Today one of my virtual machines didn't deliver content any more: Apache had port 80 open, but did not answer any request. Looking into Parallels, the privvmpages parameter was close to the hard limit and oomguarpages was way over the soft limit (no hard limit given). I couldn't login via SSH any more. SSH returned a key exchange error.
After a machine restart, all services on the VM worked again, but soon both mentioned parameters grew critical again. Spiders were crawling the site with many parallel connections.
Recently, the served website changed from Typo3 to Drupal. The settings of Apache were tuned for optimal performance of Typo3, but the requirements for the new Drupal are somewhat higher. In apache.conf, I decreased the number of allowed parallel connections and the Apache instances in general. The problem does not seem to occur any more. The performance did not suffer, even in the presence of crawlers.
I wonder, why the problem occured at all: Less than a quarter of the memory of the VM was used when I realized the problem in the beginning. Did the kernel try to optimize RAM usage by using paging too excessively...?
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