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@xxxxxxxxx> 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. :( -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>