Re: Streaming replica hangs periodically for ~ 1 second - how to diagnose/debug

On Thu, 21 Aug 2025 at 11:03, hubert depesz lubaczewski
<depesz(at)depesz(dot)com> wrote:
>
> On Wed, Aug 20, 2025 at 06:30:00PM +0100, Thom Brown wrote:
> > Do you have THP enabled? Can you use mpstat and see what %steal shows as?
>
> Sorry, what is "THP"? I tried searching for "what is thp", and most
> common search results are related to some chemical compound.

Ah, yeah I meant transparent hugepage:

cat /sys/kernel/mm/transparent_hugepage/enabled

This should show it being set as "never".

>
> mpstat, yes, we have it. Let me quickly show what I get:
>
> Two days ago, at ~ 10:10pm UTC I saw this in Pg logs:
>
> # grep -oP '^2025-08-19 22:09:2\d\.\d+ UTC' postgresql-2025-08-19_220000.csv | uniq -c | grep -C3 -P '^\s*\d\d'
> 2 2025-08-19 22:09:29.084 UTC
> 1 2025-08-19 22:09:29.094 UTC
> 2 2025-08-19 22:09:29.097 UTC
> 70 2025-08-19 22:09:29.109 UTC
> 90 2025-08-19 22:09:29.110 UTC
> 6 2025-08-19 22:09:29.111 UTC
> 1 2025-08-19 22:09:29.153 UTC
> 1 2025-08-19 22:09:29.555 UTC
>
> As you can see we have 70, and then 90 messages all logged with the same
> timestamp. All of them (160) were "duration:x" lines.
>
> At the time, mpstat (called using `mpstat 5`) looked like this:
>
> CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
> 22:09:04 all 2.25 0.00 0.27 0.24 0.00 0.10 0.00 0.00 0.00 97.14
> 22:09:09 all 2.70 0.00 0.28 0.27 0.00 0.12 0.00 0.00 0.00 96.63
> 22:09:14 all 2.83 0.00 0.63 0.23 0.00 0.09 0.00 0.00 0.00 96.22
> 22:09:19 all 2.66 0.00 0.28 0.23 0.00 0.10 0.00 0.00 0.00 96.74
> 22:09:24 all 2.36 0.00 0.27 0.21 0.00 0.10 0.00 0.00 0.00 97.06
> 22:09:29 all 2.49 0.00 0.40 0.40 0.00 0.11 0.00 0.00 0.00 96.60
> 22:09:34 all 2.65 0.00 0.28 0.23 0.00 0.11 0.00 0.00 0.00 96.73
> 22:09:39 all 2.26 0.00 0.26 0.21 0.00 0.09 0.00 0.00 0.00 97.19
> 22:09:44 all 2.31 0.00 0.25 0.22 0.00 0.09 0.00 0.00 0.00 97.13
> 22:09:49 all 2.44 0.00 0.22 0.24 0.00 0.08 0.00 0.00 0.00 97.02
> 22:09:54 all 2.23 0.00 0.23 0.28 0.00 0.10 0.00 0.00 0.00 97.16
> 22:09:59 all 2.48 0.00 0.26 0.30 0.00 0.11 0.00 0.00 0.00 96.85
> 22:10:04 all 2.29 0.00 0.28 0.26 0.00 0.12 0.00 0.00 0.00 97.06
> 22:10:09 all 2.31 0.00 0.22 0.21 0.00 0.11 0.00 0.00 0.00 97.14
> 22:10:14 all 2.38 0.00 0.25 0.21 0.00 0.10 0.00 0.00 0.00 97.05
> 22:10:19 all 2.54 0.00 0.24 0.20 0.00 0.11 0.00 0.00 0.00 96.91
> 22:10:24 all 2.26 0.00 0.25 0.22 0.00 0.10 0.00 0.00 0.00 97.17
> 22:10:29 all 2.12 0.00 0.27 0.24 0.00 0.09 0.00 0.00 0.00 97.27
> 22:10:34 all 2.56 0.00 0.26 0.26 0.00 0.12 0.00 0.00 0.00 96.80
> 22:10:39 all 2.16 0.00 0.23 0.24 0.00 0.10 0.00 0.00 0.00 97.27
> 22:10:44 all 2.37 0.00 0.26 0.22 0.00 0.10 0.00 0.00 0.00 97.05
> 22:10:49 all 2.25 0.00 0.25 0.61 0.00 0.09 0.00 0.00 0.00 96.80
> 22:10:54 all 2.41 0.00 0.28 0.22 0.00 0.10 0.00 0.00 0.00 96.99
> 22:10:59 all 2.83 0.00 0.29 0.19 0.00 0.12 0.00 0.00 0.00 96.57

This output looks fine, so it doesn't show anything concerning, so
suggests the issue is somehow on the Postgres side.

Did you happen to poll pg_stat_activity at the time to see whether you
had lots of IPC waits? I'm wondering whether the storage layer is
freezing up for a moment.

Thom