RaniandAnthony Eid
a41ccd1206
Early draft for #32051 (multi-cursor editing is very slow, and basically hangs at high cursor counts). Opening it early like @Anthony-Eid suggested so we can agree on direction before I go further. ## Root cause Typing with a cursor on every line is mostly O(N)-per-keystroke work spread across a few places. I profiled it with `sample` and per-phase timers around `handle_input`. At 1000 cursors (~24ms before this PR): the post-edit display-map sync is ~11.8ms, the CRDT edit (`apply_local_edit`) ~3.7ms, resolving all N selections plus the per-cursor input loop ~3.5ms, the post-edit selection round-trip ~2.6ms, and `change_selections` plus transact machinery ~2.4ms. The display sync re-runs per-edit `SumTree` work through every layer even when nothing transforms (the plain-text case), and selections get re-resolved through the full display round-trip several times per keystroke. ## What this does - Display fast paths: `InlayMap::sync` (no inlays) and `WrapMap::interpolate` (no soft-wrap) skip the O(edits) transform-tree rebuild and return the passthrough snapshot, gated so a pending inlay splice or existing wraps still take the slow path. - Selection fast path: when nothing collapses buffer content (`!has_folds() && !has_replacement_blocks()`), resolve `Anchor` to `Point` to `Offset` batched and skip the per-selection display round-trip (the `todo(lw)`). - Render and autoscroll resolve only the first/last/newest selections instead of all N per frame. - A parameterized `Multi-cursor input/cursors/{1000,10000,100000}` benchmark. ## Results - Typing (`handle_input`): ~2.1x faster (24ms to 11.7ms at 1k). - Type plus two word-deletes (criterion): 29% faster (89.9 to 63.8ms) at 1k, 37% (959 to 606ms) at 10k. - Post-edit display sync alone: 11.8 to 3.8ms at 1k, 133 to 45ms at 10k. All `editor` (759), `display_map` (67), `multi_buffer` (58), and `text` (36) tests pass, and `./script/clippy` is clean. ## Direction This is ~2x, and I can get the current architecture to roughly 3.5-4.5x with a few more safe changes (hoisting the per-cursor language/editability checks, cheaper snapshot clones). Genuine VS Code numbers (~1-2us/cursor) aren't reachable while the buffer is a CRDT rope of fragments with anchor selections and a 5-layer transform stack. That would need a plain-offset cursor model and/or decoupling display layout from the edit path, which is a bigger effort I'd want to design with you. One thing I'd need your call on: `set_active_selections` sends an `UpdateSelections` collab op every keystroke (building N anchors); can that be debounced to transaction end, or do presence/follow-mode features rely on per-keystroke cursor broadcast? Release Notes: - editor: Improved multi cursor editing performance --------- Co-authored-by: Anthony Eid <anthony@zed.dev>
Zed
Welcome to Zed, a high-performance, multiplayer code editor from the creators of Atom and Tree-sitter.
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