performance degradation with diskcache enabled

[bangert]

i have a grayproxy without -dataDir running with about 800 msg/s. grayproxy is using about a third of a core for this - we have been very happy.

We now wanted to enable the disk cache, but when we do that grayproxy stops forwarding any messages. cpu usage drops a little, probably due to the iowait that is also present now.

we did not experience any problem with the diskcache when running with a low number of messages/s.