Re: HIGH IO and Less CPU utilization

Hi,

Thanks for providing all this info.

On Tue, Mar 29, 2022 at 11:34:18PM +0530, Rambabu g wrote:
> Hi All,
>
> We have an issue with high load and IO Wait's but less cpu on postgres
> Database, The emp Table size is around 500GB, and the connections are very
> less.

What indexes are defined on this table ?
How large are they ?

> Red Hat Enterprise Linux Server release 7.9 (Maipo)
> PostgreSQL 11.15 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit
>
> shared_buffers=12GB
> work_mem=128MB

> 14428 | 04:45:59.712892 | active | INSERT INTO target (empno, name)
> SELECT empno, '' AS name FROM (select distinct empno from emp where sname='test'
> and tp='EMP NAME 1' LIMIT 10) AS query ;

Is the only only problem query, or just one example or ??
Are your issues with loading data, querying data or both ?

> -bash-4.2$ iostat -x

It shows that you only have a few filesystems in use.
It's common to have WAL and temp_tablespaces on a separate FS.
That probably wouldn't help your performance at all, but it would help to tell
what's doing I/O. Is there anything else running on the VM besides postgres ?

You can also check:
SELECT COUNT(1), wait_event FROM pg_stat_activity GROUP BY 2 ORDER BY 1 DESC;

And the pg_buffercache extension:
SELECT COUNT(nullif(isdirty,'f')) dirty, COUNT(1) all, COALESCE(c.relname, b.relfilenode::text) FROM pg_buffercache b LEFT JOIN pg_class c ON b.relfilenode=pg_relation_filenode(c.oid) GROUP BY 3 ORDER BY 1 DESC,2 DESC LIMIT 9;

> Hypervisor vendor: KVM

Are KSM or THP enabled on the hypervisor ?

tail /sys/kernel/mm/ksm/run /sys/kernel/mm/transparent_hugepage/khugepaged/defrag /sys/kernel/mm/transparent_hugepage/enabled /sys/kernel/mm/transparent_hugepage/defrag

--
Justin