We have problem with our Qt based production server for our business application. When total SSL connections increases with time, some clients does not manage to connect at all.
QSslSocket::waitForEncrypted() starts to fail with no QSslError, regardless of that timeout where set. There are more then ~100 active connections when this problem starts to kick in.
So there are ~170 connections, twice of threads, and “lsof” mentions a little more then 1000 opened files (we had to increase file “ulimit” for that..).
It does not look like it’s clients problem, since IPs that are failing and reconnecting changes with time (some “leaps in” with success, but then other don’t).
As mentioned, this happens in Ubuntu Server (Zentyal 10.04 and “vanilla” 9.10), but does NOT in Ubuntu Desktop 9.10.
Everything runs inside VMWare ESX 4.1, systems there tested with same resources attached. System loads stays below 1.0. Daemon runs with root permissions.
It looks like it’s something with “server”/”desktop” kernel or other configuration differences, but I couldn’t tell what exactly could make SSL connection not to handshake… in “server editions”…
We are using Qt 4.5.3 compiled by ourselves.
EDIT: after all it’s the same on any Linux I tried. It feels like it’s some kind socket limit per process, witch is about 1016 – other_opened_files. I’ll try to create new question about that.
EDIT 2: It’s select and FD_SETSIZE limit problem…
Problem is with fact that Qt uses select() which is limited with FD_SETSIZE macro for maximum selected sockets/files. I had to change FD_SETSIZE value inside /usr/include/bits/typesizes.h before compiling libQtNetwork and libQtCore.