背景：压测ck性能一直上不去，但是ck负载不大。尝试调整参数，验证是否有性能提升
1、background_common_pool_size
Sets the number of threads performing background non-specialized operations like cleaning the filesystem etc. for tables with MergeTree engines. Could be increased at runtime and could be applied at server startup from the default profile for backward compatibility.
2 、 max_threads 验证有效
The maximum number of threads to execute the request. By default, it is determined automatically.
3、background_pool_size
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. You can only increase the number of threads at runtime. To lower the number of threads you have to restart the server. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as number_of_free_entries_in_pool_to_lower_max_size_of_merge and number_of_free_entries_in_pool_to_execute_mutation.
查看参数；
select * from system.settings where name like '%background_common_pool_size%'
SHOW [CHANGED] SETTINGS LIKE|ILIKE <name>
show settings like '%background_common_pool_size%'
4、设置参数
SET param = value
set background_common_pool_size = 128
加个globa执行也没报错 官方文档上没有global参数。但是阿里云dms 需要加上global 才能全局生效
set global background_common_pool_size = 128
https://help.aliyun.com/zh/clickhouse/user-guide/set-parameters-in-the-user-xml-file?spm=a2c4g.11186623.0.0.6bdc5d7fnAdCqs
https://clickhouse.com/docs/zh/sql-reference/statements/set
如果更改配置文件config以后
查询system.settings里面的参数没生效时因为
这里没改是因为system.settings查询的时候值是从user.xml中获取的，所以一直都原来的值，其实生效的是在config.xml里
https://clickhouse.com/docs/en/operations/server-configuration-parameters/settings#background_pool_size
https://blog.csdn.net/qq_35423190/article/details/109999576