Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Aug 17, 2010 at 11:16:55AM +0100, Mel Gorman wrote:
> Well, the drift can be either direction because drift can be due to pages
> being either freed or allocated. e.g. it could be something like
> 
> NR_FREE_PAGES		CPU 0			CPU 1		Actual Free
> 128			-32			 +64		   160
> 
> Because CPU 0 was allocating pages while CPU 1 was freeing them but that
> is not what is important here. At any given time, the NR_FREE_PAGES can be
> wrong by as much as
> 
> num_online_cpus * (threshold - 1)

That's the answer I expected.
As I mentioned previous mail, we need to consider allocation path.
But you already have been considered it by partially in here. 
Yes. It looks good to me. :)

Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

> 
> As kswapd goes back to sleep when the high watermark is reached, it's important
> that it has actually reached the watermark before sleeping.  Similarly,
> if an allocator is checking the low watermark, it needs an accurate count.
> Hence a more careful accounting for NR_FREE_PAGES should happen when the
> number of free pages is within
> 
> high_watermark + (num_online_cpus * (threshold - 1))
> 
> Only checking when kswapd is awake still leaves a window between the low
> and min watermark when we could breach the watermark but I'm expecting it
> can only happen for at worst one allocation. After that, kswapd wakes
> and the count becomes accurate again.

I can't understand the point. 
Now kswapd starts from below low wmark and stops until high wmark.
So if VM has pages of below low wmark, it could always check by zone_nr_free_pages 
regardless of min. 

What's a window low and min wmark? Maybe I can miss your point. 

-- 
Kind regards,
Minchan Kim

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