From 411f32c7669596912d656584622493a3764184f0 Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Sun, 17 Nov 2024 03:14:59 +0200
Subject: [PATCH] WindowServer: Make select timeout more precise to hit target
 FPS

Previously window server was only running at around 40 FPS while
targetting 60. This makes the select timeout properly sleep so that
target FPS is reached!
---
 userspace/programs/WindowServer/main.cpp | 28 ++++++++++++++----------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/userspace/programs/WindowServer/main.cpp b/userspace/programs/WindowServer/main.cpp
index 64b897ac..7fc8736f 100644
--- a/userspace/programs/WindowServer/main.cpp
+++ b/userspace/programs/WindowServer/main.cpp
@@ -198,19 +198,23 @@ int main()
 		if (auto ret = window_server.set_background_image(BAN::move(config.background_image)); ret.is_error())
 			dwarnln("Could not set background image: {}", ret.error());
 
+	const auto get_current_us =
+		[]() -> uint64_t
+		{
+			timespec current_ts;
+			clock_gettime(CLOCK_MONOTONIC, &current_ts);
+			return (current_ts.tv_sec * 1'000'000) + (current_ts.tv_nsec / 1'000);
+		};
+
 	constexpr uint64_t sync_interval_us = 1'000'000 / 60;
-	timespec last_sync { .tv_sec = 0, .tv_nsec = 0 };
+	uint64_t last_sync_us = 0;
 	while (!window_server.is_stopped())
 	{
-		timespec current_ts;
-		clock_gettime(CLOCK_MONOTONIC, &current_ts);
-
-		uint64_t us_since_last_sync = (current_ts.tv_sec - last_sync.tv_sec) * 1'000'000 + (current_ts.tv_nsec - last_sync.tv_nsec) / 1000;
-		if (us_since_last_sync > sync_interval_us)
+		const auto current_us = get_current_us();
+		if (current_us - last_sync_us > sync_interval_us)
 		{
 			window_server.sync();
-			us_since_last_sync = 0;
-			last_sync = current_ts;
+			last_sync_us += sync_interval_us;
 		}
 
 		int max_fd = server_fd;
@@ -230,10 +234,10 @@ int main()
 		}
 		max_fd = BAN::Math::max(max_fd, window_server.get_client_fds(fds));
 
-		timeval select_timeout {
-			.tv_sec = 0,
-			.tv_usec = static_cast<long>(sync_interval_us - us_since_last_sync)
-		};
+		timeval select_timeout {};
+		if (auto current_us = get_current_us(); current_us - last_sync_us < sync_interval_us)
+			select_timeout.tv_usec = sync_interval_us - (current_us - last_sync_us);
+
 		int nselect = select(max_fd + 1, &fds, nullptr, nullptr, &select_timeout);
 		if (nselect == 0)
 			continue;