[squid-users] squid often crashes

We have 7 squids in use. 3 of them are squid 3.1.11, 4 of them are
3.1.12. The squids often crash.

The following is cited from the cache.log of squid 3.1.11

2012/07/28 11:28:54| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/28 13:08:03| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/28 14:47:05| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/29 13:37:34| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/29 15:19:55| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/29 16:26:18| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/07/30 13:07:23| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...

The active connections on squid are about 2000.

We have upgraded one 3.1.12 to 3.1.20, but the problem still exists.

2012/07/28 09:50:37| Starting Squid Cache version 3.1.20 for
x86_64-redhat-linux-gnu...
2012/07/28 09:50:54| Starting Squid Cache version 3.1.20 for
x86_64-redhat-linux-gnu...
2012/07/30 11:27:04| Starting Squid Cache version 3.1.20 for
x86_64-redhat-linux-gnu...
2012/07/30 11:27:11| Starting Squid Cache version 3.1.20 for
x86_64-redhat-linux-gnu...

How can I find out the reason ? It seems there is no clue in the cache.log

2012/07/31 11:23:55| WARNING: unparseable HTTP header field {: 211.137.119.10}
2012/07/31 11:23:55| WARNING: unparseable HTTP header field {: 211.137.119.10}
2012/07/31 11:26:46| WARNING: HTTP: Invalid Response: No object data
received for http://mydomain.com/ajax/book.getUserBookFans
Score.do AKA mydomain.com/ajax/book.getUserBookFansScore.do
2012/07/31 11:28:15| WARNING: HTTP: Invalid Response: No object data
received for http://mydomain.com/ajax/book.getUserBookFans
Score.do AKA mydomain.com/ajax/book.getUserBookFansScore.do
2012/07/31 11:43:10| NETDB state saved; 0 entries, 0 msec
2012/07/31 11:49:09| Starting Squid Cache version 3.1.20 for
x86_64-redhat-linux-gnu...
2012/07/31 11:49:09| Process ID 19635

Another problem is the squids in the same cluster almost crash at the
same time. The 7 squids are in 2 clusters, 3 squids are in one cluster
and 4 squids are in another cluster. The squids are load-balanced
using DNS round robin.

squid1 cache.log
2012/08/01 12:09:19| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/01 15:26:36| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 11:05:27| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 13:43:20| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/03 13:29:41| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 09:33:05| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 11:13:05| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 12:40:19| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...

squid2 cache.log
2012/08/01 12:09:17| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/01 15:26:39| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 11:05:25| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 13:43:18| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 21:08:27| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/03 13:29:39| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 09:33:08| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 12:40:17| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...

squid3 cache.log
2012/08/01 12:09:20| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/01 13:43:17| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/01 15:26:38| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 11:05:23| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/02 13:43:21| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/03 12:14:33| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/03 13:29:51| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 09:33:06| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
2012/08/04 12:40:20| Starting Squid Cache version 3.1.11 for
x86_64-koji-linux-gnu...
Received on Sun Aug 05 2012 - 04:07:47 MDT