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When using a large number of Virtual Hosts, Apache may run out of available file descriptors (sometimes called file handles if each Virtual Host specifies different log files. The total number of file descriptors used by Apache is one for each distinct error log file, one for every other log file directive, plus 10-20 for internal use. Unix operating systems limit the number of file descriptors that may be used by a process; the limit is typically 64, and may usually be increased up to a large hard-limit.
Although Apache attempts to increase the limit as required, this may not work if:
ulimit -S -n 100
Please see the Descriptors and Apache document containing further details about file descriptor problems and how they can be solved on your operating system.