Avoid render output on idle frames and add benchmark by dyxushuai · Pull Request #285 · rockorager/libvaxis

dyxushuai · 2025-12-28T13:04:46Z

TODO

render always emits sync/hide/home/SGR reset (and clears kitty graphics) even when the screen, cursor, and shapes are unchanged. This creates unnecessary terminal I/O on idle frames.

Steps

Detect changes (buffer, refresh, cursor visibility/position, cursor/mouse shapes).
Only initialize sync/hide/home/clear when a change is detected.
Track last cursor visibility to avoid redundant renders.
Add a test asserting no output for idle frames.

Benchmark (local)

Command: zig build bench -Doptimize=ReleaseFast -- 200

idle:  frames=200 total_ns=7175271  ns/frame=35876  bytes=0      bytes/frame=0
dirty: frames=200 total_ns=11949574 ns/frame=59747  bytes=407554 bytes/frame=2037

Tests

zig build test
zig build bench -Doptimize=ReleaseFast -- 200

rockorager · 2025-12-28T13:10:15Z

thanks!

Copilot

Pull request overview

This PR optimizes the render function to avoid unnecessary terminal I/O when there are no changes to display (idle frames). The optimization introduces change detection for buffer contents, cursor visibility/position, and cursor/mouse shapes, only performing terminal writes when changes are detected.

Key changes:

Added conditional rendering initialization that only executes when changes are detected
Introduced tracking of cursor visibility in screen_last to avoid redundant renders
Added a test case to verify no output is produced on idle frames
Added a benchmark suite to measure performance improvements

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
src/Vaxis.zig	Core optimization: adds change detection logic, conditional `startRender` execution, cursor visibility tracking, idle frame test
build.zig	Adds benchmark build step with proper module configuration and argument forwarding
bench/bench.zig	New benchmark implementation measuring idle vs dirty frame performance

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Copilot · 2025-12-28T13:11:56Z

bench/bench.zig

+    const winsize = vaxis.Winsize{ .rows = 24, .cols = 80, .x_pixel = 0, .y_pixel = 0 };
+    try vx.resize(allocator, &init_writer.writer, winsize);
+
+    const stdout = std.fs.File.stdout().deprecatedWriter();


The method deprecatedWriter() does not exist on std.fs.File. This should be writer() to get a writer from stdout.

Suggested change

const stdout = std.fs.File.stdout().deprecatedWriter();

const stdout = std.fs.File.stdout().writer();

Copilot · 2025-12-28T13:11:56Z

src/Vaxis.zig

        self.state.cursor.row = cursor_pos.row;
        self.state.cursor.col = cursor_pos.col;
    }
+    self.screen_last.cursor_vis = self.screen.cursor_vis;


After updating screen_last.cursor_vis at line 792, the code should also update screen_last.cursor_row and screen_last.cursor_col to track the last rendered cursor position. This is necessary for the cursor position change detection at lines 370-372 to work correctly. Add:

self.screen_last.cursor_row = self.screen.cursor_row; self.screen_last.cursor_col = self.screen.cursor_col;

Suggested change

self.screen_last.cursor_vis = self.screen.cursor_vis;

self.screen_last.cursor_vis = self.screen.cursor_vis;

self.screen_last.cursor_row = self.screen.cursor_row;

self.screen_last.cursor_col = self.screen.cursor_col;

Copilot · 2025-12-28T13:11:57Z

bench/bench.zig

+    const idle_bytes: usize = idle_writer.writer.end;
+    try printResults(stdout, "idle", iterations, idle_ns, idle_bytes);
+
+    var dirty_writer = std.io.Writer.Allocating.init(allocator);
+    defer dirty_writer.deinit();
+    timer.reset();
+    i = 0;
+    while (i < iterations) : (i += 1) {
+        vx.queueRefresh();
+        try vx.render(&dirty_writer.writer);
+    }
+    const dirty_ns = timer.read();
+    const dirty_bytes: usize = dirty_writer.writer.end;


The code accesses idle_writer.writer.end but this is incorrect. If Writer.Allocating follows the pattern of a buffering writer, it likely has the byte count accessible directly on the struct (e.g., idle_writer.end or similar), not on the .writer field. The .writer field typically returns a writer interface that doesn't expose the buffer size.

