Re: [PATCH] mm: Skip the reserved bootmem for compaction

[Mel Gorman]

On Mon, Sep 02, 2024 at 08:24:43PM +0800, Rong Qianfeng wrote:
> Reserved pages are basically non-lru pages. This kind of memory can't be
> used as migration sources and targets, skip it can bring some performance
> benefits.
> 
> Because some drivers may also use PG_reserved, we just set PB_migrate_skip
> for those clustered reserved bootmem during memory initialization.
> 
> Signed-off-by: Rong Qianfeng <rongqianfeng@xxxxxxxx>

I'm not convinced the savings due to skipping a few pages during the scan
would justify the additional code. There would have to be a large number
of reserved pages scattered throughout the zone to make a difference and
even that situation would be a big surprise. I'm not even sure this can be
explicitly tested unless you artifically create reserved pages throughout the
zone, which would not be convincing, or know if a driver that exhibits such
behaviour in which case my first question is -- what is that driver doing?!?

-- 
Mel Gorman
SUSE Labs