why FreeBSD swap they?

[Ken Chen]

I have some web servers to do serve by DNS load-sharing. But one of the
machine always swaped the PHP processes after days, others don't. There PHPs
are running under fastcgi mode.

web4# uname -a
FreeBSD web4.top1069.com 6.2-RELEASE FreeBSD 6.2-RELEASE #0: Thu Apr 19
09:26:59 CST 2007     root at web4.xxxx.com:/usr/src/sys/i386/compile/WEB4
i386
web4# ps -ax | grep -i php | awk '{print $3}' | sort | uniq -c | sort -n
   1 S+
   2 Is
   2 S
   3 IW<
  13 S<
  15 IW<s
  16 I<
  17 I<s
web4#

All machines are with 1GB RAM Only this machine is 6.2, others are 6.0 or
6.1.

$ uname -a
FreeBSD web3.top1069.com 6.1-RELEASE FreeBSD 6.1-RELEASE #1: Mon Mar 19
02:34:10 CST 2007     root at web3.xxxx.com:/usr/src/sys/i386/compile/WEB3
i386
$ ps -ax | grep -i php | awk '{print $3}' | sort | uniq -c | sort -n
   1 RL+
   2 Is
   2 S
   3 I<
  14 S<
  17 I<s