linux-kernel - Re: [PATCH] mm: consider all swapped back pages in used-once logic

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Date:	Mon, 25 Jun 2012 10:08:32 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Zheng Liu <gnehzuil.liu@...il.com>, Michal Hocko <mhocko@...e.cz>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: consider all swapped back pages in used-once logic

On Mon, Jun 25, 2012 at 08:53:11AM +0900, Minchan Kim wrote:
> Hi Hannes,
> 
> On 06/23/2012 08:04 PM, Johannes Weiner wrote:
> 
> > On Mon, May 21, 2012 at 07:07:00PM +0800, Zheng Liu wrote:
> >> On Mon, May 21, 2012 at 11:37:05AM +0200, Johannes Weiner wrote:
> >> [snip]
> >>>>> Is it because the read()/write() IO is high throughput and pushes
> >>>>> pages through the LRU lists faster than the mmap pages are referenced?
> >>>>
> >>>> Yes, in this application, one query needs to access mapped file page
> >>>> twice and file page cache twice.  Namely, one query needs to do 4 disk
> >>>> I/Os.  We have used fadvise(2) to reduce file page cache accessing to
> >>>> only once.  For mapped file page, in fact them are accessed only once
> >>>> because in one query the same data is accessed twice.  Thus, one query
> >>>> causes 2 disk I/Os now.  The size of read/write is quite larger than
> >>>> mmap/munmap.  So, as you see, if we can keep mmap/munmap file in memory
> >>>> as much as possible, we will gain the better performance.
> >>>
> >>> You access the same unmapped cache twice, i.e. repeated reads or
> >>> writes against the same file offset?
> >>
> >> No.  We access the same mapped file twice.
> >>
> >>>
> >>> How do you use fadvise?
> >>
> >> We access the header and content of the file respectively using read/write.
> >> The header and content are sequentially.  So we use fadivse(2) with
> >> FADV_WILLNEED flag to do a readahead.
> >>
> >>>> In addition, another factor also has some impacts for this application.
> >>>> In inactive_file_is_low_global(), it is different between 2.6.18 and
> >>>> upstream kernel.  IMHO, it causes that mapped file pages in active list
> >>>> are moved into inactive list frequently.
> >>>>
> >>>> Currently, we add a parameter in inactive_file_is_low_global() to adjust
> >>>> this ratio.  Meanwhile we activate every mapped file pages for the first
> >>>> time.  Then the performance gets better, but it still doesn't reach the
> >>>> performance of 2.6.18.
> >>>
> >>> 2.6.18 didn't have the active list protection at all and always
> >>> forcibly deactivated pages during reclaim.  Have you tried fully
> >>> reverting to this by making inactive_file_is_low_global() return true
> >>> unconditionally?
> >>
> >> No, I don't try it.  AFAIK, 2.6.18 didn't protect the active list.  But
> >> it doesn't always forcibly deactivate the pages.  I remember that in
> >> 2.6.18 kernel we calculate 'mapped_ratio' in shrink_active_list(), and
> >> then we get 'swap_tendency' according to 'mapped_ratio', 'distress', and
> >> 'sc->swappiness'.  If 'swap_tendency' is not greater than 100.  It
> >> doesn't reclaim mapped file pages.  By this equation, if the sum of the
> >> anonymous pages and mapped file pages is not greater than the 50% of
> >> total pages, we don't deactivate these pages.  Am I missing something?
> > 
> > I think we need to go back to protecting mapped pages based on how
> > much of reclaimable memory they make up, one way or another.
> 
> 
> I partly agreed it with POV regression.
> But I would like to understand rationale of "Why we should handle specially mmapped page".
> In case of code pages(VM_EXEC), we already have handled it specially and
> I understand why we did. At least, my opinion was that our LRU algorithm doesn't consider
> _frequency_ fully while it does _recency_ well. I thought code page would be high frequency of access
> compared to other pages.
> But in case of mapped data pages, why we should handle specially?
> I guess mapped data pages would have higher access chance than unmapped page because
> unmapped page doesn't have any owner(it's just for caching for reducing I/O) while mapped page
> has a owner above.
> 
> Doesn't it make sense?

I agree that the reason behind protecting VM_EXEC pages was that our
frequency information for mapped pages is at LRU cycle granularity.

But I don't see why you think this problem wouldn't apply to all
mapped pages in general.
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