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[SDK] Circular buffer tweaks + cpu pressure test #3349

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Merged
merged 7 commits into from
Jun 11, 2022
Merged

[SDK] Circular buffer tweaks + cpu pressure test #3349

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12d1612
CircularBuffer tweaks and cpu pressure test.
CodeBlanch Jun 7, 2022
673ea1d
Switch to Volatile.Read.
CodeBlanch Jun 7, 2022
7bda54c
Merge branch 'main' into circular-buffer-cpupressuretest
cijothomas Jun 8, 2022
dd0a640
Perf tweaks.
CodeBlanch Jun 9, 2022
248d559
Merge branch 'circular-buffer-cpupressuretest' of https://github.com/…
CodeBlanch Jun 9, 2022
223e255
Remove race check in debug after doing more testing.
CodeBlanch Jun 10, 2022
fe3a40f
Merge branch 'main' into circular-buffer-cpupressuretest
CodeBlanch Jun 11, 2022
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50 changes: 27 additions & 23 deletions src/OpenTelemetry/Internal/CircularBuffer.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,9 @@
// limitations under the License.
// </copyright>

#nullable enable

using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading;

Expand All @@ -26,7 +29,7 @@ namespace OpenTelemetry.Internal
internal class CircularBuffer<T>
where T : class
{
private readonly T[] trait;
private readonly T?[] trait;
private long head;
private long tail;

Expand Down Expand Up @@ -54,20 +57,20 @@ public int Count
{
get
{
var tailSnapshot = this.tail;
return (int)(this.head - tailSnapshot);
var tailSnapshot = Volatile.Read(ref this.tail);
return (int)(Volatile.Read(ref this.head) - tailSnapshot);
}
}

/// <summary>
/// Gets the number of items added to the <see cref="CircularBuffer{T}"/>.
/// </summary>
public long AddedCount => this.head;
public long AddedCount => Volatile.Read(ref this.head);

/// <summary>
/// Gets the number of items removed from the <see cref="CircularBuffer{T}"/>.
/// </summary>
public long RemovedCount => this.tail;
public long RemovedCount => Volatile.Read(ref this.tail);

/// <summary>
/// Adds the specified item to the buffer.
Expand All @@ -83,22 +86,23 @@ public bool Add(T value)

while (true)
{
var tailSnapshot = this.tail;
var headSnapshot = this.head;
var tailSnapshot = Volatile.Read(ref this.tail);
var headSnapshot = Volatile.Read(ref this.head);

if (headSnapshot - tailSnapshot >= this.Capacity)
{
return false; // buffer is full
}

var head = Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot);
if (head != headSnapshot)
if (Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot) != headSnapshot)
{
continue;
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@utpilla utpilla Jun 8, 2022

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Would it be useful to add Spinwait.SpinOnce() here before continue?

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@CodeBlanch CodeBlanch Jun 8, 2022

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Defer to @reyang on this one, but I don't think so. This thread isn't so much waiting on another thread to finish as it is learning that some other thread took the head. It should retry immediately and just take the next head/index available.

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@cijothomas cijothomas Jun 8, 2022

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SpinOnce might be more smart yielding if singlecore etc?

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@CodeBlanch CodeBlanch Jun 9, 2022

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I think a yield here would be even worse than a spin 😄 Because it doesn't need to wait on anything. If there is only 1 core this thread should just loop around, take the next index, and continue on doing its thing. Same as if there were many cores, really.

There should probably be a SpinOnce here though:

opentelemetry-dotnet/src/OpenTelemetry/Internal/CircularBuffer.cs

Line 171 in 91d7e83

continue;

Because that logic is actually waiting on the writer thread to finish. On single core, it should yield immediately because a spin won't accomplish anything other than delay letting the writer get the CPU back to finish its job 🤣

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@reyang reyang Jun 10, 2022

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I think a yield here would be even worse than a spin

+1

}

var index = (int)(head % this.Capacity);
this.trait[index] = value;
var previous = Interlocked.Exchange(ref this.trait[headSnapshot % this.Capacity], value);

Debug.Assert(previous == null, "Race: Another thread wrote to index.");

return true;
}
}
Expand All @@ -125,16 +129,15 @@ public bool TryAdd(T value, int maxSpinCount)

while (true)
{
var tailSnapshot = this.tail;
var headSnapshot = this.head;
var tailSnapshot = Volatile.Read(ref this.tail);
var headSnapshot = Volatile.Read(ref this.head);

if (headSnapshot - tailSnapshot >= this.Capacity)
{
return false; // buffer is full
}

var head = Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot);
if (head != headSnapshot)
if (Interlocked.CompareExchange(ref this.head, headSnapshot + 1, headSnapshot) != headSnapshot)
{
if (spinCountDown-- == 0)
{
Expand All @@ -144,8 +147,10 @@ public bool TryAdd(T value, int maxSpinCount)
continue;
}

var index = (int)(head % this.Capacity);
this.trait[index] = value;
var previous = Interlocked.Exchange(ref this.trait[headSnapshot % this.Capacity], value);

Debug.Assert(previous == null, "Race: Another thread wrote to index.");

return true;
}
}
Expand All @@ -161,19 +166,18 @@ public bool TryAdd(T value, int maxSpinCount)
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public T Read()
{
var index = (int)(this.tail % this.Capacity);
var index = (int)(Volatile.Read(ref this.tail) % this.Capacity);
while (true)
{
var value = this.trait[index];
if (value == null)
var previous = Interlocked.Exchange(ref this.trait[index], null);
if (previous == null)
{
// If we got here it means a writer isn't done.
continue;
}

this.trait[index] = null;
this.tail++;
return value;
Interlocked.Increment(ref this.tail);
return previous;
}
}
}
Expand Down
64 changes: 64 additions & 0 deletions test/OpenTelemetry.Tests/Internal/CircularBufferTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,9 @@
// </copyright>

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Xunit;

namespace OpenTelemetry.Internal.Tests
Expand Down Expand Up @@ -109,5 +112,66 @@ public void CheckAddedCountAndCount()
Assert.Equal(1, circularBuffer.RemovedCount);
Assert.Equal(1, circularBuffer.Count);
}

[Fact]
public async Task CpuPressureTest()
{
if (Environment.ProcessorCount < 2)
{
return;
}

var circularBuffer = new CircularBuffer<string>(2048);

List<Task> tasks = new();

int numberOfItemsPerWorker = 100_000;

for (int i = 0; i < Environment.ProcessorCount; i++)
{
int tid = i;

tasks.Add(Task.Run(async () =>
{
await Task.Delay(2000).ConfigureAwait(false);

if (tid == 0)
{
for (int i = 0; i < numberOfItemsPerWorker * (Environment.ProcessorCount - 1); i++)
{
SpinWait wait = default;
while (true)
{
if (circularBuffer.Count > 0)
{
circularBuffer.Read();
break;
}

wait.SpinOnce();
}
}
}
else
{
for (int i = 0; i < numberOfItemsPerWorker; i++)
{
SpinWait wait = default;
while (true)
{
if (circularBuffer.Add("item"))
{
break;
}

wait.SpinOnce();
}
}
}
}));
}

await Task.WhenAll(tasks).ConfigureAwait(false);
}
}
}