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I use Nagios to monitor my systems and occasionally get alerts that I had high memory utilization on some of my machines. But using top and free -m, all I found was confusion.

Until recently, I thought the only way to free up the memory was to reboot the server. When it comes to how OS's manage their memory, it can be kind of unclear for some people.

The reason top and free -m often don't seem to line up is because of the different states of memory. There are two states of used memory, inactive and active.

Active memory is memory that is being used by a particular process. Inactive memory is memory that was allocated to a process that is no longer running.

top and free -m do not differentiate between these two types of memory. This is why free -m may show only 2% memory available. while top may only show a few processes using 1 or 2%.

To better understand the differences between inactive and active memory, think of it this way. If...

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It's been almost 3 years since PHP 7.0 was released. At that time, I quickly updated my servers. When I did that, I killed my old site by killing mysql_*. I elected not to update that site, in favor of rebuilding.

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