Re: Logical Replication, CPU load and Locking contention

> On 10 Mar 2021, at 11:25, Ron <ronljohnsonjr(at)gmail(dot)com> wrote:
>
> On 3/10/21 2:10 AM, Radoslav Nedyalkov wrote:
>>
>> On Wed, Mar 10, 2021 at 3:56 AM Martín Fernández <fmartin91(at)gmail(dot)com <mailto:fmartin91(at)gmail(dot)com>> wrote:
>> Hello,
>>
>> I’m troubleshooting a problem at my company with a pg 12 cluster that we run.
>>
>> We are using Amazon DMS to replicate data from our database into S3 buckets. DMS replicates data by using logical replication slots.
>>
>> After introducing DMS in our environment, we have seen an increase in CPU load of 20 points at business hours (from ~60% to ~80%).
>>
>> The other thing that we have identified is that AccessShareLocks increase considerably when DMS running.
>>
>> Based on this information, I’m trying to understand if this is something expected when running logical replication or not. We’ve been running physical replication for several years and we haven’t seen nothing like this. It could be the case that the issue is not related at all with logical replication and is purely a DMS artifact.
>>
>> Thanks before hand!
>>
>> Best,
>> Martín
>>
>>
>> Hi,
>>
>> I would check in pg_stat_activity what those logical replication slots do. I guess COPY.
>
> If it's a full-load or full-load-and-cdc, then it's almost certainly a COPY.

We are doing full-load and full-load-and-cdc. At this point it’s just cdc since the full load was done month ago.

>
>> Are you doing one shot copy ? every day ? Then copying all the tables will lead to load increase.
>> How many tables at a time DMS copies? It should be configurable.
>
> It definitely is, with the MaxFullLoadSubTasks parameter. The default is 8.

We are copying a lot of tables. 100+

>
>> AccessShareLock is absolutely normal. You have a transaction doing SELECT (COPY) over a table.
>
> If DMS with Postgresql as the source is anything like when Oracle is the source (we're testing Oracle -> RDS Postgresql) then it starts a SERIALIZABLE transaction.

My guess at this point is that the CPU load increases due to the increase of AccessShareLocks, is that a fair assumption ? Have you seen a similar behavior in Oracle ?

>
>> Physical replication is a different thing. It happens at another level.
>>
>> Regards
>> Rado
>
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