Have you ever edited a document in LibreOffice in more than one window? Right, neither have I. Who'd think about LibreOffice and more than one user at the same time, right? Except ... somebody did and that's how collaborative editing based on LibreOffice works. For whatever strange reason, somewhen in the past somebody thought that implementing multiple views for one document in OpenOffice (StarOffice?) was a good idea. Just select Window->New Window in the menu and you can edit your favourite document in 50 views that each show a different part of the document and update in real time. And that, in fact, is how collaborative editing such as with Collabora Online works - open a document, create a new view for every user, and there you go.
But, given that this has never really been used that much, how well did the original relevant code perform and scale for more users? Well, not much, it turns out. Not a big surprise, considering that presumably back when that code was written nobody thought the same document could be edited by numerous users at the same time. But I've been looking exactly into this recently as part of optimizing Collabora Online performance, and boy, are there were gems in there. You thought that showing the same document in more views would just mean more painting also in those views? Nah, think again, this is OpenOffice code, the land of programming wonders.
Profiling the code
When running Online's perf-test, which simulates several users typing in the same document, most of the time is actually spent in SwEditShell::EndAllAction(). It's called whenever Writer finishes an action such as adding another typed characters, and one of the things it does is telling other views about the change. So here LO spends a little time adding the character and then the rest of the time is spent in various code parts "talking" about it. A good part of that is that whenever an action is finished, that view tells the others about it happening, and then all those views tell all other views about how they reacted to it, making every change O(n^2) with the number of views. That normally does not matter, since on the desktop n generally tends to be 1, but hey, add few more views, and it can be a magnitude slower or more.
Redrawing, for example, is rather peculiar. When a part of the document changes, relevant areas of the view need redrawing. So all views get told about the rectangles that need repainting. In the desktop case those can be cropped by the window area, but for tiled rendering used by Online the entire document is the "window" area, so every view gets told about every change. And each view collects such rectangles, and later on it processes them and tells all other views about the changes. Yes, again. And it seems that in rare cases each view really needs its own repaint …
