Xpra: Ticket #2091: xdummy: xorg server slow to parse modeline config

Now that we have thousands of modelines in the xdummy config (r16160), the server can take too long to startup. ie: r20970 This is more of a problem on low-power devices: #1777

This was recorded in ticket:1557#comment:11: This makes the Xdummy server a little bit slower to start, so r16252 increases the timeout used in the automated tests.

Looking at the Xorg server code, there is a 3-level nested loop on modelines in xf86ValidateModes.

The middle loop reads: If the supplied mode names don't produce a valid mode, scan through unconsidered modePool members until one survives validation. This is done in decreasing order by mode pixel area. The innermost loop, never actually triggers anything, its docstring says: Deal with the case where this mode wasn't considered because of a builtin mode of the same name With a thousand modeline entries, we can end up running the strcmp millions of times...

We should be able to re-work that code to achieve the same results with a lot less CPU time. On a decent CPU this can take multiple seconds, it should be doable under 100ms.

Thu, 27 Dec 2018 20:45:55 GMT - Antoine Martin: status changed

This also explains why the unit tests have been failing occasionally.

Sun, 04 Aug 2019 04:21:59 GMT - Antoine Martin: milestone changed


Thu, 05 Mar 2020 10:49:29 GMT - Antoine Martin: milestone changed

Less of a problem recently? Did this get fixed or improved already?

See also #1467

Wed, 13 Jan 2021 03:24:01 GMT - Antoine Martin: milestone changed

Less of a problem as Debian builds have reverted to Xvfb: #2834

Sat, 23 Jan 2021 05:41:58 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2091