On Sat, Mar 19, 2022 at 5:15 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> > + 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;
> > + else if (young * MIN_NR_GENS > total)
> > + *need_aging = true;
>
> Could we have some doc here?
Will do.
> Given MIN_NR_GENS=2 and MAX_NR_GENS=4,
> it seems you mean if we have three generations and the youngest pages are more
> than 1/2 of the total pages, we need aging?
Yes.
> > + else if (old * (MIN_NR_GENS + 2) < total)
> > + *need_aging = true;
>
> it seems you mean if the oldest pages are less than 1/4 of the total pages,
> we need aging?
Yes.
> Can we have comments to explain why here?
>
> your commit message only says " The aging produces young generations.
> Given an lruvec, it increments max_seq when max_seq-min_seq+1
> approaches MIN_NR_GENS." it can't explain what the code is doing
> here.
Fair enough. Approaching MIN_NR_GENS=2 means getting close to it. From
the consumer's POV, if it *reaches* 2, the eviction will have to
stall, because the two youngest generations are not yet fully aged,
i.e., the second chance policy similar to the active/inactive lru.
