Re: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1 GiB

[Michal Hocko]

On Mon 31-08-26 10:56:35, Nimrod Oren wrote:
> When THP is enabled, set_recommended_min_free_kbytes() may raise
> min_free_kbytes using a heuristic that scales with pageblock_nr_pages.
> With MIGRATE_PCPTYPES equal to 3, the formula accounts for 11
> pageblocks for every eligible populated zone before capping the result
> at 5% of low memory.
> 
> This is reasonable when a pageblock is 2 MiB, as on common 4 KiB page
> configurations, but scales poorly with larger base page sizes. With
> the default arm64 pageblock sizes, the contribution per eligible zone
> before the 5% cap is:
> 
>   4 KiB pages:   2 MiB pageblock,   22 MiB per zone
>  16 KiB pages:  32 MiB pageblock,  352 MiB per zone
>  64 KiB pages: 512 MiB pageblock,  5.5 GiB per zone
> 
> Consequently, min_free_kbytes can reach excessive and unwanted levels.
> 
> Add an absolute 1 GiB cap to the recommendation, in addition to the
> existing percentage cap. This bounds the automatic recommendation to a
> sane value on systems with large pageblocks while preserving existing
> behavior for typical systems with 2 MiB pageblocks.

I would argue that the whole model of increasing min_free_kbytes for THP
is wrong. This will trigger memory reclaim sooner and make the THP
availability more likely but this was at times where we didn't have
pro-active compaction and many changes in the compaction. So is this
actually needed in general resp. only large pageblocks systems?

-- 
Michal Hocko
SUSE Labs