PS. One more piece of data: we just switched that server back to poll, still on Twisted 2.5. The CPU is at 30-45% stable, with exactly the same usage. So this is either caused by the epoll() itself, or by the epoll reactor in twisted. From: Alec Matusis [mailto:matusis@matusis.com] Sent: Wednesday, April 11, 2007 12:52 AM To: 'Twisted general discussion' Subject: epoll reactor problems We just switched 2 types of production servers to epoll reactor( Twisted 2.5) from poll reactor (Twisted 2.2). The CPU%% utilization of the first type of server that does not do much except occasionally pushing messages to about 5000 clients dropped from about 40% to 8%, which is very good. The second type of server is more complicated. The CPU utilization of that server (as measured by top) went down from 40% to about 15% after switching to epoll. Here is the problem: after about 10min of running that server with CPU%% staying at about 15%, the CPU suddenly jumps to 99.9% for that process and just stays there. We reproduced this several times. The server remains responsive, even when top shows 99.9% CPU. (Which is very different from 99.9% CPU from real load when we used poll- we are intimately familiar with the performance in that regime unfortunately.) The kernel is 2.6.11.4-21.12-smp Can anybody help with this 99.9% CPU epoll problem?