tune tcmalloc for benchmarks

Author: anonrig

src/workerd/tests/BUILD.bazel

@@ -6,12 +6,21 @@ load("//:build/wd_test.bzl", "wd_test")
 wd_cc_library(
     name = "bench-tools",
     hdrs = ["bench-tools.h"],
+    local_defines = select({
+        "//src/workerd/server:really_use_tcmalloc": ["WD_USE_TCMALLOC"],
+        "//conditions:default": [],
+    }),
     tags = ["workerd-benchmark"],
     visibility = ["//visibility:public"],
     deps = [
         "@capnp-cpp//src/kj:kj-test",
         "@google_benchmark//:benchmark",
-    ],
+    ] + select({
+        # tcmalloc is only available on Linux and when use_tcmalloc is enabled.
+        # We use the malloc_extension API to configure tcmalloc for deterministic benchmarks.
+        "//src/workerd/server:really_use_tcmalloc": ["@tcmalloc//tcmalloc:malloc_extension"],
+        "//conditions:default": [],
+    }),
 )
 
 wd_cc_library(

src/workerd/tests/bench-tools.h

@@ -10,6 +10,34 @@
 
 #include <kj/test.h>
 
+// Configure tcmalloc for deterministic benchmarks on Linux.
+// tcmalloc uses probabilistic heap sampling which can introduce variance in benchmark results.
+// WD_USE_TCMALLOC is defined when tcmalloc is enabled (Linux + use_tcmalloc flag).
+#if defined(WD_USE_TCMALLOC) && defined(WD_IS_BENCHMARK)
+#include "tcmalloc/malloc_extension.h"
+
+namespace workerd::bench {
+
+struct TcmallocBenchmarkConfig {
+  TcmallocBenchmarkConfig() {
+    // Disable heap profiling sampling by setting interval to max value.
+    // Default is ~512KB which causes probabilistic sampling of allocations.
+    tcmalloc::MallocExtension::SetProfileSamplingInterval(std::numeric_limits<int64_t>::max());
+
+    // Disable GWP-ASan guarded sampling. A negative value disables it.
+    tcmalloc::MallocExtension::SetGuardedSamplingInterval(-1);
+
+    // Disable background memory release actions that can cause timing variance.
+    tcmalloc::MallocExtension::SetBackgroundProcessActionsEnabled(false);
+  }
+};
+
+// Global instance ensures configuration runs before main().
+inline TcmallocBenchmarkConfig tcmallocBenchmarkConfig;
+
+}  // namespace workerd::bench
+#endif  // defined(WD_USE_TCMALLOC) && defined(WD_IS_BENCHMARK)
+
 // Define a benchmark. Use microseconds instead of nanoseconds by default, most tests run long
 // enough to not need ns precision.
 #define WD_BENCHMARK(X) BENCHMARK(X)->Unit(benchmark::kMicrosecond)