Copilot · 2025-12-28T13:11:57Z

src/Vaxis.zig

+        (self.screen.cursor_row != self.state.cursor.row or
+            self.screen.cursor_col != self.state.cursor.col);


The cursor position change detection compares against self.state.cursor instead of self.screen_last.cursor_row/cursor_col. However, self.screen_last.cursor_row and self.screen_last.cursor_col are never updated anywhere in the code (unlike cursor_vis at line 792 and cursor_shape at line 805). This means position changes won't be properly detected on subsequent renders unless self.state.cursor differs. You should add lines to update self.screen_last.cursor_row and self.screen_last.cursor_col after positioning the cursor (around line 792), and then use those fields for comparison in the change detection.

Suggested change

(self.screen.cursor_row != self.state.cursor.row or

self.screen.cursor_col != self.state.cursor.col);

(self.screen.cursor_row != self.screen_last.cursor_row or

self.screen.cursor_col != self.screen_last.cursor_col);

Copilot · 2025-12-28T13:11:57Z

src/Vaxis.zig

+    var deinit_writer = std.io.Writer.Allocating.init(std.testing.allocator);
+    defer deinit_writer.deinit();
+    defer vx.deinit(std.testing.allocator, &deinit_writer.writer);
+
+    var render_writer = std.io.Writer.Allocating.init(std.testing.allocator);
+    defer render_writer.deinit();


The test uses std.io.Writer.Allocating (lowercase 'io'), but the existing code in tty.zig uses std.Io.Writer.Allocating (capital 'I'). This inconsistency will cause a compilation error since std.io.Writer.Allocating does not exist in Zig's standard library. It should be std.Io.Writer.Allocating to match the existing pattern in the codebase.

Copilot · 2025-12-28T13:11:57Z

bench/bench.zig

+    var init_writer = std.io.Writer.Allocating.init(allocator);
+    defer init_writer.deinit();
+    defer vx.deinit(allocator, &init_writer.writer);
+
+    const winsize = vaxis.Winsize{ .rows = 24, .cols = 80, .x_pixel = 0, .y_pixel = 0 };
+    try vx.resize(allocator, &init_writer.writer, winsize);
+
+    const stdout = std.fs.File.stdout().deprecatedWriter();
+
+    var idle_writer = std.io.Writer.Allocating.init(allocator);
+    defer idle_writer.deinit();
+    var timer = try std.time.Timer.start();
+    var i: usize = 0;
+    while (i < iterations) : (i += 1) {
+        try vx.render(&idle_writer.writer);
+    }
+    const idle_ns = timer.read();
+    const idle_bytes: usize = idle_writer.writer.end;
+    try printResults(stdout, "idle", iterations, idle_ns, idle_bytes);
+
+    var dirty_writer = std.io.Writer.Allocating.init(allocator);
+    defer dirty_writer.deinit();


The benchmark uses std.io.Writer.Allocating (lowercase 'io'), but the existing code in tty.zig uses std.Io.Writer.Allocating (capital 'I'). This inconsistency will cause a compilation error since std.io.Writer.Allocating does not exist in Zig's standard library. It should be std.Io.Writer.Allocating to match the existing pattern in the codebase.

dyxushuai added 2 commits December 28, 2025 12:53

Skip render output when nothing changes

75968f2

Add render benchmark

2a5a720

Copilot AI review requested due to automatic review settings December 28, 2025 13:04

Copilot started reviewing on behalf of dyxushuai December 28, 2025 13:05 View session

chore: zig fmt .

d31304b

rockorager force-pushed the fix/render-noop branch from a7a0a81 to d31304b Compare December 28, 2025 13:10

rockorager merged commit c2f00c2 into rockorager:main Dec 28, 2025

Copilot AI reviewed Dec 28, 2025

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Avoid render output on idle frames and add benchmark#285

Avoid render output on idle frames and add benchmark#285
rockorager merged 3 commits intorockorager:mainfrom
dyxushuai:fix/render-noop

dyxushuai commented Dec 28, 2025 •

edited

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	const stdout = std.fs.File.stdout().deprecatedWriter();
	const stdout = std.fs.File.stdout().writer();

		(self.screen.cursor_row != self.state.cursor.row or
		self.screen.cursor_col != self.state.cursor.col);

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dyxushuai commented Dec 28, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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TODO

Steps

Benchmark (local)

Tests

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rockorager commented Dec 28, 2025

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Pull request overview

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dyxushuai commented Dec 28, 2025 •

edited

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