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 Thu, Aug 19, 2010 at 11:34:39PM +0900, Minchan Kim wrote:
> On Thu, Aug 19, 2010 at 03:09:46PM +0100, Mel Gorman wrote:
> > On Thu, Aug 19, 2010 at 11:01:50PM +0900, Minchan Kim wrote:
> > > On Thu, Aug 19, 2010 at 11:38:39AM +0100, Mel Gorman wrote:
> > > > On Thu, Aug 19, 2010 at 07:33:57PM +0900, Minchan Kim wrote:
> > > > > On Thu, Aug 19, 2010 at 5:06 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> > > > > > 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
> > > > > 
> > > > > Yes. I understood this as livelock but at last VM will kill victim
> > > > > process then it can allocate free pages.
> > > > 
> > > > And if the exit path for the OOM kill needs to allocate a page what
> > > > should it do?
> > > 
> > > Yeah. It might be livelock. 
> > > Then, let's rethink the problem. 
> > > 
> > > The problem is following as. 
> > > 
> > > 1. Process A try to allocate the page
> > > 2. VM try to reclaim the page for process A
> > > 3. VM reclaims some pages but it remains on PCP so can't allocate pages for A
> > > 4. VM try to kill process B
> > > 5. The exit path need new pages for exiting process B
> > > 6. Livelock happens(I am not sure but we need any warning if it really happens at least)
> > > 
> > 
> > The problem this patch is concerned with is about the vmstat counters, not
> > the pages on the per-cpu lists. The issue being dealt with is that the page
> > allocator grants a page going below the min watermark because NR_FREE_PAGES
> > can be inaccurate. The patch aims to fix that but taking greater care
> > with NR_FREE_PAGES when memory is low.
> 
> Your goal is to protect _min_ pages which is reserved. Right?
> I thought your final goal is to protect the livelock problem. 
> Hmm.. Sorry for the noise. :(
> 

Emm, it's the same thing. If the min watermark is not properly
preserved, the system is in danger of being live-locked.

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

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