I ran some tests and couldn't reproduce in 5.5.1

I run mariadb in docker from mariadb/columnstore image id 5a61255d059f.
Mariadb 10.5.5
Columnstore 1.5.4
The bug is still present:

$ ps -eLf | grep ExeMgr | wc -l

1214

If you need any additional information, I am glad to provide it

Where is this docker image coming from?

Please use the official release here:

https://hub.docker.com/r/mariadb/columnstore

I am using the official image, with few basic settings

The image id specified above was the latest one at the moment of downloading official image from docker hub

I just tried the latest image (97f7df0d1068) and did not see the reported issue.

docker run -d -p 3306:3306 --name mcs_container mariadb/columnstore
docker exec -it mcs_container bash

[root@f23ec9d9a05f columnstore]# cat exemgr.log
Starting ExeMgr: st = 50, qs = 20, mx = 95, cf = /etc/columnstore/Columnstore.xml
[root@f23ec9d9a05f columnstore]#

We've tried a number of configurations, with and without docker. We specifically pulled the image from https://hub.docker.com/r/mariadb/columnstore. In all cases we couldn't get the number of threads to compound.

The original poster (OP) says "The threads seem to spawn for each job". Not being sure what OP meant by "job", we went with the assumption it could mean either "mysql session" or"query". After hundreds of connections and thousands of queries, we saw no explosion of threads.

It is possible that OP is using an older version and the problem has been fixed in the version we tested. We suggest OP download the latest (should be 5.5.1 by now) and try to reproduce the symptom.