[Unison-hackers] [patch] high system time with BTRFS and compression...

[Daniel J Blueman]

With unison 2.27.57 on Linux (Ubuntu 10.04 x86-64, client and server),
performing synchronisation with a BTRFS filesystem with compression
enabled takes higher system time, due to many threads performing
smaller (~4K) synchronous reads.

Increasing the buffer size to 64K helps alleviate this, and will
provide a smaller win on other filesystems. Also, on faster networks
with larger MTU, 64K read/write to the socket provides more
throughput.

Please consider and thanks,
 Daniel

diff -ur unison-2.27.57/remote.ml unison-2.27.57-new/remote.ml
--- unison-2.27.57/remote.ml    2008-01-18 13:53:35.000000000 +0000
+++ unison-2.27.57-new/remote.ml        2010-05-26 12:49:00.146269467 +0100
@@ -79,7 +79,7 @@
 let receivedBytes = ref 0.
 let emittedBytes = ref 0.

-let inputBuffer_size = 8192
+let inputBuffer_size = 65536

 let fill_inputBuffer conn =
  assert (conn.inputLength = 0);
@@ -127,7 +127,7 @@

 (****)

-let outputBuffer_size = 8192
+let outputBuffer_size = 65536

 let rec send_output conn =
  catch_io_errors
diff -ur unison-2.27.57/transfer.ml unison-2.27.57-new/transfer.ml
--- unison-2.27.57/transfer.ml  2007-04-02 04:03:20.000000000 +0100
+++ unison-2.27.57-new/transfer.ml      2010-05-26 12:51:12.676894227 +0100
@@ -234,8 +234,8 @@
 let send infd length showProgress transmit =
  debug (fun() -> Util.msg "sending file\n");
  let timer = Trace.startTimer "Sending file using generic transmission" in
-  let bufSz = 8192 in
-  let bufSzFS = Uutil.Filesize.ofInt 8192 in
+  let bufSz = 65536 in
+  let bufSzFS = Uutil.Filesize.ofInt 65536 in
  let buf = String.create bufSz in
  let q = makeQueue length in
  let rec sendSlice length =
@@ -298,14 +298,14 @@
  (*** PREPROCESS ***)

  (* Preprocess buffer size *)
-  let preproBufSize = 8192
+  let preproBufSize = 65536

  (* Incrementally build arg by executing f on successive blocks (of size
     'blockSize') of the input stream (pointed by 'infd').
     The procedure uses a buffer of size 'bufferSize' to load the input,
     and eventually handles the buffer update. *)
  let blockIter infd f arg maxCount =
-    let bufferSize = 8192 + blockSize in
+    let bufferSize = 65536 + blockSize in
    let buffer = String.create bufferSize in
    let rec iter count arg offset length =
      if count = maxCount then arg else begin
@@ -363,7 +363,7 @@
  (*** DECOMPRESSION ***)

  (* Decompression buffer size *)
-  let decomprBufSize = 8192
+  let decomprBufSize = 65536

  (* For each transfer instruction, either output a string or copy one or
     several blocks from the old file. *)
@@ -517,8 +517,8 @@

  (* Compression buffer size *)
  (* MUST be >= 2 * blockSize *)
-  let comprBufSize = 8192
-  let comprBufSizeFS = Uutil.Filesize.ofInt 8192
+  let comprBufSize = 65536
+  let comprBufSizeFS = Uutil.Filesize.ofInt 65536

  (* Compress the file using the algorithm described in the header *)
  let rsyncCompress sigs infd srcLength showProgress transmit =
-- 
Daniel J Blueman