Ben Jackson wrote:
> I have been investigating some pathological performance problems with
> YouCompleteMe (YCM). YCM is a vim plugin which, in effect calls
> complete() on every keystroke in insert mode. I have noticed that
> sometimes, vim can become extremely slow while typing in this
> scenario.
>
> I did some profiling with callgrind and found that 99% of the time was
> redrawing, and in particular syntax highlighting regex code. Of
> course, doing something very often makes it appear “slow”.
>
> Anyway, my solution has been to reduce the number of times I call
> complete(), but I’m still curious if this can be improved. I’m not
> super clear on the exact set of calls here, but looking at the code it
> _appears_ that we force a redraw of all windows whenever the pum is
> undisplayed (pum_undisplay calls redraw_all_later(UPD_NOT_VALID),
> which is called by set_completion, via ins_compl_free, etc.).
>
> I think this also happens when any key is pressed while the pum is
> displayed (however it is displayed).
>
> Am I understanding the behaviour correctly - that a full redraw is
> requested whenever the pum is visible and a key is pressed, or
> complete() is called?
> Is there a way this redraw can be reduced? I seem to recall that popup
> windows have a clever way to minimise the redraw by storing what they
> are obscuring (or did I imagine that?).
If a popup window is closed, moved or resized, the windows under it are
updated with UPD_NOT_VALID. The window contents (with text and
attributes) isn't cached, it needs to be recomputed. Normally this is
fast enough, but with complex syntax highlighting it can be a bit slow.
Caching the text and attributes under the popup might be possible, but
it is tricky. Especially with completion, some text might be put in the
line (which is removed again when not using the completion) which
requires redrawing text around it. The caching only helps in limited
situations.
What would help in general is to make syntax highlighting faster. At
least avoid bottlenecks.
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