Emacs Lisp Text Processing: find-file vs with-temp-buffer
This page gives a detailed speed comparion of using emacs lisp's
with-temp-buffer for processing 5 thousand files.
find-file to open 5565 files, takes 63 seconds.
with-temp-buffer, 16 seconds. (4 times faster.)
Moral: when doing batch text processing of thousands of files, don't use
with-temp-file instead. (use the latter when you need to make changes to the file.)
(if you don't know how, see basics at: Emacs Lisp Idioms for Text Processing in Batch Style.)
Here's the test that you can run.
For this testing purpose, the input dir used is the HTML version of GNU Emacs Lisp Reference Manual. A total of ~900 files. If you like to run the test, you can download it at: http://www.gnu.org/software/emacs/manual/elisp.html. (download the “with one web page per node” version)
Here's the actual code.
;; 2011-12-20 ;; Speed test script ;; ;; What the script do: ;; Creates a 〔sitemap.xml〕 file. ;; Open each files in a dir, if the file doesn't contain the word “refresh”, add a entry of the file to 〔sitemap.xml〕. ;; Must end in a slash. Must not start with ~ (setq webroot "/Users/h3/web/xahlee_org/emacs_manual/elisp/") ;; ------------------------ (defun my-process-file (fPath destBuff) "Process the file at fullpath FPATH. Write result to buffer DESTBUFF." (with-temp-buffer (insert-file-contents fPath) (goto-char 1) (when (not (search-forward "refresh" nil "noerror")) (with-current-buffer destBuff (insert "<url><loc>") (insert (concat "http://example.org/" (substring fPath (length webroot)))) (insert "</loc></url>\n") )) ) ) ;; ------------------------ (print (benchmark-run 1 ;; create sitemap buffer (let (filePath sitemapBuf) (setq filePath (concat webroot "sitemap.xml")) (setq sitemapBuf (find-file filePath)) (erase-buffer) (set-buffer-file-coding-system 'unix) (insert "<?xml version=\"1.0\" encoding=\"UTF-8\"?> <urlset xmlns=\"http://www.sitemaps.org/schemas/sitemap/0.9\"> ") (require 'find-lisp) (mapc (lambda (ξx) (my-process-file ξx sitemapBuf)) (find-lisp-find-files webroot "\\.html$")) (insert "</urlset>") (save-buffer) ) )) (message "%s" "Yay, Done!")
To run it:
- Copy and Paste and save this file as
- Change the “webroot” variable to a directory on your computer that has lots HTML files.
- In terminal, run it with “--script”, like this:
emacs --script ~/speedtest_temp-buff.el. This won't load your init files. You init files might contain hooks or other things that effect the speed.
- When the program is finished, it'll create a file named
sitemap.xmlin the same dir of “webroot”.
benchmark-run's output will be printed on the screen.
What the script does is very simple:
- Open each HTML file in a directory
- If the file contains the string “refresh”, then do nothing.
- Else, add the file name as a entry to a file
sitemap.xml. (this file is created by the script.)
This version gets file content by using a temp buffer, like this:
(with-temp-buffer (insert-file-contents fPath) ;; do processing here )
The script is a simplified version of generating a sitemap. 〔►see Emacs Lisp: Create Sitemap〕
find-file version is identical except the
my-process-file function. Like this:
(defun my-process-file (fPath destBuff) "Process the file at fullpath FPATH. Write result to buffer DESTBUFF." (let (myBuffer) (setq myBuffer (find-file fPath)) (goto-char 1) (when (not (search-forward "refresh" nil "noerror")) (with-current-buffer destBuff (insert "<url><loc>") (insert (concat "http://example.org/" (substring fPath (length webroot)))) (insert "</loc></url>\n") )) (kill-buffer myBuffer) ) )
Here's the test results (all timing are in seconds, rounded).
|Script Version||Script Running Time||Garbage Collection||Garbage Collection Time||Actual Time|
Do Not use find-file or write-file
write-file, or any function that visits a file has many unwanted side-effects, and it can be up to 40 times slower (i tested before). Here's example of side-effects:
- It keeps undo info.
- It syntax color the buffer.
- It displays the file. (very slow if you have
- It may have tons of hooks added by others. (
- It may do backup.
Jon Snader (jcs) suggested using the “benchmark-run” for timing report and garbage collection info. http://irreal.org/blog/?p=400. The “benchmark-run” is tremendously useful. Thanks Jon.
Thanks to Trey Jackson for a major correction on this article. In my previous report, the timing difference was a factor of 45, because i had various personal hooks.