Re: [PATCH] vmscan: raise the bar to PAGEOUT_IO_SYNC stalls

[Andrea Arcangeli <aarcange@redhat.com>]

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. 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.

Not sure why people insists on lumpy when we've memory compaction that
won't alter the working set and it's more effective.