Re: Conflict detection for update_deleted in logical replication

On Thu, Dec 19, 2024 at 11:11 PM Nisha Moond <nisha.moond412@gmail.com> wrote:
>
> Here is further performance test analysis with v16 patch-set.
>
>
> In the test scenarios already shared on -hackers [1], where pgbench was run only on the publisher node in a pub-sub setup, no performance degradation was observed on either node.
>
>
>
> In contrast, when pgbench was run only on the subscriber side with detect_update_deleted=on [2], the TPS performance was reduced due to dead tuple accumulation. This performance drop depended on the wal_receiver_status_interval—larger intervals resulted in more dead tuple accumulation on the subscriber node. However, after the improvement in patch v16-0002, which dynamically tunes the status request, the default TPS reduction was limited to only 1%.
>
>
>
> We performed more benchmarks with the v16-patches where pgbench was run on both the publisher and subscriber, focusing on TPS performance. To summarize the key observations:
>
>  - No performance impact on the publisher as dead tuple accumulation does not occur on the publisher.

Nice. It means that frequently getting in-commit-phase transactions by
the subscriber didn't have a negative impact on the publisher's
performance.

>
>  - The performance is reduced on the subscriber side (TPS reduction (~50%) [3] ) due to dead tuple retention for the conflict detection when detect_update_deleted=on.
>
>  - Performance reduction happens only on the subscriber side, as workload on the publisher is pretty high and the apply workers must wait for the amount of transactions with earlier timestamps to be applied and flushed before advancing the non-removable XID to remove dead tuples.

Assuming that the performance dip happened due to dead tuple retention
for the conflict detection, would TPS on other databases also be
affected?

>
>
> [3] Test with pgbench run on both publisher and subscriber.
>
>
>
> Test setup:
>
> - Tests performed on pgHead + v16 patches
>
> - Created a pub-sub replication system.
>
> - Parameters for both instances were:
>
>
>
>    share_buffers = 30GB
>
>    min_wal_size = 10GB
>
>    max_wal_size = 20GB
>
>    autovacuum = false

Since you disabled autovacuum on the subscriber, dead tuples created
by non-hot updates are accumulated anyway regardless of
detect_update_deleted setting, is that right?

> Test Run:
>
> - Ran pgbench(read-write) on both the publisher and the subscriber with 30 clients for a duration of 120 seconds, collecting data over 5 runs.
>
> - Note that pgbench was running for different tables on pub and sub.
>
> (The scripts used for test "case1-2_measure.sh" and case1-2_setup.sh" are attached).
>
>
>
> Results:
>
>
>
> Run#                   pub TPS              sub TPS
>
> 1                         32209   13704
>
> 2                         32378   13684
>
> 3                         32720   13680
>
> 4                         31483   13681
>
> 5                         31773   13813
>
> median               32209   13684
>
> regression          7%         -53%

What was the TPS on the subscriber when detect_update_deleted = false?
And how much were the tables bloated compared to when
detect_update_deleted = false?

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com