所有的网站都崩溃了，当我重新启动Nginx的时候，他们就工作了。 看到日志中的工作进程崩溃

突然在这个VPS的所有网站停止响应。 服务器已启动，内存使用情况正常。 但在Nginx错误日志中看到这个：

2014/07/20 07:10:24 [alert] 13671#0: worker process 15020 exited on signal 5 (core dumped) 2014/07/20 07:15:11 [alert] 13671#0: worker process 16327 exited on signal 5 (core dumped) 2014/07/20 07:15:11 [alert] 13671#0: worker process 16036 exited on signal 5 (core dumped) 2014/07/20 07:15:11 [alert] 13671#0: worker process 16369 exited on signal 5 (core dumped) 2014/07/20 07:15:17 [alert] 17646#0: [ngx_pagespeed 1.7.30.1-3609] [0720/071517:FATAL:critical_selector_finder.cc(118)] Check failed: driver->property_page() != NULL. Backtrace: nginx: worker process() [0x4c6c23] nginx: worker process() [0x4c3dc3] nginx: worker process() [0x850851] nginx: worker process() [0x850e57] nginx: worker process() [0x84bd63] nginx: worker process() [0x5270fa] nginx: worker process() [0x539953] nginx: worker process() [0x6977fd] nginx: worker process() [0x52d239] nginx: worker process() [0x5051ad] nginx: worker process() [0x501b50] nginx: worker process() [0x508446] nginx: worker process() [0x655772] nginx: worker process() [0x6e97dd] nginx: worker process() [0x6ec0e7] nginx: worker process() [0x655772] nginx: worker process() [0x6f546b] nginx: worker process() [0x6f78cc] /lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a) [0x7faee2be0e9a] /lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7faee03e43fd] Backtrace: nginx: worker process() [0x4c6c23] nginx: worker process() [0x4ace7e] nginx: worker process() [0x4c3e9f] nginx: worker process() [0x850851] nginx: worker process() [0x850e57] nginx: worker process() [0x84bd63] nginx: worker process() [0x5270fa] nginx: worker process() [0x539953] nginx: worker process() [0x6977fd] nginx: worker process() [0x52d239] nginx: worker process() [0x5051ad] nginx: worker process() [0x501b50] nginx: worker process() [0x508446] nginx: worker process() [0x655772] nginx: worker process() [0x6e97dd] nginx: worker process() [0x6ec0e7] nginx: worker process() [0x655772] nginx: worker process() [0x6f546b] nginx: worker process() [0x6f78cc] /lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a) [0x7faee2be0e9a] /lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7faee03e43fd] 2014/07/20 07:15:17 [alert] 17645#0: worker process 17646 exited on signal 5 (core dumped) 

可能是什么问题？ 如何防止它再次发生？