Re: [PATCH -mm] vmscan: bail out of page reclaim after swap_cluster_max pages

[KOSAKI Motohiro]

>> Rik, sorry, I nak current your patch. because it don't fix old akpm issue.
>
> You are right.  We do need to keep pressure between zones
> equivalent to the size of the zones (or more precisely, to
> the number of pages the zones have on their LRU lists).

Oh, sorry.
you are right. but I talked about reverse thing.

1. shrink_zones() doesn't have any shortcut exiting way.
    it always call all zone's shrink_zone()
2. balance_pgdat also doesn't have shortcut.

simple shrink_zone() shortcut and lite memory pressure cause following
bad scenario.

1. reclaim 32 page from ZONE_HIGHMEM
2. reclaim 32 page from ZONE_NORMAL
3. reclaim 32 page from ZONE_DMA
4. exit reclaim
5. another task call page alloc and it cause try_to_free_pages()
6. reclaim 32 page from ZONE_HIGHMEM
7. reclaim 32 page from ZONE_NORMAL
8. reclaim 32 page from ZONE_DMA

oops, all zone reclaimed the same pages although ZONE_HIGHMEM have
much memory than ZONE_DMA.
IOW, ZONE_DMA's reclaim scanning rate is much than ZONE_HIGHMEM largely.

it isn't intentionally.



Actually, try_to_free_pages don't need pressure fairness. it is the
role of the balance_pgdat().


> However, having dozens of direct reclaim tasks all getting
> to the lower priority levels can be disastrous, causing
> extraordinarily large amounts of memory to be swapped out
> and minutes-long stalls to applications.

agreed.

>
> I think we can come up with a middle ground here:
> - always let kswapd continue its rounds

agreed.

> - have direct reclaim tasks continue when priority == DEF_PRIORITY

disagreed.
it cause above bad scenario, I think.

> - break out of the loop for direct reclaim tasks, when
>  priority < DEF_PRIORITY and enough pages have been freed
>
> Does that sound like it would mostly preserve memory pressure
> between zones, while avoiding the worst of the worst when it
> comes to excessive page eviction?
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