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

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Aug 18, 2010 at 11:57:26PM +0900, Minchan Kim wrote:
> On Wed, Aug 18, 2010 at 09:51:23AM +0100, Mel Gorman wrote:
> > > What's a window low and min wmark? Maybe I can miss your point. 
> > > 
> > 
> > The window is due to the fact kswapd is not awake yet. The window is because
> > kswapd might not be awake as NR_FREE_PAGES is higher than it should be. The
> > system is really somewhere between the low and min watermark but we are not
> > taking the accurate measure until kswapd gets woken up. The first allocation
> > to notice we are below the low watermark (be it due to vmstat refreshing or
> > that NR_FREE_PAGES happens to report we are below the watermark regardless of
> > any drift) wakes kswapd and other callers then take an accurate count hence
> > "we could breach the watermark but I'm expecting it can only happen for at
> > worst one allocation".
> 
> Right. I misunderstood your word. 
> One more question. 
> 
> Could you explain live lock scenario?
> 

Lets say

NR_FREE_PAGES     = 256
Actual free pages = 8

The PCP lists get refilled in patch taking all 8 pages. Now there are
zero free pages. Reclaim kicks in but to reclaim any pages it needs to
clean something but all the pages are on a network-backed filesystem. To
clean them, it must transmit on the network so it tries to allocate some
buffers.

The livelock is that to free some memory, an allocation must succeed but
for an allocation to succeed, some memory must be freed. The system
might still remain alive if a process exits and does not need to
allocate memory while exiting but by and large, the system is in a
dangerous state.

> I looked over the code. Although the VM pass zone_watermark_ok by luck,
> It can't allocate the page from buddy and then might go OOM. 
> When do we meet live lock case?
> 
> I think the description in change log would be better to understand 
> this patch in future. 
> 

Is the above description useful? If so, I can put it in the leader.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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