Fix available memory reporting on macOS

src/BenchmarkDotNet/Environments/PhysicalMemoryInfo.cs

@@ -361,23 +361,10 @@ private static bool IsInstalledMemoryDevice(ReadOnlySpan<byte> device)
             if (string.IsNullOrEmpty(sysctlOutput) || !long.TryParse(sysctlOutput!.Trim(), out total) || total == 0)
                 return null;
 
-            // 2. Get Free Memory using vm_stat
+            // 2. Get Available Memory using vm_stat
             string? vmStatOutput = ProcessHelper.RunAndReadOutput("vm_stat");
             if (!string.IsNullOrEmpty(vmStatOutput))
-            {
-                long pageSize = 4096;
-                var pageSizeMatch = Regex.Match(vmStatOutput, @"page size of (\d+) bytes");
-                if (pageSizeMatch.Success && long.TryParse(pageSizeMatch.Groups[1].Value, out long parsedPageSize))
-                {
-                    pageSize = parsedPageSize;
-                }
-
-                var match = Regex.Match(vmStatOutput, @"Pages free:\s+(\d+)");
-                if (match.Success && long.TryParse(match.Groups[1].Value, out long pagesFree))
-                {
-                    available = pagesFree * pageSize;
-                }
-            }
+                available = ParseMacAvailableMemory(vmStatOutput!);
 
             var (speed, type) = GetMacMemoryDetails();
             return new PhysicalMemoryInfo(total, available, speed, type);
@@ -453,5 +440,30 @@ void ReadMemoryDevice(JsonElement device)
                 }
             }
         }
+
+        /// <summary>
+        /// Available memory is free + speculative + inactive pages, the pages macOS can reuse for new allocations.
+        /// vm_stat reports speculative pages separately from "Pages free". Only "Pages free" is required.
+        /// </summary>
+        internal static long? ParseMacAvailableMemory(string output)
+        {
+            long pageSize = 4096;
+            var pageSizeMatch = Regex.Match(output, @"page size of (\d+) bytes");
+            if (pageSizeMatch.Success && long.TryParse(pageSizeMatch.Groups[1].Value, out long parsedPageSize))
+                pageSize = parsedPageSize;
+
+            if (TryGetPages("free") is not long availablePages)
+                return null;
+
+            availablePages += TryGetPages("speculative") ?? 0;
+            availablePages += TryGetPages("inactive") ?? 0;
+            return availablePages * pageSize;
+
+            long? TryGetPages(string pageName)
+            {
+                var match = Regex.Match(output, $@"Pages {pageName}:\s+(\d+)");
+                return match.Success && long.TryParse(match.Groups[1].Value, out long pages) ? pages : null;
+            }
+        }
     }
 }

tests/BenchmarkDotNet.Tests/Environments/PhysicalMemoryInfoTests.cs

@@ -308,5 +308,71 @@ private static byte[] WithStrings(byte[] formatted, string[] strings)
             bytes.Add(0);
             return bytes.ToArray();
         }
+
+        [Fact]
+        public void MacAvailableMemoryIncludesReusablePages()
+        {
+            const string vmStat = """
+                Mach Virtual Memory Statistics: (page size of 16384 bytes)
+                Pages free:                               100.
+                Pages active:                           2000.
+                Pages inactive:                          300.
+                Pages speculative:                        50.
+                Pages throttled:                           0.
+                Pages wired down:                       500.
+                """;
+
+            Assert.Equal(7_372_800, SystemMemory.ParseMacAvailableMemory(vmStat));
+        }
+
+        [Fact]
+        public void MacAvailableMemoryReturnsZeroWhenAllAvailablePagesAreZero()
+        {
+            const string vmStat = """
+                Mach Virtual Memory Statistics: (page size of 4096 bytes)
+                Pages free:                                 0.
+                Pages inactive:                             0.
+                Pages speculative:                           0.
+                """;
+
+            Assert.Equal(0, SystemMemory.ParseMacAvailableMemory(vmStat));
+        }
+
+        [Fact]
+        public void MacAvailableMemoryReturnsNullWhenFreePagesAreMissing()
+        {
+            const string vmStat = """
+                Mach Virtual Memory Statistics: (page size of 4096 bytes)
+                Pages inactive:                          300.
+                Pages speculative:                        50.
+                """;
+
+            Assert.Null(SystemMemory.ParseMacAvailableMemory(vmStat));
+        }
+
+        [Fact]
+        public void MacAvailableMemoryTreatsMissingReusablePagesAsZero()
+        {
+            const string vmStat = """
+                Mach Virtual Memory Statistics: (page size of 16384 bytes)
+                Pages free:                               100.
+                Pages inactive:                          300.
+                """;
+
+            Assert.Equal(400 * 16384, SystemMemory.ParseMacAvailableMemory(vmStat));
+        }
+
+        [Fact]
+        public void MacAvailableMemoryDefaultsTo4KPagesWhenPageSizeIsMissing()
+        {
+            const string vmStat = """
+                Mach Virtual Memory Statistics:
+                Pages free:                               100.
+                Pages inactive:                          300.
+                Pages speculative:                        50.
+                """;
+
+            Assert.Equal(450 * 4096, SystemMemory.ParseMacAvailableMemory(vmStat));
+        }
     }
 }