Regression: Items are not being evicted from MemoryCache in .NET 6

[RobSiklos]

Description

This works in .NET Framework, but in .NET 6, I'm seeing an issue where items added to a MemoryCache are not evicted based on the cache memory limit.

Likely related, no matter how much I add to the cache, MemoryCache.GetLastSize() always returns 0 in .NET 6.

Reproduction Steps

Run the code below, and notice that items are never evicted from the cache, and that the result of GetLastSize() is always 0.

using System.Collections.Specialized;
using System.Globalization;
using System.Runtime.Caching;

private static readonly TimeSpan CachePollingInterval = TimeSpan.FromSeconds(1);
private const long CacheMemoryLimitMegabytes = 1024;

void Main()
{
    NameValueCollection cacheConfig = new NameValueCollection();
    cacheConfig.Add("cacheMemoryLimitMegabytes", CacheMemoryLimitMegabytes.ToString(CultureInfo.InvariantCulture));
    cacheConfig.Add("physicalMemoryLimitPercentage", "0");
    cacheConfig.Add("pollingInterval", CachePollingInterval.ToString(@"hh\:mm\:ss", CultureInfo.InvariantCulture));

    using (var cache = new MemoryCache("RobTest", cacheConfig))
    {
        Console.WriteLine($"Memory limit: {cache.CacheMemoryLimit} MB");
        Console.WriteLine($"Polling interval: {cache.PollingInterval}");

        int num = 40; // The number of elements we're going to add to the cache.

        for (int i = 0; i < num; i++)
        {
            // Create an array and initialize it with non-default values so that it actually takes up space in memory.
            var arr = new byte[100_000_000];
            for (int j = 0; j < arr.Length; j++) { arr[j] = 0xF; }

            // Add the item to the cache.
            cache.Add(i.ToString(), arr, DateTimeOffset.Now + TimeSpan.FromMinutes(5));

            Console.WriteLine();
            Console.WriteLine($"Elements in the cache: {cache.GetCount()}");
            Console.WriteLine($"Cache size: {cache.GetLastSize()}%");

            Thread.Sleep(1000);
        }
    }
}

Expected behavior

1. At some point, elements are evicted from the cache, in consideration of the specified polling interval.
2. At some point, the result of GetLastSize() is non-zero.

Actual behavior

In .NET 6, items are not evicted from the cache, and the result of GetLastSize() is always zero.

Regression?

Yes, this worked as expected in .NET Framework.

Known Workarounds

No response

Configuration

Running .NET 6.0.8 on Windows 10.0.19044.0 (x64)

Other information

No response

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Issue Details

---

Description

This works in .NET Framework, but in .NET 6, I'm seeing an issue where items added to a MemoryCache are not evicted based on the cache memory limit.

Likely related, no matter how much I add to the cache, MemoryCache.GetLastSize() always returns 0 in .NET 6.

Reproduction Steps

Run the code below, and notice that items are never evicted from the cache, and that the result of GetLastSize() is always 0.

using System.Collections.Specialized;
using System.Globalization;
using System.Runtime.Caching;

private static readonly TimeSpan CachePollingInterval = TimeSpan.FromSeconds(1);
private const long CacheMemoryLimitMegabytes = 1024;

void Main()
{
    NameValueCollection cacheConfig = new NameValueCollection();
    cacheConfig.Add("cacheMemoryLimitMegabytes", CacheMemoryLimitMegabytes.ToString(CultureInfo.InvariantCulture));
    cacheConfig.Add("physicalMemoryLimitPercentage", "0");
    cacheConfig.Add("pollingInterval", CachePollingInterval.ToString(@"hh\:mm\:ss", CultureInfo.InvariantCulture));

    using (var cache = new MemoryCache("RobTest", cacheConfig))
    {
        Console.WriteLine($"Memory limit: {cache.CacheMemoryLimit} MB");
        Console.WriteLine($"Polling interval: {cache.PollingInterval}");

        int num = 40; // The number of elements we're going to add to the cache.

        for (int i = 0; i < num; i++)
        {
            // Create an array and initialize it with non-default values so that it actually takes up space in memory.
            var arr = new byte[100_000_000];
            for (int j = 0; j < arr.Length; j++) { arr[j] = 0xF; }

            // Add the item to the cache.
            cache.Add(i.ToString(), arr, DateTimeOffset.Now + TimeSpan.FromMinutes(5));

            Console.WriteLine();
            Console.WriteLine($"Elements in the cache: {cache.GetCount()}");
            Console.WriteLine($"Cache size: {cache.GetLastSize()}%");

            Thread.Sleep(1000);
        }
    }
}

Expected behavior

1. At some point, elements are evicted from the cache, in consideration of the specified polling interval.
2. At some point, the result of GetLastSize() is non-zero.

Actual behavior

In .NET 6, items are not evicted from the cache, and the result of GetLastSize() is always zero.

Regression?

Yes, this worked as expected in .NET Framework.

Known Workarounds

No response

Configuration

Running .NET 6.0.8 on Windows 10.0.19044.0 (x64)

Other information

No response

Author:	RobSiklos
Assignees:	-
Labels:	area-Extensions-Caching
Milestone:	-

[StephenMolloy]

This may tie in with #62516 which has been labeled as a documentation issue. I know there are some features of MemoryCache from Framework that don't work on Core... but I thought MemoryCache.GetLastSize() was supposed to be non-zero here. I think this probably warrants a closer look.

[KalleOlaviNiemitalo]

MemoryCache.GetLastSize() calls MemoryCacheStatistics.GetLastSize(), which reads MemoryMonitor.PressureLast, which reads _pressureHist[_i0], to which the data comes from CacheMemoryMonitor.GetCurrentPressure(), which computes the result from SRefMultiple.ApproximateSize, which is hardcoded to return 0

runtime/src/libraries/System.Runtime.Caching/src/System/Runtime/Caching/SRef.cs

Lines 13 to 26 in 9295993

	// until then we provide a stub
	internal sealed class SRefMultiple
	{
	internal SRefMultiple()
	{
	}
	#pragma warning disable CA1822
	internal long ApproximateSize => 0;
	internal void Dispose()
	{
	}
	#pragma warning restore CA1822
	}

because System.SizedReference is not available.

[akoeplinger]

This depends on some solution being implemented for #24200