Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[wangzicheng]

On 2026-07-30 06:36 +0800, Barry Song wrote:
> On Tue, Jul 28, 2026 at 8:27 PM Zicheng Wang <wangzicheng@xxxxxxxxx> wrote:
> >
> > Hi Barry,
> >
> > > The original code is a bit odd, as it compares a single young
> > > generation and a single old generation against the total, rather than
> > > comparing all young generations with all old generations.
> > >
> > > By comparing all young generations against all old generations, we no
> > > longer depend on the exact value of MIN_NR_GENS. The idea is to
> > > emulate inactive_is_low().
> >
> > Makes sense, the inactive_is_low() framing fits.
> >
> > One gap: inactive_is_low()'s ratio is dynamic calculated.
> > int_sqrt(10*1024) = 101 (ages only when inactive drops below ~1%),
> > while patch 4/5's fixed 4 ages once old falls under 20% of young.
> > It seems there will be considerable cost on big memcgs, or should
> > the ratio scale with size?
> >
> > Looking forward to the perf data.
> 
> I don't have access to a machine with 1 TB of memory—or even

Me neither.

> more than 20 GB—to test this. But I think your comment makes
> sense. We shouldn't let a machine with a huge amount of memory
> in a single lruvec age too aggressively.
> So maybe something like this:
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ed35c502923d..a6768b9ec8d6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4968,6 +4968,7 @@ static bool lru_gen_imbalanced(struct lruvec
> *lruvec, int type,
>  {
>         struct lru_gen_folio *lrugen = &lruvec->lrugen;
>         unsigned long young = 0, old = 0, seq;
> +       unsigned long old_ratio, gb;
> 
>         /*
>          * reclaim is forced to a single type in those cases, so there is
> @@ -4989,7 +4990,16 @@ static bool lru_gen_imbalanced(struct lruvec
> *lruvec, int type,
>                 else
>                         old += size;
>         }
> -       return young > old * 4;
> +
> +       /*
> +        * Copied from the active/inactive LRU heuristic, but uses a higher
> +        * old_ratio so fewer folios remain in the old generations, avoiding
> +        * overly aggressive aging.
> +        */
> +       gb = (young + old) >> (30 - PAGE_SHIFT);
> +       old_ratio = gb ? int_sqrt(10 * gb) : 1;
> +       old <<= 2;
> +       return young > old * old_ratio;
>  }
Look good to me. 

Also, this heuristic makes me believe more that MGLRU's `aging' could be
exposed as a standalone feature, allowing users to tune it more flexibly.

> Best Regards
> Barry
> 

Best,
Zicheng