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bug#49127: Performance degradation in encode_coding_object
From: |
Eli Zaretskii |
Subject: |
bug#49127: Performance degradation in encode_coding_object |
Date: |
Sun, 20 Jun 2021 12:04:59 +0300 |
> Date: Sun, 20 Jun 2021 08:30:24 +0200
> From: Victor Nawothnig via "Bug reports for GNU Emacs,
> the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>
> With gprof/prof_events I have nailed the problem to be encode_coding_object
> looping over all markers. In degenerate cases this list can contain millions
> of markers. Traversing this list is particularly slow because of the
> indirection being a singly linked list. Based on the fact that a GC remedies
> this, I’m assuming this list contains mostly unreachable markers. When
> stepping through encode_coding_object with GDB after a GC this list of
> markers shrinks to small double digit numbers from millions.
>
> The source of these markers appears to be looking-at in the font locking code
> of haskell-mode, this assumption is based on the fact that commenting out the
> uses of looking-at in haskell-mode prevents the accumulation of markers and
> thus the slowdown.
Do you understand why using looking-at causes creation of markers? If
so, can you show the details of why this happens?
> One contributing factor to all of this, is that for lsp-mode to perform
> adequately, one needs a relatively high gc-cons-threshold, which means GCs
> that would clean up the markers run more rarely, leading to higher
> accumulation of markers over time.
Yes, playing with GC threshold is usually a bad idea, but it is hard
to explain to people why, and they keep doing that, to their cost.
> This problem only triggers in terminal frames, but not in GUI frames. Setting
> GDB breakpoints suggests that the GUI frame never even calls into
> encode_coding_object.
Can you should a backtrace from the call to encode_coding_object,
including the Lisp backtrace (via the "xbacktrace" command)?
> So far I’m torn on whether this is a bug in the haskell-mode font locking
> code or in Emacs. What do you think?
Let's revisit this question after we have all the data I requested
above, okay?
Thanks.