GNU bug report logs - #78766
100-4000x redisplay slowdown with vscroll>0 and make-cursor-line-fully-visible=t

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From: JD Smith <jdtsmith <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>, 78766 <at> debbugs.gnu.org
Subject: bug#78766: 100-4000x redisplay slowdown with vscroll>0 and make-cursor-line-fully-visible=t
Date: Fri, 13 Jun 2025 10:07:05 -0400

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Quick followup.  I was able to instrument the single function `set_iterator_to_next' to track total call count and distribution of call times of this core function.  Since the instrumentation slowed the test down so much, I profiled moving (forward-char) by just 3 chars with redisplay in a partially visible top line, using the slow setting of make-cursor-line-fully-visible=t.  

The total call count per character moved in the slow case is enormous: >800K.

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I compared this against the fast case in which make-cursor-line-fully-visible=nil.  Here I moved 56 * 20 iterations = 1,120 chars.  You can see the call count is much, much smaller (although with similar run times per function call):

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I.e. about 280 calls per char moved.  In both cases the time series of calls looks fairly normal:

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