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    <p>The suggestion of setting <code>debug-on-quit</code> to <code>t</code> so that you can find out what Emacs is up to is a good one. You can think of this as being a form of sampling profiling with a single sample: often a single sample is all you need.</p> <hr> <p><strong>Update:</strong> Starting with version 24.3, Emacs contains <em>two</em> profilers. There's a (new) sampling profiler in <code>profiler.el</code>, and an (old) instrumenting profiler in <code>elp.el</code>.</p> <p>The sampling profiler is <a href="http://www.gnu.org/software/emacs/manual/html_node/elisp/Profiling.html" rel="noreferrer">documented here</a>. It's pretty straightforward to use:</p> <blockquote> <p>To begin profiling, type <code>M-x profiler-start</code>. You can choose to profile by processor usage, memory usage, or both. After doing some work, type <code>M-x profiler-report</code> to display a summary buffer for each resource that you chose to profile. When you have finished profiling, type <code>M-x profiler-stop</code>.</p> </blockquote> <p>Here's some example output from a <code>cpu+mem</code> profiler session with the <a href="https://github.com/gareth-rees/p4.el" rel="noreferrer">Perforce/Emacs integration</a> that I maintain. I've expanded the topmost function (<code>progn</code>) in order to find out <a href="https://github.com/gareth-rees/p4.el/issues/112" rel="noreferrer">where the CPU time and memory use is coming from</a>.</p> <pre class="lang-none prettyprint-override"><code>Function Bytes % - progn 26,715,850 29% - let 26,715,850 29% - while 26,715,850 29% - let 26,715,850 29% - cond 26,715,850 29% - insert 26,715,850 29% + c-after-change 26,713,770 29% + p4-file-revision-annotate-links 2,080 0% + let 20,431,797 22% + call-interactively 12,767,261 14% + save-current-buffer 10,005,836 11% + while 8,337,166 9% + p4-annotate-internal 5,964,974 6% + p4-annotate 2,821,034 3% + let* 2,089,810 2% </code></pre> <p>You can see that the culprit is <code>c-after-change</code>, so it looks as though I could save a lot of CPU time and memory by <a href="https://github.com/gareth-rees/p4.el/commit/4e015793877a56c4ccb1bfbb6e2d278be7c078d8" rel="noreferrer">locally binding <code>inhibit-modification-hooks</code> to <code>t</code> around this code</a>.</p> <hr> <p>You can also use the Emacs Lisp Profiler. This is rather under-documented: you'll have to read the comments in <code>elp.el</code> for the details, but basically you run <code>elp-instrument-package</code> to turn on profiling for all the functions with a given prefix, and then <code>elp-results</code> to see the results.</p> <p>Here's some typical output after typing <code>M-x elp-instrument-package RET c- RET</code>, fontifying 4,000 lines of C, and then running <code>elp-results</code> (and using <code>elp-sort-by-function</code> to sort by call count):</p> <pre class="lang-none prettyprint-override"><code>Function Name Call Count Elapsed Time Average Time ============================= ========== ============ ============ c-skip-comments-and-strings 107 0.0 0.0 c-valid-offset 78 0.0 0.0 c-set-offset 68 0.031 0.0004558823 c-end-of-macro 52 0.0 0.0 c-neutralize-CPP-line 52 0.0 0.0 c-font-lock-invalid-string 20 0.0 0.0 c-set-style-1 19 0.031 0.0016315789 ... </code></pre> <p>In your particular case the profiler doesn't help immediately, because you don't know which package is at fault. But if you can make a guess (or use <code>debug-on-quit</code> to find it for sure) then the profiler can help you diagnose the problem in detail.</p>
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