On Wed, Jul 28, 2010 at 4:17 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > Fix "system goes unresponsive under memory pressure and lots of > dirty/writeback pages" bug. > > http://lkml.org/lkml/2010/4/4/86 > > In the above thread, Andreas Mohr described that > > Invoking any command locked up for minutes (note that I'm > talking about attempted additional I/O to the _other_, > _unaffected_ main system HDD - such as loading some shell > binaries -, NOT the external SSD18M!!). > > This happens when the two conditions are both meet: > - under memory pressure > - writing heavily to a slow device > > OOM also happens in Andreas' system. The OOM trace shows that 3 > processes are stuck in wait_on_page_writeback() in the direct reclaim > path. One in do_fork() and the other two in unix_stream_sendmsg(). They > are blocked on this condition: > > (sc->order && priority < DEF_PRIORITY - 2) > > which was introduced in commit 78dc583d (vmscan: low order lumpy reclaim > also should use PAGEOUT_IO_SYNC) one year ago. That condition may be too > permissive. In Andreas' case, 512MB/1024 = 512KB. If the direct reclaim > for the order-1 fork() allocation runs into a range of 512KB > hard-to-reclaim LRU pages, it will be stalled. > > It's a severe problem in three ways. > > Firstly, it can easily happen in daily desktop usage. vmscan priority > can easily go below (DEF_PRIORITY - 2) on _local_ memory pressure. Even > if the system has 50% globally reclaimable pages, it still has good > opportunity to have 0.1% sized hard-to-reclaim ranges. For example, a > simple dd can easily create a big range (up to 20%) of dirty pages in > the LRU lists. > > Secondly, once triggered, it will stall unrelated processes (not doing IO > at all) in the system. This "one slow USB device stalls the whole system" > avalanching effect is very bad. > > Thirdly, once stalled, the stall time could be intolerable long for the > users. When there are 20MB queued writeback pages and USB 1.1 is > writing them in 1MB/s, wait_on_page_writeback() will stuck for up to 20 > seconds. Not to mention it may be called multiple times. > > So raise the bar to only enable PAGEOUT_IO_SYNC when priority goes below > DEF_PRIORITY/3, or 6.25% LRU size. As the default dirty throttle ratio is > 20%, it will hardly be triggered by pure dirty pages. We'd better treat > PAGEOUT_IO_SYNC as some last resort workaround -- its stall time is so > uncomfortably long (easily goes beyond 1s). > > The bar is only raised for (order < PAGE_ALLOC_COSTLY_ORDER) allocations, > which are easy to satisfy in 1TB memory boxes. So, although 6.25% of > memory could be an awful lot of pages to scan on a system with 1TB of > memory, it won't really have to busy scan that much. > > Reported-by: Andreas Mohr <andi@xxxxxxxx> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx> The description and code both look good to me. Thanks for great effort, Wu. -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
