Add normalized equivalent of YieldProcessor, retune some spin loops (…

…#13670)

* Add normalized equivalent of YieldProcessor, retune some spin loops

Part of fix for https://github.com/dotnet/coreclr/issues/13388

Normalized equivalent of YieldProcessor
- The delay incurred by YieldProcessor is measured once lazily at run-time
- Added YieldProcessorNormalized that yields for a specific duration (the duration is approximately equal to what was measured for one YieldProcessor on a Skylake processor, about 125 cycles). The measurement calculates how many YieldProcessor calls are necessary to get a delay close to the desired duration.
- Changed Thread.SpinWait to use YieldProcessorNormalized

Thread.SpinWait divide count by 7 experiment
- At this point I experimented with changing Thread.SpinWait to divide the requested number of iterations by 7, to see how it fares on perf. On my Sandy Bridge processor, 7 * YieldProcessor == YieldProcessorNormalized. See numbers in PR below.
- Not too many regressions, and the overall perf is somewhat as expected - not much change on Sandy Bridge processor, significant improvement on Skylake processor.
  - I'm discounting the SemaphoreSlim throughput score because it seems to be heavily dependent on Monitor. It would be more interesting to revisit SemaphoreSlim after retuning Monitor's spin heuristics.
  - ReaderWriterLockSlim seems to perform worse on Skylake, the current spin heuristics are not translating well

Spin tuning
- At this point, I abandoned the experiment above and tried to retune spins that use Thread.SpinWait
- General observations
  - YieldProcessor stage
    - At this stage in many places we're currently doing very long spins on YieldProcessor per iteration of the spin loop. In the last YieldProcessor iteration, it amounts to about 70 K cycles on Sandy Bridge and 512 K cycles on Skylake.
    - Long spins on YieldProcessor don't let other work run efficiently. Especially when many scheduled threads all issue a long YieldProcessor, a significant portion of the processor can go unused for a long time.
    - Long spins on YieldProcessor is in some cases helping to reduce contention in high-contention cases, effectively taking away some threads into a long delay. Sleep(1) works much better but has a much higher delay so it's not always appropriate. In other cases, I found that it's better to do more iterations with a shorter YieldProcessor. It would be even better to reduce the contention in the app or to have a proper wait in the sync object, where appropriate.
    - Updated the YieldProcessor measurement above to calculate the number of YieldProcessorNormalized calls that amount to about 900 cycles (this was tuned based on perf), and modified SpinWait's YieldProcessor stage to cap the number of iterations passed to Thread.SpinWait. Effectively, the first few iterations have a longer delay than before on Sandy Bridge and a shorter delay than before on Skylake, and the later iterations have a much shorter delay than before on both.
  - Yield/Sleep(0) stage
    - Observed a couple of issues:
      - When there are no threads to switch to, Yield and Sleep(0) become no-op and it turns the spin loop into a busy-spin that may quickly reach the max spin count and cause the thread to enter a wait state, or may just busy-spin for longer than desired before a Sleep(1). Completing the spin loop too early can cause excessive context switcing if a wait follows, and entering the Sleep(1) stage too early can cause excessive delays.
      - If there are multiple threads doing Yield and Sleep(0) (typically from the same spin loop due to contention), they may switch between one another, delaying work that can make progress.
    - I found that it works well to interleave a Yield/Sleep(0) with YieldProcessor, it enforces a minimum delay for this stage. Modified SpinWait to do this until it reaches the Sleep(1) threshold.
  - Sleep(1) stage
    - I didn't see any benefit in the tests to interleave Sleep(1) calls with some Yield/Sleep(0) calls, perf seemed to be a bit worse actually. If the Sleep(1) stage is reached, there is probably a lot of contention and the Sleep(1) stage helps to remove some threads from the equation for a while. Adding some Yield/Sleep(0) in-between seems to add back some of that contention.
      - Modified SpinWait to use a Sleep(1) threshold, after which point it only does Sleep(1) on each spin iteration
    - For the Sleep(1) threshold, I couldn't find one constant that works well in all cases
      - For spin loops that are followed by a proper wait (such as a wait on an event that is signaled when the resource becomes available), they benefit from not doing Sleep(1) at all, and spinning in other stages for longer
      - For infinite spin loops, they usually seemed to benefit from a lower Sleep(1) threshold to reduce contention, but the threshold also depends on other factors like how much work is done in each spin iteration, how efficient waiting is, and whether waiting has any negative side-effects.
      - Added an internal overload of SpinWait.SpinOnce to take the Sleep(1) threshold as a parameter
- SpinWait - Tweaked the spin strategy as mentioned above
- ManualResetEventSlim - Changed to use SpinWait, retuned the default number of iterations (total delay is still significantly less than before). Retained the previous behavior of having Sleep(1) if a higher spin count is requested.
- Task - It was using the same heuristics as ManualResetEventSlim, copied the changes here as well
- SemaphoreSlim - Changed to use SpinWait, retuned similarly to ManualResetEventSlim but with double the number of iterations because the wait path is a lot more expensive
- SpinLock - SpinLock was using very long YieldProcessor spins. Changed to use SpinWait, removed process count multiplier, simplified.
- ReaderWriterLockSlim - This one is complicated as there are many issues. The current spin heuristics performed better even after normalizing Thread.SpinWait but without changing the SpinWait iterations (the delay is longer than before), so I left this one as is.
- The perf (see numbers in PR below) seems to be much better than both the baseline and the Thread.SpinWait divide by 7 experiment
  - On Sandy Bridge, I didn't see many significant regressions. ReaderWriterLockSlim is a bit worse in some cases and a bit better in other similar cases, but at least the really low scores in the baseline got much better and not the other way around.
  - On Skylake, some significant regressions are in SemaphoreSlim throughput (which I'm discounting as I mentioned above in the experiment) and CountdownEvent add/signal throughput. The latter can probably be improved later.

