On Fri, Jul 30, 2010 at 03:17:35PM +0200, Andrea Arcangeli wrote: > On Wed, Jul 28, 2010 at 03:17:05PM +0800, Wu Fengguang 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. > > Lumpy reclaim just made the system totally unusable with frequent > order 9 allocations. Yes, it's very disruptive and has been for a while. It was not much of a problem when resizing the static hugepage pool but is a disaster for transparent huge pages. > I nuked it long ago and replaced it with mem > compaction. You may try aa.git to test how thing goes without lumpy > reclaim. I recently also started to use mem compaction for order 1/2/3 > allocations as there's no point not to use it for them, and to call > mem compaction from kswapd to satisfy order 2 GFP_ATOMIC in > replacement of blind responsiveness-destroyer lumpy. > A full-scale replacement is overkill but I can see why it would be done in the short-term. There are times when lumpy reclaim is still needed - specifically when the allocation failure is due to a lack of memory rather than fragmentation. There will also be cases where compaction can't work because there are too many movable pages to move into too few pageblocks. > Not sure why people insists on lumpy when we've memory compaction that > won't alter the working set and it's more effective. > Compaction is preferred, no doubt about it but lumpy reclaim cannot be dismissed. I know lumpy reclaim is too disruptive and Kosaki noticed the same and it's currently doing some pretty stupid things. There are a few ideas knocking around publicly on how to reduce its impact while increasing its effectiveness. I have a few old ideas knocking around as well that I just need the time to get around to. I hope to get at it after the fuss over writeback is addressed. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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
