[horde] still having problems with dropped sessions

[Kevin Konowalec]

Horde: 3.1.2
Imp: H3 (4.1.2)
Turba: H3 (2.1.1)
PHP Version: 4.4.1

So I've added a second memcached instance of 512MB (for a total of  
1024MB) but we're still seeing sessions being dropped fairly  
frequently (and no messages in the logs are even remotely helpful).   
By "dropped" I mean the user will be logged in and reading or writing  
mail and suddenly he/she will find themselves booted back to the  
login screen without explanation.  We have roughly 3600-4000  
concurrent users at peak volume - is 1Gig enough memcached size?  And  
if it's not a memcached problem, where else can anyone suggest we  
look as the cause?  Since this is the busiest time of the year I  
suspect that it's a function of load... but I'm not sure which part  
of the system is failing because of it.  And it's strange in that  
sometimes you can leave yourself logged in all night and your session  
will still be there (as you'd expect) but other times you can be  
booted out several times repeatedly without warning.

I'm not sure if it's an indicator of the way students use the system  
(ie logging in to check their mail several times a day) or if it's a  
result of this repeated login problem but my log analysis showed that  
the number of distinct users logging in per day is dwarfed by the  
total number of logins per day at a ratio of about 3:1 and growing.