On Wed, 27 Jul 2011, Hugh Dickins wrote: > On Wed, 27 Jul 2011, Andrew Morton wrote: > > On Tue, 19 Jul 2011 15:54:23 -0700 (PDT) > > Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > > > But it was a shock to find swapoff of a 500MB file 20 times slower > > > on my laptop, taking 10 minutes; and at that rate it significantly > > > slows down my testing. > > > > So it used to take half a minute? That was already awful. > > Why? Was it IO-bound? It doesn't sound like it. > > No, not IO-bound at all. I oversimplified: about 10 seconds of that was waiting for IO, the rest (of 10 minutes or of half a minute) was cpu. It's the cpu part of it which the change of radix tree has affected, for the worse. > > How much did that 10 minutes improve? > > To 1 minute: still twice as slow as before. I believe that's because of > the smaller nodes and greater height of the generic radix tree. I ought > to experiment with a bigger RADIX_TREE_MAP_SHIFT to verify that belief > (though I don't think tmpfs swapoff would justify raising it): will do. Yes, raising RADIX_TREE_MAP_SHIFT from 6 to 10 (so on 32-bit the rnode is just over 4kB, comparable with the old shmem's use of pages for this) brings the time down considerably: still slower than before, but 12% slower instead of twice as slow (or 20% slower instead of 3 times as slow when comparing sys times). Not that making a radix_tree_node need order:1 page would be sensible. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>