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From:   Yu Zhao <yuzhao@google.com>
Date:   Mon, 21 Mar 2022 22:02:57 -0600
Message-ID: <CAOUHufa1nuyJ1MawqBTRZS78EFOGTw0_qh5k3XvDo9XQCvan7g@mail.gmail.com>
Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Mar 21, 2022 at 6:52 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
>  +
> > +static long get_nr_evictable(struct lruvec *lruvec, unsigned long max_seq,
> > +                          unsigned long *min_seq, bool can_swap, bool *need_aging)
> > +{
> > +     int gen, type, zone;
> > +     long old = 0;
> > +     long young = 0;
> > +     long total = 0;
> > +     struct lru_gen_struct *lrugen = &lruvec->lrugen;
> > +
> > +     for (type = !can_swap; type < ANON_AND_FILE; type++) {
> > +             unsigned long seq;
> > +
> > +             for (seq = min_seq[type]; seq <= max_seq; seq++) {
> > +                     long size = 0;
> > +
> > +                     gen = lru_gen_from_seq(seq);
> > +
> > +                     for (zone = 0; zone < MAX_NR_ZONES; zone++)
> > +                             size += READ_ONCE(lrugen->nr_pages[gen][type][zone]);
> > +
> > +                     total += size;
> > +                     if (seq == max_seq)
> > +                             young += size;
> > +                     if (seq + MIN_NR_GENS == max_seq)
> > +                             old += size;
> > +             }
> > +     }
> > +
> > +     /* try to spread pages out across MIN_NR_GENS+1 generations */
> > +     if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq)
> > +             *need_aging = true;
> > +     else if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS < max_seq)
> > +             *need_aging = false;
>
> Can you explain/document the reason for the considering the below
> conditions for ageing?
>
> > +     else if (young * MIN_NR_GENS > total)
> > +             *need_aging = true;
>
> Are we trying to consdier the case of more than half the total pages
> young as needing ageing? If so should MIN_NR_GENS be 2 instead of using
> that #define? Or
>
> > +     else if (old * (MIN_NR_GENS + 2) < total)
> > +             *need_aging = true;
>
> What is the significance of '+ 2' ?

Will improve the comment according to my previous reply here [1].

[1] https://lore.kernel.org/linux-mm/CAOUHufYmUPZY0gCC+wYk6Vr1L8KEx+tJeEAhjpBfUnLJsAHq5A@mail.gmail.com/