Re: [PATCH 2/3] vmscan: make mapped executable pages the first class citizen

[KOSAKI Motohiro]

Hi

2009/5/19 Wu Fengguang <fengguang.wu@xxxxxxxxx>:
> On Tue, May 19, 2009 at 04:06:35PM +0800, KOSAKI Motohiro wrote:
>> > > > Like the console mode, the absolute nr_mapped drops considerably - to 1/13 of
>> > > > the original size - during the streaming IO.
>> > > >
>> > > > The delta of pgmajfault is 3 vs 107 during IO, or 236 vs 393 during the whole
>> > > > process.
>> > >
>> > > hmmm.
>> > >
>> > > about 100 page fault don't match Elladan's problem, I think.
>> > > perhaps We missed any addional reproduce condition?
>> >
>> > Elladan's case is not the point of this test.
>> > Elladan's IO is use-once, so probably not a caching problem at all.
>> >
>> > This test case is specifically devised to confirm whether this patch
>> > works as expected. Conclusion: it is.
>>
>> Dejection ;-)
>>
>> The number should address the patch is useful or not. confirming as expected
>> is not so great.
>
> OK, let's make the conclusion in this way:
>
> The changelog analyzed the possible beneficial situation, and this
> test backs that theory with real numbers, ie: it successfully stops
> major faults when the active file list is slowly scanned when there
> are partially cache hot streaming IO.
>
> Another (amazing) finding of the test is, only around 1/10 mapped pages
> are actively referenced in the absence of user activities.
>
> Shall we protect the remaining 9/10 inactive ones? This is a question ;-)

Unfortunately, I don't reproduce again.
I don't apply your patch yet. but mapped ratio is reduced only very little.

I think smem can show which library evicted.  Can you try it?

download:  http://www.selenic.com/smem/
usage:   ./smem -m -r --abbreviate


We can't decide 9/10 is important or not. we need know actual evicted file list.

Thanks.


> Or, shall we take the "protect active VM_EXEC mapped pages" approach,
> or Christoph's "protect all mapped pages all time, unless they grow
> too large" attitude?  I still prefer the best effort VM_EXEC heuristics.
>
> 1) the partially cache hot streaming IO is far more likely to happen
>   on (file) servers. For them, evicting the 9/10 inactive mapped
>   pages over night should be acceptable for sysadms.
>
> 2) for use-once IO on desktop, we have Rik's active file list
>   protection heuristics, so nothing to worry at all.
>
> 3) for big working set small memory desktop, the active list will
>   still be scanned, in this situation, why not evict some of the
>   inactive mapped pages? If they have not been accessed for 1 minute,
>   they are not likely be the user focus, and the tight memory
>   constraint can only afford to cache the user focused working set.
>
> Does that make sense?
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