Merge branch 'bp/read-cache-parallel'

A new extension to the index file has been introduced, which allows
the file to be read in parallel.

* bp/read-cache-parallel:
  read-cache: load cache entries on worker threads
  ieot: add Index Entry Offset Table (IEOT) extension
  read-cache: load cache extensions on a worker thread
  config: add new index.threads config setting
  eoie: add End of Index Entry (EOIE) extension
  read-cache: clean up casting and byte decoding
  read-cache.c: optimize reading index format v4

1 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index db3572626b..7c4d67aa6a 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -314,3 +314,44 @@ The remaining data of each directory block is grouped by type:
 
   - An ewah bitmap, the n-th bit indicates whether the n-th index entry
     is not CE_FSMONITOR_VALID.
+
+== End of Index Entry
+
+  The End of Index Entry (EOIE) is used to locate the end of the variable
+  length index entries and the begining of the extensions. Code can take
+  advantage of this to quickly locate the index extensions without having
+  to parse through all of the index entries.
+
+  Because it must be able to be loaded before the variable length cache
+  entries and other index extensions, this extension must be written last.
+  The signature for this extension is { 'E', 'O', 'I', 'E' }.
+
+  The extension consists of:
+
+  - 32-bit offset to the end of the index entries
+
+  - 160-bit SHA-1 over the extension types and their sizes (but not
+	their contents).  E.g. if we have "TREE" extension that is N-bytes
+	long, "REUC" extension that is M-bytes long, followed by "EOIE",
+	then the hash would be:
+
+	SHA-1("TREE" + <binary representation of N> +
+		"REUC" + <binary representation of M>)
+
+== Index Entry Offset Table
+
+  The Index Entry Offset Table (IEOT) is used to help address the CPU
+  cost of loading the index by enabling multi-threading the process of
+  converting cache entries from the on-disk format to the in-memory format.
+  The signature for this extension is { 'I', 'E', 'O', 'T' }.
+
+  The extension consists of:
+
+  - 32-bit version (currently 1)
+
+  - A number of index offset entries each consisting of:
+
+    - 32-bit offset from the begining of the file to the first cache entry
+	in this block of entries.
+
+    - 32-bit count of cache entries in this block