Signed-off-by: dotnet-bot <[email protected]>

src/System.Private.CoreLib/shared/System/Threading/SpinWait.cs

@@ -69,9 +69,26 @@ public struct SpinWait
         // numbers may seem fairly arbitrary, but were derived with at least some
         // thought in the design document.  I fully expect they will need to change
         // over time as we gain more experience with performance.
-        internal const int YIELD_THRESHOLD = 10; // When to switch over to a true yield.
-        internal const int SLEEP_0_EVERY_HOW_MANY_TIMES = 5; // After how many yields should we Sleep(0)?
-        internal const int SLEEP_1_EVERY_HOW_MANY_TIMES = 20; // After how many yields should we Sleep(1)?
+        internal const int YieldThreshold = 10; // When to switch over to a true yield.
+        private const int Sleep0EveryHowManyYields = 5; // After how many yields should we Sleep(0)?
+        internal const int DefaultSleep1Threshold = 20; // After how many yields should we Sleep(1) frequently?
+
+        /// <summary>
+        /// A suggested number of spin iterations before doing a proper wait, such as waiting on an event that becomes signaled
+        /// when the resource becomes available.
+        /// </summary>
+        /// <remarks>
+        /// These numbers were arrived at by experimenting with different numbers in various cases that currently use it. It's
+        /// only a suggested value and typically works well when the proper wait is something like an event.
+        /// 
+        /// Spinning less can lead to early waiting and more context switching, spinning more can decrease latency but may use
+        /// up some CPU time unnecessarily. Depends on the situation too, for instance SemaphoreSlim uses double this number
+        /// because the waiting there is currently a lot more expensive (involves more spinning, taking a lock, etc.). It also
+        /// depends on the likelihood of the spin being successful and how long the wait would be but those are not accounted
+        /// for here.
+        /// </remarks>
+        internal static readonly int SpinCountforSpinBeforeWait = PlatformHelper.IsSingleProcessor ? 1 : 35;
+        internal const int Sleep1ThresholdForSpinBeforeWait = 40; // should be greater than SpinCountforSpinBeforeWait
 
         // The number of times we've spun already.
         private int _count;
@@ -81,7 +98,12 @@ public struct SpinWait
         /// </summary>
         public int Count
         {
-            get { return _count; }
+            get => _count;
+            internal set
+            {
+                Debug.Assert(value >= 0);
+                _count = value;
+            }
         }
 
         /// <summary>
@@ -94,10 +116,7 @@ public int Count
         /// On a single-CPU machine, <see cref="SpinOnce"/> always yields the processor. On machines with
         /// multiple CPUs, <see cref="SpinOnce"/> may yield after an unspecified number of calls.
         /// </remarks>
-        public bool NextSpinWillYield
-        {
-            get { return _count > YIELD_THRESHOLD || PlatformHelper.IsSingleProcessor; }
-        }
+        public bool NextSpinWillYield => _count >= YieldThreshold || PlatformHelper.IsSingleProcessor;
 
