On Mon, Mar 15, 2010 at 01:09:35PM -0700, Andrew Morton wrote: > On Mon, 15 Mar 2010 13:34:50 +0100 > Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> wrote: > > > c) If direct reclaim did reasonable progress in try_to_free but did not > > get a page, AND there is no write in flight at all then let it try again > > to free up something. > > This could be extended by some kind of max retry to avoid some weird > > looping cases as well. > > > > d) Another way might be as easy as letting congestion_wait return > > immediately if there are no outstanding writes - this would keep the > > behavior for cases with write and avoid the "running always in full > > timeout" issue without writes. > > They're pretty much equivalent and would work. But there are two > things I still don't understand: > > 1: Why is direct reclaim calling congestion_wait() at all? If no > writes are going on there's lots of clean pagecache around so reclaim > should trivially succeed. What's preventing it from doing so? > > 2: This is, I think, new behaviour. A regression. What caused it? > I looked at this a bit closer using an iozone test very similar to Christian's. Despite buying a number of disks, I still can't reproduce his problem but I instrumented congestion_wait counts and times similar to what he did. 2.6.29-instrument:congestion_waittime 990 2.6.30-instrument:congestion_waittime 2823 2.6.31-instrument:congestion_waittime 193169 2.6.32-instrument:congestion_waittime 228890 2.6.33-instrument:congestion_waittime 785529 2.6.34-rc1-instrument:congestion_waittime 797178 So in the problem window, there was *definite* increases in the time spent in congestion_wait and the number of times it was called. I'll look closer at this tomorrow and Monday and see can I pin down what is happening. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>