PHP fatal error: Codeigniter: Can't create a new thread; Out of available memory

I keep getting these error messages in my error log:

PHP Fatal error:  Uncaught exception 'Doctrine_Connection_Exception' with message 'PDO Connection Error: SQLSTATE[00000] [1135] Can't create a new thread (errno 12); if you are not out of available memory, you can consult the manual for a possible OS-dependent开发者_Python百科 bug

PHP Fatal error:  Class 'Doctrine_Connection_Exception' not found in /var/www/vhosts/domain.com/httpdocs/application/plugins/doctrine/lib/Doctrine/Connection.php on line 474

PHP Fatal error:  require_once() [<a href='function.require'>function.require</a>]: Failed opening required '/var/www/vhosts/domain.com/httpdocs/system/codeigniter/CodeIgniter.php' (include_path='.:') in /var/www/vhosts/domain.com/httpdocs/index.php on line 140

PHP Warning:  require_once(/var/www/vhosts/domain.com/httpdocs/system/codeigniter/CodeIgniter.php) [<a href='function.require-once'>function.require-once</a>]: failed to open stream: Too many open files in system in /var/www/vhosts/domain.com/httpdocs/index.php on line 140, referer: ''

I do not know if these issues are related to each other or not. Just thought I'd mention it.

Everytime one of this error occurs, the corresponding page on my site does not load. Please help me decode the issue.

Some more information - The site is hosted on Media temple's DV plan.

Thanks in advance, Kamalesh

For the first error: Your SQL server is out of memory. This isn't fixable by PHP, you need to (let someone) configure your SQL server so it has more available memory -OR- profile and optimize your queries (if possible).

The second error: Some PHP file can't find the file that contains the Doctrine_Connection_Exception, from the first error I can see that it does exists and sometimes can be found, I'm guessing it's a problem with your include path,

Thanks for your suggestions. I think I have figured out the problem.

We switched on 'keep_alive' in httpd.conf recently. The errors have stopped propping up on switching it off.