--- /dev/null
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace System.Threading
+{
+ internal static class ProcessorIdCache
+ {
+ // The upper bits of t_currentProcessorIdCache are the currentProcessorId. The lower bits of
+ // the t_currentProcessorIdCache are counting down to get it periodically refreshed.
+ // TODO: Consider flushing the currentProcessorIdCache on Wait operations or similar
+ // actions that are likely to result in changing the executing core
+ [ThreadStatic]
+ private static int t_currentProcessorIdCache;
+
+ private const int ProcessorIdCacheShift = 16;
+ private const int ProcessorIdCacheCountDownMask = (1 << ProcessorIdCacheShift) - 1;
+ // Refresh rate of the cache. Will be derived from a speed check of GetCurrentProcessorNumber API.
+ private static int s_processorIdRefreshRate;
+ // We will not adjust higher than this though.
+ private const int MaxIdRefreshRate = 5000;
+
+ private static int RefreshCurrentProcessorId()
+ {
+ int currentProcessorId = Thread.GetCurrentProcessorNumber();
+
+ // On Unix, GetCurrentProcessorNumber() is implemented in terms of sched_getcpu, which
+ // doesn't exist on all platforms. On those it doesn't exist on, GetCurrentProcessorNumber()
+ // returns -1. As a fallback in that case and to spread the threads across the buckets
+ // by default, we use the current managed thread ID as a proxy.
+ if (currentProcessorId < 0)
+ currentProcessorId = Environment.CurrentManagedThreadId;
+
+ Debug.Assert(s_processorIdRefreshRate <= ProcessorIdCacheCountDownMask);
+
+ // Mask with int.MaxValue to ensure the execution Id is not negative
+ t_currentProcessorIdCache = ((currentProcessorId << ProcessorIdCacheShift) & int.MaxValue) | s_processorIdRefreshRate;
+
+ return currentProcessorId;
+ }
+
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ internal static int GetCurrentProcessorId()
+ {
+ int currentProcessorIdCache = t_currentProcessorIdCache--;
+ if ((currentProcessorIdCache & ProcessorIdCacheCountDownMask) == 0)
+ {
+ return RefreshCurrentProcessorId();
+ }
+
+ return currentProcessorIdCache >> ProcessorIdCacheShift;
+ }
+
+ // If GetCurrentProcessorNumber takes any nontrivial time (compared to TLS access), return false.
+ // Check more than once - to make sure it was not because TLS was delayed by GC or a context switch.
+ internal static bool ProcessorNumberSpeedCheck()
+ {
+ // NOTE: We do not check the frequency of the Stopwatch.
+ // The frequency often does not match the actual timer refresh rate anyways.
+ // If the resolution, precision or access time to the timer are inadequate for our measures here,
+ // the test will fail anyways.
+
+ double minID = double.MaxValue;
+ double minTLS = double.MaxValue;
+
+ // warm up the code paths.
+ UninlinedThreadStatic();
+ // also check if API is actually functional (-1 means not supported)
+ if (Thread.GetCurrentProcessorNumber() < 0)
+ {
+ s_processorIdRefreshRate = ProcessorIdCacheCountDownMask;
+ return false;
+ }
+
+ long oneMicrosecond = Stopwatch.Frequency / 1000000 + 1;
+ for (int i = 0; i < 10; i++)
+ {
+ // we will measure at least 16 iterations and at least 1 microsecond
+ long t;
+ int iters = 8;
+ do
+ {
+ iters *= 2;
+ t = Stopwatch.GetTimestamp();
+ for (int j = 0; j < iters; j++)
+ {
+ Thread.GetCurrentProcessorNumber();
+ }
+ t = Stopwatch.GetTimestamp() - t;
+ } while (t < oneMicrosecond);
+
+ minID = Math.Min(minID, (double)t / iters);
+
+ // we will measure at least 1 microsecond,
+ // and use at least 1/2 of ProcID iterations
+ // we assume that TLS can't be more than 2x slower than ProcID
+ iters /= 4;
+ do
+ {
+ iters *= 2;
+ t = Stopwatch.GetTimestamp();
+ for (int j = 0; j < iters; j++)
+ {
+ UninlinedThreadStatic();
+ }
+ t = Stopwatch.GetTimestamp() - t;
+ } while (t < oneMicrosecond);
+
+ minTLS = Math.Min(minTLS, (double)t / iters);
+ }
+
+ // A few words about choosing cache refresh rate:
+ //
+ // There are too reasons why data structures use core affinity:
+ // 1) To improve locality - avoid running on one core and using data in other core's cache.
+ // 2) To reduce sharing - avoid multiple threads using the same piece of data.
+ //
+ // Scenarios with large footprint, like striped caches, are sensitive to both parts. It is desirable to access
+ // large data from the "right" core.
+ // In scenarios where the state is small, like a striped counter, it is mostly about sharing.
+ // Otherwise the state is small and occasionally moving counter to a different core via cache miss is not a big deal.
