On Wed, Dec 08, 2010 at 05:23:24PM -0800, Andrew Morton wrote: > On Wed, 8 Dec 2010 16:36:21 -0800 Simon Kirby <sim@xxxxxxxxxx> wrote: > > > On Wed, Dec 08, 2010 at 04:16:59PM +0100, Johannes Weiner wrote: > > > > > Kswapd tries to rebalance zones persistently until their high > > > watermarks are restored. > > > > > > If the amount of unreclaimable pages in a zone makes this impossible > > > for reclaim, though, kswapd will end up in a busy loop without a > > > chance of reaching its goal. > > > > > > This behaviour was observed on a virtual machine with a tiny > > > Normal-zone that filled up with unreclaimable slab objects. > > > > > > This patch makes kswapd skip rebalancing on such 'hopeless' zones and > > > leaves them to direct reclaim. > > > > Hi! > > > > We are experiencing a similar issue, though with a 757 MB Normal zone, > > where kswapd tries to rebalance Normal after an order-3 allocation while > > page cache allocations (order-0) keep splitting it back up again. It can > > run the whole day like this (SSD storage) without sleeping. > > People at google have told me they've seen the same thing. A fork is > taking 15 minutes when someone else is doing a dd, because the fork > enters direct-reclaim trying for an order-one page. It successfully > frees some order-one pages but before it gets back to allocate one, dd > has gone and stolen them, or split them apart. > Is there a known test case for this or should I look at doing a streaming-IO test with a basic workload constantly forking in the background to measure the fork latency? > This problem would have got worse when slub came along doing its stupid > unnecessary high-order allocations. > > Billions of years ago a direct-reclaimer had a one-deep cache in the > task_struct into which it freed the page to prevent it from getting > stolen. > > Later, we took that out because pages were being freed into the > per-cpu-pages magazine, which is effectively task-local anyway. But > per-cpu-pages are only for order-0 pages. See slub stupidity, above. > > I expect that this is happening so repeatably because the > direct-reclaimer is dong a sleep somewhere after freeing the pages it > needs - if it wasn't doing that then surely the window wouldn't be wide > enough for it to happen so often. But I didn't look. > > Suitable fixes might be > > a) don't go to sleep after the successful direct-reclaim. > I submitted a patch for this a long time ago but at the time we didn't have a test case that made a difference to it. Might be worth revisiting. I can't find the related patch any more but it was fairly trivial. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>