How many CPU sockets? Is NUMA exposed? This could be MySQL/MariaDB
"swapping insanity".
On Mon, Sep 23, 2024 at 2:44 PM cyusedfzfb via discuss
Hi all!
New here, signed up just new, to discuss an interesting mariadb behaviour we are seeing, related to mariadb unexpectedly using swap space.
Look at this example:
RHEL 8.10, running mariadb-server-utils.x86_64 3:10.5.22-1.module+el8.8.0+20134+a92c7654:
[user@db ~]$ free -g total used free shared buff/cache available Mem: 187 9 11 0 165 175 Swap: 3 3 0
and:
top - 08:52:46 up 39 days, 15:50, 2 users, load average: 1.70, 2.05, 1.99 Tasks: 616 total, 2 running, 614 sleeping, 0 stopped, 0 zombie %Cpu(s): 6.0 us, 0.8 sy, 0.0 ni, 88.2 id, 4.7 wa, 0.1 hi, 0.1 si, 0.0 st MiB Mem : 191529.7 total, 11761.7 free, 9764.9 used, 170003.1 buff/cache MiB Swap: 4096.0 total, 1006.1 free, 3089.9 used. 179429.5 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND SWAP 1952811 mysql 20 0 14.4g 5.0g 15172 S 65.7 2.7 9663:14 mysqld 1.7g 2934963 root 20 0 5208 2056 1408 S 8.9 0.0 10:39.29 gzip 0 2934962 root 20 0 33612 8268 6960 S 7.9 0.0 2:35.31 mysqldump 0
while:
[user@db ~]$ cat /proc/sys/vm/swappiness 1
We could (we should..) allocate more ram to mariadb, but the point is: there is plenty of RAM available, and there always has been, since boot. And yet, mariadb is using all swap.
I guess this has no performance impact, probably the swapped pages are not actually used, but zabbix does not like 100% swap usage. And frankly: neither do I.
Why? Can anyone here explain? Swappiness set to 1...is mariadb ignoring swappiness..? Is there something else we can configure..?
I read some older posts on ram/swap, and here is some data, requested in that older thread:
[user@db ~]$ sudo ps -o pid,vsz,comm `pgrep mysqld` PID VSZ COMMAND 1952811 15117612 mysqld
and
[user@db ~]$ for file in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{
print ""}' $file; done | sort -k 2 -n -r | grep mysql mysqld 1982672 kB
We are seeing similar behaviour on another machines, running more recent mariadb version: 10.5.22-MariaDB MariaDB Server:
[user@db_another ~]$ free -g total used free shared buff/cache available Mem: 62 40 0 1 21 20 Swap: 4 3 1
Again: enough RAM available, and swap is used 3/4, mostly by MariaDB.
Hope the provided information is enough. Let me know if there is anything else I can provide. Looking forward for any insight or help on the matter. :-)
Thanks! MJ
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