I'm experiencing some strange memory management behaviour on Squeeze with Kernel 2.6.  I'm trying to get to the bottom of this with nmon.Sometimes this will freeze up the system for 30 seconds to 5 minutes.The server has 4GB of RAM.  The figures from nmon are:+1.6GB memfree.  cached dropped by ~350MBswapcached dropped by ~80MB swap free jumped by ~200Mactive memory dropped by ~1.3GBWhen this happens, the server isn't near any limits.  Very soon after it happens, the active memory creeps back up, along with cached... and naturally memfree drops.This doesn't appear to be a runaway process.  The OS just seems to reallocate a bunch of ram very suddenly, then it slowly reallocates it back.  Swappiness shouldn't be a factor because swap is barely ever touched.Is there any way to track what is happening?  Why does this free memory suddenly appear in seconds, only to be given back to the cache within 30 minutes?  
Sudden burst in free memory
memory;virtual memory
Turns out that it was a DoS.  Some 60 php5-cgi processes appear and die within a 30 second period, sucking back 1.6GB of RAM then are killed.It didn't show up in my logs because it was not a single process sucking up the 1.6GB, it was 60-someodd processes.  And the oom killer doesn't kill it, something in Apache is killing it.I finally found it when I adjusted my process logging to get the breadth of processes and wrote some awk scripts to add up their memory.  It seems obvious now.