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diff --git a/indra/llcommon/coro_scheduler.h b/indra/llcommon/coro_scheduler.h
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+/**
+ * @file   coro_scheduler.h
+ * @author Nat Goodspeed
+ * @date   2024-08-05
+ * @brief  Custom scheduler for viewer's Boost.Fibers (aka coroutines)
+ *
+ * $LicenseInfo:firstyear=2024&license=viewerlgpl$
+ * Copyright (c) 2024, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_CORO_SCHEDULER_H)
+#define LL_CORO_SCHEDULER_H
+
+#include "workqueue.h"
+#include <boost/fiber/fiber.hpp>
+#include <boost/fiber/algo/round_robin.hpp>
+
+/**
+ * llcoro::scheduler is specifically intended for the viewer's main thread.
+ * Its role is to ensure that the main coroutine, responsible for UI
+ * operations and coordinating everything else, doesn't get starved by
+ * secondary coroutines -- however many of those there might be.
+ *
+ * The simple boost::fibers::algo::round_robin scheduler could result in
+ * arbitrary time lag between resumptions of the main coroutine. Of course
+ * every well-behaved viewer coroutine must be coded to yield before too much
+ * real time has elapsed, but sheer volume of secondary coroutines could still
+ * consume unreasonable real time before cycling back to the main coroutine.
+ */
+
+namespace llcoro
+{
+
+class scheduler: public boost::fibers::algo::round_robin
+{
+    using super = boost::fibers::algo::round_robin;
+public:
+    // If the main fiber is ready, and it's been at least this long since the
+    // main fiber last ran, jump the main fiber to the head of the queue.
+    static const F64 DEFAULT_TIMESLICE;
+
+    scheduler();
+    void awakened( boost::fibers::context*) noexcept override;
+    boost::fibers::context* pick_next() noexcept override;
+
+    static void use();
+
+private:
+    LL::WorkQueue::ptr_t getWorkQueue();
+
+    // This is the fiber::id of the main fiber.
+    boost::fibers::fiber::id mMainID;
+    // This context* is nullptr while the main fiber is running or suspended,
+    // but is set to the main fiber's context each time the main fiber is ready.
+    boost::fibers::context* mMainCtx{};
+    // Remember the context returned by the previous pick_next() call.
+    boost::fibers::context* mPrevCtx{};
+    // If it's been at least this long since the last time the main fiber got
+    // control, jump it to the head of the queue.
+    F64 mTimeslice{ DEFAULT_TIMESLICE };
+    // Time when we resumed the most recently running fiber
+    F64 mResumeTime{ 0 };
+    // Timestamp as of the last time we suspended the main fiber.
+    F64 mMainLast{ 0 };
+    // Timestamp of start time
+    F64 mStart{ 0 };
+    // count context switches
+    U64 mSwitches{ 0 };
+    // WorkQueue for deferred logging
+    LL::WorkQueue::weak_t mQueue;
+};
+
+} // namespace llcoro
+
+#endif /* ! defined(LL_CORO_SCHEDULER_H) */