Loading ways in non-slim mode is slower than with --slim. #2013

What version of osm2pgsql are you using?

osm2pgsql --version
2023-07-23 22:08:09 osm2pgsql version 1.8.1 (1.8.1-183-g5dba2405)
Build: RelWithDebInfo
Compiled using the following library versions:
Libosmium 2.19.0
Proj [API 6] 8.2.1
Lua 5.1.4 (LuaJIT 2.1.0-beta3)

What operating system and PostgreSQL/PostGIS version are you using?

Postgres version: 15.3 (Ubuntu 15.3-1.pgdg22.04+1)
PostGIS version: POSTGIS="3.3.3 2355e8e" [EXTENSION] PGSQL="150" GEOS="3.10.2-CAPI-1.16.0" PROJ="8.2.1" LIBXML="2.9.13" LIBJSON="0.15" LIBPROTOBUF="1.3.3" WAGYU="0.5.0 (Internal)"

Tell us something about your system

Bare metal 512 GB RAM, 2x Intel Xeon E5-2699 v4

What did you do exactly?

I have been running multiple tests with both --slim and non-slim and Planet size imports in the past couple of days after upgrading to the latest master on a machine with enough RAM to hold all data in RAM if choosing to import in a non-slim mode with --flat-nodes specified.

One thing I have noticed now is that consistently, the way loading in non-slim mode is about 15% slower than with --slim and --flat-nodes, about 70k/s for non-slim versus 85k/s for --slim and --flat-nodes.

I didn't expect this, as all data is stored in RAM with non-slim mode. Both nodes and especially relations do load (much) faster (for relations I see an about 2.5x times speed increase). Admittedly, the --flat-nodes file is stored on a very capable NVMe RAID 0, but even so, I would assume data in RAM to be faster accessible.

Is there any plausible explanation for this difference and the slower loading of ways in non-slim mode?

What did you expect to happen?

Loading ways using non-slim mode is as fast or faster than with --slim.

What did happen instead?

Loading ways using non-slim mode is about 15% slower than with --slim.

What did you do to try analyzing the problem?

Run multiple import sessions in both --slim and non-slim mode to verify the witnessed speed difference was consistent. It was.