ambient raytracing in ServerSystemEntitySimulation exists only for OnBeingLookedAt. Providing a configuration setting to disable ambient hover raytracing, or executing it on a slow background timer, removes up to 44% of main thread load at 2,000+ players.

• Option B: Ray-chunk bounding box broadphase in FindNearestRayIntersectingEntity
  Before enumerating chunk.Entities in FindNearestRayIntersectingEntity, test whether the ray actually intersects the 32x32x32 bounding box of the chunk. In a typical 3x3x3 chunk volume, a directional picking ray intersects only 3 to 5 chunks, allowing 80% of chunks (and their entity collections) to be skipped immediately.

• Option C: Load-adaptive dynamic skip intervals
  Dynamically increase StratumSelectionMaxSkipTicks from 3 up to 20 or 30 ticks as connected player count grows past thresholds (such as 200, 500, and 1,000 players).
  Trade-off: Retains vanilla behavior at low counts while automatically protecting tick rates under load.