Index: include/llvm/Support/CachePruning.h
===================================================================
--- include/llvm/Support/CachePruning.h
+++ include/llvm/Support/CachePruning.h
@@ -52,8 +52,13 @@
   /// 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.
+  /// Some file systems have a limit on the 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;
 };