         /// <summary>
         /// Performs a single spin.
@@ -108,7 +127,27 @@ public bool NextSpinWillYield
         /// </remarks>
         public void SpinOnce()
         {
-            if (NextSpinWillYield)
+            SpinOnce(DefaultSleep1Threshold);
+        }
+
+        internal void SpinOnce(int sleep1Threshold)
+        {
+            Debug.Assert(sleep1Threshold >= YieldThreshold || PlatformHelper.IsSingleProcessor); // so that NextSpinWillYield behaves as requested
+
+            // (_count - YieldThreshold) % 2 == 0: The purpose of this check is to interleave Thread.Yield/Sleep(0) with
+            // Thread.SpinWait. Otherwise, the following issues occur:
+            //   - When there are no threads to switch to, Yield and Sleep(0) become no-op and it turns the spin loop into a
+            //     busy-spin that may quickly reach the max spin count and cause the thread to enter a wait state, or may
+            //     just busy-spin for longer than desired before a Sleep(1). Completing the spin loop too early can cause
+            //     excessive context switcing if a wait follows, and entering the Sleep(1) stage too early can cause
+            //     excessive delays.
+            //   - If there are multiple threads doing Yield and Sleep(0) (typically from the same spin loop due to
+            //     contention), they may switch between one another, delaying work that can make progress.
+            if ((
+                    _count >= YieldThreshold &&
+                    (_count >= sleep1Threshold || (_count - YieldThreshold) % 2 == 0)
+                ) ||
+                PlatformHelper.IsSingleProcessor)
             {
                 //
                 // We must yield.
@@ -125,19 +164,21 @@ public void SpinOnce()
                 // configured to use the (default) coarse-grained system timer.
                 //
 
-                int yieldsSoFar = (_count >= YIELD_THRESHOLD ? _count - YIELD_THRESHOLD : _count);
-
-                if ((yieldsSoFar % SLEEP_1_EVERY_HOW_MANY_TIMES) == (SLEEP_1_EVERY_HOW_MANY_TIMES - 1))
+                if (_count >= sleep1Threshold)
                 {
                     RuntimeThread.Sleep(1);
                 }
-                else if ((yieldsSoFar % SLEEP_0_EVERY_HOW_MANY_TIMES) == (SLEEP_0_EVERY_HOW_MANY_TIMES - 1))
-                {
-                    RuntimeThread.Sleep(0);
-                }
                 else
                 {
-                    RuntimeThread.Yield();
+                    int yieldsSoFar = _count >= YieldThreshold ? (_count - YieldThreshold) / 2 : _count;
+                    if ((yieldsSoFar % Sleep0EveryHowManyYields) == (Sleep0EveryHowManyYields - 1))
+                    {
+                        RuntimeThread.Sleep(0);
+                    }
+                    else
+                    {
+                        RuntimeThread.Yield();
+                    }
                 }
             }
             else
@@ -153,11 +194,24 @@ public void SpinOnce()
                 // number of spins we are willing to tolerate to reduce delay to the caller,
                 // since we expect most callers will eventually block anyway.
                 //
-                RuntimeThread.SpinWait(4 << _count);
+                // Also, cap the maximum spin count to a value such that many thousands of CPU cycles would not be wasted doing
+                // the equivalent of YieldProcessor(), as that that point SwitchToThread/Sleep(0) are more likely to be able to
+                // allow other useful work to run. Long YieldProcessor() loops can help to reduce contention, but Sleep(1) is
+                // usually better for that.
+                //
+                // RuntimeThread.OptimalMaxSpinWaitsPerSpinIteration:
+                //   - See Thread::InitializeYieldProcessorNormalized(), which describes and calculates this value.
+                //
+                int n = RuntimeThread.OptimalMaxSpinWaitsPerSpinIteration;
+                if (_count <= 30 && (1 << _count) < n)
+                {
+                    n = 1 << _count;
+                }
+                RuntimeThread.SpinWait(n);
             }
 
             // Finally, increment our spin counter.
-            _count = (_count == int.MaxValue ? YIELD_THRESHOLD : _count + 1);
+            _count = (_count == int.MaxValue ? YieldThreshold : _count + 1);
         }
 
         /// <summary>
@@ -299,9 +353,7 @@ internal static int ProcessorCount
         /// <summary>
         /// Gets whether the current machine has only a single processor.
         /// </summary>
-        internal static bool IsSingleProcessor
-        {
-            get { return ProcessorCount == 1; }
-        }
+        /// <remarks>This typically does not change on a machine, so it's checked only once.</remarks>
+        internal static readonly bool IsSingleProcessor = ProcessorCount == 1;
     }
 }