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When memory and swap run out, the Linux kernel picks a process and kills it to save the system. It is called the OOM killer (out of memory). It usually picks one of the biggest, and on a web server that is often the database. The symptom you see is a service that vanishes without warning and “comes back” once restarted. To confirm, look for the mark in the kernel log; to fix it, find out who is using the memory and shrink it, or give the machine more.
Confirming it was this
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The remedies, cheapest first
| What to do | When it fits |
| Reduce what runs | Turn off services you do not use; look at what starts with the server (systemctl list-unit-files --state=enabled). |
| Tune the web server and PHP workers | Each PHP-FPM or Apache process takes memory; if the allowed maximum (in PHP-FPM, pm.max_children) times the size of each exceeds the memory, the server dies in a peak. Lower the maximum. |
| Tune the database | MariaDB and MySQL reserve memory for cache (in InnoDB, innodb_buffer_pool_size). A value meant for a bigger machine smothers a small one. |
| Cap one service | In systemd, MemoryMax= in a unit (create an override with sudo systemctl edit name). A runaway service is stopped before it drags the others down. For containers, Docker has the --memory option. |
| Fix the cause in the application | A memory leak, a query that loads everything, a plugin that got heavy. The OOM is just the warning. |
| Have swap | Gives slack for short peaks, but it is disk, and far slower. A cushion, not a cure. How to add it: adding swap to a VPS. |
| More memory | When normal use no longer fits. See the plans at VPS servers and what your service allows in Meu Interweb. |
| If the database was killed mid-write, check it after starting it. In the database log look for recovery messages or “crashed” tables, and check the site. See tables marked as crashed. A recent copy is what saves it: backing up with mysqldump and pg_dump. |
| It is not the same as PHP’s “Allowed memory size exhausted”. That is PHP’s own limit for one script, and the server stays up: PHP’s memory limit. The OOM is the whole system’s. |
| Before buying more memory, measure. Many cases are solved by lowering one configuration number. And if you plan to run heavy programs (Docker, several sites, browser automation), add up what each needs, according to their documentation, before choosing the plan. See also common VPS and Cloud errors. |
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A service keeps dying and you cannot see why? Paste us the kernel log lines and the plan name, and we will tell you what we see from our side. Open a support ticket |
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SEE ALSO Seeing what uses CPU, memory and disk |
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