Apache threads locked/100% CPU

[theartofweb]

Last night two Apache threads hung using ~100% CPU and needing to be KILLed:

root     20139  0.0  0.6 221440 28140 ?        Ss   Mar31   3:19 /usr/sbin/apache2 -k start

www-data 31091 99.5  0.4 265340 19172 ?        Sl   Apr24 781:38  \_ /usr/sbin/apache2 -k start

www-data 31404 99.6  0.5 337932 21660 ?        Sl   Apr24 780:24  \_ /usr/sbin/apache2 -k start

In the Apache error.log are the following mod_pagespeed errors:

[Wed Apr 24 22:47:25 2013] [error] [mod_pagespeed 1.5.27.1-2857 @32668] Serf status 110(Connection timed out) polling for 3 threaded fetches for 0.05 seconds

[Wed Apr 24 22:47:25 2013] [error] [mod_pagespeed 1.5.27.1-2857 @32668] Serf status 110(Connection timed out) polling for 2 threaded fetches for 0.05 seconds

Munin graphs confirm a spike in "CPU usage" and "Interrupts" starting at the same timestamp.

We are running two instances of Apache with a total of up to 100 children.  Other threads were unaffected.