Re: Heavy LWLockTranche buffer_mapping in Postgres 9.6.10 server

Hi,

On 2020-02-01 16:17:13 -0700, Justin Lu wrote:
> We are seeing very heavy LWLockTranche buffer_mapping in db recently.
> 
> There server had 24 core, 128GB of RAM, SSD data file system, on Unbuntu
> 16.04.6.
> The shared_buffers was at 32GB. 1/4 of over RAM size. No issue on
> checkpoints (avg time 29 min apart).
> 
> After seeing the heavy wait, we added 64GB more RAM and increased
> shared_buffers to 48GB, effective_cache_size to 90GB. But it seems there is
> no impact on the buffer mapping waits at all.

I suggest doing a perf profile with --call-graph dwarf, to see where
this is mostly coming from.

One thing I've seen causing symptoms like this before, is if there's
suddenly a larger amount of table truncations, dropping, etc - dropping
/ truncating a table / index needs to scan all of shared buffers...

Greetings,

Andres Freund