bug#49127: Performance degradation in encode_coding_object

[Eli Zaretskii]

Ping!  Could you please respond to my requests below?  I'd like to
make some progress with this bug report.

> Date: Sun, 20 Jun 2021 12:04:59 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 49127@debbugs.gnu.org
> 
> > 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.
> 
> 
> 
>