fix members of a multibar always rendering

[remi-dupre]

Hi!

I noticed a rather inconvenient performance issue with MultiBar, contrary to regular progress bar no rate limiter will prevent computing the string that will displayed for a given member ProgressBar. The cost of this "hidden rendering" can quickly add up if unicode+ANSI decoding features are enabled.

My implementation relies on actually rejecting calls to DrawTarget::drawable for multi-bar targets but marking the member as "delayed" for later redraw. When a call for drawable is actually accepted, all delayed member or first redrawn.

Here is an example flamegraph from the application I'm building, using the main branch :

A recurring call to set_length takes 30% of the running time in the example above. With my patch it is down to <4% :

[djc]

This seems like a pretty roundabout way of doing it. Could we instead keep the Arc<Mutex<BarState>> in MultiStateMember and let all the members draw only when the MultiState::draw_target rate limit allows it?

[remi-dupre]

Think I get what you mean, I'll try something 👍

[remi-dupre]

Actually i'm not so sure I get it 😞

Do you mean removing the "delayed" logic altogether? If I do that, a low frequency bar might never redraw if a high frequency bar gets all the allowances 😕

[djc]

In my mind the ideal way of doing this is that we should only draw all of the multistate members together into a single drawable, relying on the MultiState::draw_target's rate limiter to decide when to draw. I think that this solution should not suffer from the problem of low frequency bars never redrawing because all bars would always get drawn at the same time.

It might be tricky to make this work out, though.