pagezero again

Ivan Voras ivoras at
Fri May 12 14:14:22 PDT 2006

Some time ago I reported that pagezero kernel thread sometime takes 
(what seems to me) a too large chunk of available CPU (30%) on a very 
busy web server. There were no replies :( Since then, I've reconfigured 
apache to use PHP as a fastcgi module and the problem seems to have 
gotten worse - now it *always* takes ~25-30% CPU, and my "System" time 
stat shows it (its almost always ~30%).

 From what I can understand, pagezero thread fills kernel's pool of 
zeroed pages. This looks non-threteaning, but what would cause such high 
demand of zeroed pages on my system? The number of processes is almost 
constant, there are no frequent process spawnings or forks (top shows 
"last pid" to be almost constant). FastCGI processes communicate over 
UNIX sockets.

Also, pagezero thread is (should be) executing at idle priority - does 
this mean it won't interfere much with machine's performance? Even if 
"0% idle" is not uncommon state?

(This is FreeBSD 6.1-R, 2CPU SMP).

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