linux-kernel - Re: [PATCH 3/3] Provide control over unmapped pages (v4)

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Message-ID: <AANLkTik4pXr_3hY3QoivMHHA6zq-k35f4HPNLZZC8bge@mail.gmail.com>
Date:	Thu, 17 Feb 2011 08:45:11 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org, npiggin@...nel.dk,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	kosaki.motohiro@...fujitsu.com, cl@...ux.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 3/3] Provide control over unmapped pages (v4)

On Mon, Feb 14, 2011 at 2:33 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> * MinChan Kim <minchan.kim@...il.com> [2011-02-10 14:41:44]:
>
>> I don't know why the part of message is deleted only when I send you.
>> Maybe it's gmail bug.
>>
>> I hope mail sending is successful in this turn. :)
>>
>> On Thu, Feb 10, 2011 at 2:33 PM, Minchan Kim <minchan.kim@...il.com> wrote:
>> > Sorry for late response.
>> >
>> > On Fri, Jan 28, 2011 at 8:18 PM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>> >> * MinChan Kim <minchan.kim@...il.com> [2011-01-28 16:24:19]:
>> >>
>> >>> >
>> >>> > But the assumption for LRU order to change happens only if the page
>> >>> > cannot be successfully freed, which means it is in some way active..
>> >>> > and needs to be moved no?
>> >>>
>> >>> 1. holded page by someone
>> >>> 2. mapped pages
>> >>> 3. active pages
>> >>>
>> >>> 1 is rare so it isn't the problem.
>> >>> Of course, in case of 3, we have to activate it so no problem.
>> >>> The problem is 2.
>> >>>
>> >>
>> >> 2 is a problem, but due to the size aspects not a big one. Like you
>> >> said even lumpy reclaim affects it. May be the reclaim code could
>> >> honour may_unmap much earlier.
>> >
>> > Even if it is, it's a trade-off to get a big contiguous memory. I
>> > don't want to add new mess. (In addition, lumpy is weak by compaction
>> > as time goes by)
>> > What I have in mind for preventing LRU ignore is that put the page
>> > into original position instead of head of lru. Maybe it can help the
>> > situation both lumpy and your case. But it's another story.
>> >
>> > How about the idea?
>> >
>> > I borrow the idea from CFLRU[1]
>> > - PCFLRU(Page-Cache First LRU)
>> >
>> > When we allocates new page for page cache, we adds the page into LRU's tail.
>> > When we map the page cache into page table, we rotate the page into LRU's head.
>> >
>> > So, inactive list's result is following as.
>> >
>> > M.P : mapped page
>> > N.P : none-mapped page
>> >
>> > HEAD-M.P-M.P-M.P-M.P-N.P-N.P-N.P-N.P-N.P-TAIL
>> >
>> > Admin can set threshold window size which determines stop reclaiming
>> > none-mapped page contiguously.
>> >
>> > I think it needs some tweak of page cache/page mapping functions but
>> > we can use kswapd/direct reclaim without change.
>> >
>> > Also, it can change page reclaim policy totally but it's just what you
>> > want, I think.
>> >
>
> I am not sure how this would work, moreover the idea behind
> min_unmapped_pages is to keep sufficient unmapped pages around for the
> FS metadata and has been working with the existing code for zone
> reclaim. What you propose is more drastic re-org of the LRU and I am
> not sure I have the apetite for it.

Yes. My suggestion can change LRU order totally but it can't meet your
goal so it was a bad idea. Sorry for bothering you.

I can add reviewed-by [1/3],[2/3], but still doubt [3/3].
LRU ordering problem as I mentioned is not only your problem but it's
general problem these day(ex, more aggressive compaction/lumpy
reclaim). So we may need general solution if it is real problem.
Okay. I don't oppose your approach from now on until I can prove how
much LRU-reordering makes bad effect. (But still I  raise my eyebrow
on implementation [3/3] but I don't oppose it until I suggest better
approach)

Thanks.
-- 
Kind regards,
Minchan Kim
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