[compiler-rt] [scudo] Reduce unsuccessful attempts of page releasing (PR #110583)

[Christopher Ferris via llvm-commits]

================
@@ -1354,34 +1398,45 @@ template <typename Config> class SizeClassAllocator64 {
 
     const uptr RegionPushedBytesDelta =
         BytesInFreeList - Region->ReleaseInfo.BytesInFreeListAtLastCheckpoint;
-    if (RegionPushedBytesDelta < PageSize)
-      return false;
-
-    // Releasing smaller blocks is expensive, so we want to make sure that a
-    // significant amount of bytes are free, and that there has been a good
-    // amount of batches pushed to the freelist before attempting to release.
-    if (isSmallBlock(BlockSize) && ReleaseType == ReleaseToOS::Normal)
-      if (RegionPushedBytesDelta < Region->TryReleaseThreshold)
-        return false;
 
     if (ReleaseType == ReleaseToOS::Normal) {
-      const s32 IntervalMs = atomic_load_relaxed(&ReleaseToOsIntervalMs);
-      if (IntervalMs < 0)
+      if (RegionPushedBytesDelta < Region->ReleaseInfo.TryReleaseThreshold / 2)
+        return false;
+
+      const u64 IntervalNs =
+          static_cast<u64>(atomic_load_relaxed(&ReleaseToOsIntervalMs)) *
+          1000000;
+      if (IntervalNs < 0)
         return false;
 
-      // The constant 8 here is selected from profiling some apps and the number
-      // of unreleased pages in the large size classes is around 16 pages or
-      // more. Choose half of it as a heuristic and which also avoids page
-      // release every time for every pushBlocks() attempt by large blocks.
-      const bool ByPassReleaseInterval =
-          isLargeBlock(BlockSize) && RegionPushedBytesDelta > 8 * PageSize;
-      if (!ByPassReleaseInterval) {
-        if (Region->ReleaseInfo.LastReleaseAtNs +
-                static_cast<u64>(IntervalMs) * 1000000 >
-            getMonotonicTimeFast()) {
-          // Memory was returned recently.
+      const u64 CurTimeNs = getMonotonicTimeFast();
+      const u64 DiffSinceLastReleaseNs =
+          CurTimeNs - Region->ReleaseInfo.LastReleaseAtNs;
+
+      // At here, `RegionPushedBytesDelta` is more than half of
+      // `TryReleaseThreshold`. If the last release was happened 2 release
+      // interval before, we will still try to see if there's any chance to
+      // release some memory even it doesn't exceed the threshold.
+      if (RegionPushedBytesDelta < Region->ReleaseInfo.TryReleaseThreshold) {
+        // We want the threshold to have a shorter response time to the variant
+        // memory usage patterns. By having some experiments (which was done
+        // with 1, 2, 4, 8 intervals), `2` strikes the better balance between
+        // the memory usage and number of page release attempts.
+        if (DiffSinceLastReleaseNs < 2 * IntervalNs)
           return false;
-        }
+      } else if (DiffSinceLastReleaseNs < IntervalNs) {
+        // In this case, we are over the threshold but we just did some page
----------------
cferris1000 wrote:

we just did some page -> we just released some pages in the same release interval.

https://github.com/llvm/llvm-project/pull/110583