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Nuno
Asked: 2020-10-24 01:09:24 +0800 CST

Swap being used when not necessary

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I have: vm.swappiness = 1 as suggested here to prevent using Swap when there is plenty of available memory. (0 would completely disable swap, as far as I understand)

In frequent cases, I end up with, say, 5GB swap used (total 16GB),
while I have 9GB free available memory (total 32GB)
(only 1GB free if we count cached RAM).

As my application starts to get slow (since most of the Swap is from MariaDB), I run:

swapoff -a
swapon -a

And all the swap memory is moved to the main RAM.
I end up with 5.6GB available RAM, 4GB if we count cached.

There is no apparent reason to have so much Swap.
I've been researching about this, and I cannot find a way to prevent using Swap when it's not needed.
vm.swappiness = 1 is definitely "permanently saved", since it is on the sysctl.conf file, and there have been reboots, etc.

I don't want to disable swap, but the only workaround I can think of is:

  1. disable swap
  2. have a cron running every 1 minute to check how much free RAM there is
  3. enable swap if available RAM is low

But, this is VERY risky to do, and I wish the OS was smart enough to manage this better!

Is there absolutely no way to only use swap when absolutely needed?

centos memory swap centos7 swapping

1 Answers

  1. Do you have lot of stuff running on the server? If so, maybe consider a dedicated machine or VM for the DB.

    Is this physical or VM/cloud server? Some cloud providers do NOT want you to use swap at all (so just putting that out there in case it is relevant): https://docs.rackspace.com/support/how-to/swap-space-on-cloud-servers/

    There is some good discussion on this page for troubleshooting swap for MySQL or MariaDB -- maybe check the NUMA section toward the end: https://fromdual.com/do-not-underestimate-performance-impacts-of-swapping-on-numa-database-systems

    This Q&A at MariaDB site leads to NUMA interleaving settings as a possible solution: https://mariadb.com/kb/en/swap-usage-wont-decrease/

    A lot of discussion about memory and MariaDB: https://mariadb.com/kb/en/mariadb-memory-allocation/

    And this is the authoritative youtube video for NUMA :-D https://youtu.be/KmtzQCSh6xk

    Please upvote/accept this answer if it is helpful. Thanks.

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