Performance figures for changed autorevert implementation

[Michael Albinus]

Hi,

I have collected some performance figures, comparing autorevert with and
without file notification support. All test cases have the same scenario:

- Start "emacs -Q" with exactly one file
- Enable `global-auto-revert-mode'
- Wait for 3600 sec (aka 1 h)
- Dump performance figures
- Kill Emacs

The test file was of length 0, in order to minimize the time to revert
this file if needed. When a file was changed on disk, the command
"echo >$file" was performed outside Emacs.

I did run the test on my laptop (Intel® Core™2 Duo CPU T9300 @ 2.50GHz ×2,
2 GB RAM, Ubuntu 12.04) overnight, no other application did run in parallel.

For several scenarios this test was performed twice, with and without
file notification support (`auto-revert-use-notify' set to t or nil).
Furthermore, the same tests have been performed for local files and for
remote files. In the latter case, the experimental Tramp file name
handler has been used, which is not merged with Emacs trunk yet.

As summary one could say:

* If the file is changed on disk and reverted by Emacs, the file
  notification implementation is slower

* If the file is unchanged, but another file in the same directory is
  changed, the file notification implementation is slower for local
  directories, but faster for remote directories.

* If the file is unchanged, and no other file in the same directory is
  changed as well, the file notification implementation is much faster.

For interpretation of the following numbers: `auto-revert-handler' is
the function which checks regularly all buffers whether they shall be
reverted, and which calls `revert-buffer' if needed.
`auto-revert-notify-handler' is the function for handling incoming file
notification events.

Test 1: Change the test file once per second
============================================

Local file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          6487       3.8662873909 0.0005960054

Local file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          6478       3.5982854019 0.0005554623
auto-revert-notify-handler   7142       0.4215819239 5.902...e-05

Remote file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          7208       135.01537626 0.0187313230

Remote file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          7198       142.25203243 0.0197627163
auto-revert-notify-handler   7146       4.4642421369 0.0006247190


Test 2: Don't change the test file, change another file in the same directory 
once per second
=============================================================================================

Local file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          5768       0.1576693999 2.733...e-05

Local file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-notify-handler   7134       0.3924119630 5.500...e-05
auto-revert-handler          5744       0.0264933590 4.612...e-06

Remote file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          6480       9.4174383650 0.0014533083

Remote file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-notify-handler   7134       4.3312991959 0.0006071347
auto-revert-handler          6471       0.0270106180 4.174...e-06


Test 3: Don't change any file
=============================

Local file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          5768       0.1542307049 2.673...e-05

Local file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          5768       0.0271623790 4.709...e-06

Remote file, without file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          6489       9.1625807580 0.0014120173

Remote file, with file notification
Function Name                Call Count Elapsed Time Average Time
============================ ========== ============ ============
auto-revert-handler          6480       0.0388975389 6.002...e-06