Index: llvm/trunk/include/llvm/Support/CachePruning.h
===================================================================
--- llvm/trunk/include/llvm/Support/CachePruning.h
+++ llvm/trunk/include/llvm/Support/CachePruning.h
@@ -52,8 +52,11 @@
   /// the number of files based pruning.
   ///
   /// This defaults to 1000000 because with that many files there are
-  /// diminishing returns on the effectiveness of the cache, and file
-  /// systems have a limit on total number of files.
+  /// diminishing returns on the effectiveness of the cache. Some systems have a
+  /// limit on total number of files, and some also limit the number of files
+  /// per directory, such as Linux ext4, with the default setting (block size is
+  /// 4096 and large_dir disabled), there is a per-directory entry limit of
+  /// 508*510*floor(4096/(40+8))~=20M for average filename length of 40.
   uint64_t MaxSizeFiles = 1000000;
 };