+ //
+ // In scenarios that care more about sharing precise results of GetCurrentProcessorNumber may not justify
+ // the cost unless the underlying implementation is very cheap.
+ // In such cases it is desirable to amortize the cost over multiple accesses by caching in a ThreadStatic.
+ //
+ // In addition to the data structure, the benefits also depend on use pattern and on concurrency level.
+ // I.E. if an array pool user only rents array "just in case" but does not actually use it, and concurrency level is low,
+ // a longer refresh would be beneficial since that could lower the API cost.
+ // If array is actually used, then there is benefit from higher precision of the API and shorter refresh is more attractive.
+ //
+ // Overall we do not know the ideal refresh rate and using some kind of dynamic feedback is unlikely to be feasible.
+ // Experiments have shown, however, that 5x amortization rate is a good enough balance between precision and cost of the API.
+ s_processorIdRefreshRate = Math.Min((int)(minID * 5 / minTLS), MaxIdRefreshRate);
+
+ // In a case if GetCurrentProcessorNumber is particularly fast, like it happens on platforms supporting RDPID instruction,
+ // caching is not an improvement, thus it is desirable to bypass the cache entirely.
+ // Such systems consistently derive the refresh rate at or below 2-3, while the next tier, RDTSCP based implementations result in ~10,
+ // so we use "5" as a criteria to separate "fast" machines from the rest.
+ return s_processorIdRefreshRate <= 5;
+ }
+
+ // NoInlining is to make sure JIT does not CSE and to have a better perf proxy for TLS access.
+ // Measuring inlined ThreadStatic in a loop results in underestimates and unnecessary caching.
+ [MethodImpl(MethodImplOptions.NoInlining)]
+ internal static int UninlinedThreadStatic()
+ {
+ return t_currentProcessorIdCache;
+ }
+ }
+}
}
[MethodImpl(MethodImplOptions.InternalCall)]
- private static extern int GetCurrentProcessorNumber();
+ internal static extern int GetCurrentProcessorNumber();
- // The upper bits of t_currentProcessorIdCache are the currentProcessorId. The lower bits of
- // the t_currentProcessorIdCache are counting down to get it periodically refreshed.
- // TODO: Consider flushing the currentProcessorIdCache on Wait operations or similar
- // actions that are likely to result in changing the executing core
- [ThreadStatic]
- private static int t_currentProcessorIdCache;
-
- private const int ProcessorIdCacheShift = 16;
- private const int ProcessorIdCacheCountDownMask = (1 << ProcessorIdCacheShift) - 1;
- private const int ProcessorIdRefreshRate = 5000;
-
- private static int RefreshCurrentProcessorId()
- {
- int currentProcessorId = GetCurrentProcessorNumber();
-
- // On Unix, GetCurrentProcessorNumber() is implemented in terms of sched_getcpu, which
- // doesn't exist on all platforms. On those it doesn't exist on, GetCurrentProcessorNumber()
- // returns -1. As a fallback in that case and to spread the threads across the buckets
- // by default, we use the current managed thread ID as a proxy.
- if (currentProcessorId < 0) currentProcessorId = Environment.CurrentManagedThreadId;
-
- // Add offset to make it clear that it is not guaranteed to be 0-based processor number
- currentProcessorId += 100;
-
- Debug.Assert(ProcessorIdRefreshRate <= ProcessorIdCacheCountDownMask);
-
- // Mask with int.MaxValue to ensure the execution Id is not negative
- t_currentProcessorIdCache = ((currentProcessorId << ProcessorIdCacheShift) & int.MaxValue) | ProcessorIdRefreshRate;
-
- return currentProcessorId;
- }
-
- // Cached processor id used as a hint for which per-core stack to access. It is periodically
- // refreshed to trail the actual thread core affinity.
+ // Cached processor id could be used as a hint for which per-core stripe of data to access to avoid sharing.
+ // It is periodically refreshed to trail the actual thread core affinity.
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int GetCurrentProcessorId()
{
- int currentProcessorIdCache = t_currentProcessorIdCache--;
- if ((currentProcessorIdCache & ProcessorIdCacheCountDownMask) == 0)
- {
- return RefreshCurrentProcessorId();
- }
+ if (s_isProcessorNumberReallyFast)
+ return GetCurrentProcessorNumber();
- return currentProcessorIdCache >> ProcessorIdCacheShift;
+ return ProcessorIdCache.GetCurrentProcessorId();
}
+ // a speed check will determine refresh rate of the cache and will report if caching is not advisable.
+ // we will record that in a readonly static so that it could become a JIT constant and bypass caching entirely.
+ private static readonly bool s_isProcessorNumberReallyFast = ProcessorIdCache.ProcessorNumberSpeedCheck();
+
internal void ResetThreadPoolThread()
{
// Currently implemented in unmanaged method Thread::InternalReset and
projects / platform / upstream / dotnet / runtime.git / commitdiff