Re: [PATCH] vmscan: prevent background aging of anon page in no swap system

[Minchan Kim]

On Mon, Aug 30, 2010 at 2:40 PM, Ying Han <yinghan@xxxxxxxxxx> wrote:
> On Sun, Aug 29, 2010 at 5:18 PM, Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>> Hi Ying,
>>
>> On Mon, Aug 30, 2010 at 6:23 AM, Ying Han <yinghan@xxxxxxxxxx> wrote:
>>> On Sun, Aug 29, 2010 at 1:03 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>>>> On 08/29/2010 01:45 PM, Ying Han wrote:
>>>>
>>>>> There are few other places in vmscan where we check nr_swap_pages and
>>>>> inactive_anon_is_low. Are we planning to change them to use
>>>>> total_swap_pages
>>>>> to be consistent ?
>>>>
>>>> If that makes sense, maybe the check can just be moved into
>>>> inactive_anon_is_low itself?
>>>
>>> That was the initial patch posted, instead we changed to use
>>> total_swap_pages instead. How this patch looks:
>>>
>>> @@ -1605,6 +1605,9 @@ static int inactive_anon_is_low(struct zone
>>> *zone, struct scan_control *sc)
>>>  {
>>>        int low;
>>>
>>> +       if (total_swap_pages <= 0)
>>> +               return 0;
>>> +
>>>        if (scanning_global_lru(sc))
>>>                low = inactive_anon_is_low_global(zone);
>>>        else
>>> @@ -1856,7 +1859,7 @@ static void shrink_zone(int priority, struct zone *zone,
>>>         * Even if we did not try to evict anon pages at all, we want to
>>>         * rebalance the anon lru active/inactive ratio.
>>>         */
>>> -       if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
>>> +       if (inactive_anon_is_low(zone, sc))
>>>                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
>>>
>>>        throttle_vm_writeout(sc->gfp_mask);
>>>
>>> --Ying
>>>
>>>>
>>
>> I did it intentionally since inactive_anon_is_low have been used both
>> direct reclaim and background path. In this point, your patch could
>> make side effect in swap enabled system when swap is full.
>>
>> I think we need aging in only background if system is swap full.
>> That's because if the swap space is full, we don't reclaim anon pages
>> in direct reclaim path with (nr_swap_pages < 0)  and even have been
>> not rebalance it until now.
>> I think direct reclaim path is important about latency as well as
>> reclaim's effectiveness.
>> So if you don't mind, I hope direct reclaim patch would be left just as it is.
>
> Minchan, I would prefer to make kswapd as well as direct reclaim to be
> consistent if possible.
> They both try to reclaim pages when system is under memory pressure,
> and also do not make
> much sense to look at anon lru if no swap space available. Either
> because of no swapon or run
> out of swap space.

In out of swap space, The few swap space would become more precious.
So I think we still need background aging to protect hot page swap out.
But I admit it's hard to measure it so I can't insist on.
But I wanted to maintain it as it is to avoid _unexpected_ side effect.

And your patch can't compile out inactive_anon_is_low call in non swap
configurable system. It makes unnecessary call. So I want to use
nr_swap_pages && inactive_anon_is_low.

For it, I sended following patch at last version

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1b145e6..0b8a3ce 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1747,7 +1747,7 @@ static void shrink_zone(int priority, struct zone *zone,
         * Even if we did not try to evict anon pages at all, we want to
         * rebalance the anon lru active/inactive ratio.
         */
-       if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
+       if (nr_swap_pges > 0 && inactive_anon_is_low(zone, sc))
                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);

        throttle_vm_writeout(sc->gfp_mask);

But Andrew merged middle version.
I will send this patch again.

Thanks.

-- 
Kind regards,
Minchan Kim
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