On Sat, Jul 31, 2010 at 01:44:09AM +0900, Round Robinjp wrote: > > > Please could you elaborate a little more why depending on > > > compaction to satisfy other high-order allocation is not good. > > > > > > > At the very least, it's not a situation that has been tested heavily and > > because other high-order allocations are typically not movable. In the > > worst case, if they are both frequent and long-lived they *may* eventually > > encounter fragmentation-related problems. This uncertainity is why it's > > not good. It gets worse if there is no swap as eventually all movable pages > > will be compacted as much as possible but there still might not be enough > > contiguous memory for a high-order page because other pages are pinned. > > I am interested in this topic too. > > How about using compaction for infrequent short-lived > high-order allocations? Depend on MIGRATE_RESERVE instead within fragmentation avoidance. It's objective is to keep certain blocks of pages free unless there is no other choice. How many blocks of MIGRATE_RESERVE there are depends on the value of min_free_kbytes (which can be tuned to a recommended value with hugeadm) MIGRATE_RESERVE is known to be important for short-lived high-order allocations - particularly atomic ones. > Is there any problem in that case? > (apart from the point that it is not tested for that purpose) > It's racy, you are depending on compaction to happen at the right time and with enough vigour to prevent allocation failures. > Also how about using compaction as a preparation > for partial refresh? > Hacky, but you could do it from userspace by periodically writing to /proc/sys/vm/compact_memory. In the event allocation failures are common, it would still be best to figure out how long-lived those allocations are and why MIGRATE_RESERVE was insufficient. I'm not saying pre-emptively compacting won't work, it probably will for a large number of cases but there will be failure scenarios in the field. -- 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>