On Thu, Aug 07, 2014 at 03:08:22PM +0200, Michal Hocko wrote: > On Mon 04-08-14 17:14:54, Johannes Weiner wrote: > > Instead of passing the request size to direct reclaim, memcg just > > manually loops around reclaiming SWAP_CLUSTER_MAX pages until the > > charge can succeed. That potentially wastes scan progress when huge > > page allocations require multiple invocations, which always have to > > restart from the default scan priority. > > > > Pass the request size as a reclaim target to direct reclaim and leave > > it to that code to reach the goal. > > THP charge then will ask for 512 pages to be (direct) reclaimed. That > is _a lot_ and I would expect long stalls to achieve this target. I > would also expect quick priority drop down and potential over-reclaim > for small and moderately sized memcgs (e.g. memcg with 1G worth of pages > would need to drop down below DEF_PRIORITY-2 to have a chance to scan > that many pages). All that done for a charge which can fallback to a > single page charge. > > The current code is quite hostile to THP when we are close to the limit > but solving this by introducing long stalls instead doesn't sound like a > proper approach to me. THP latencies are actually the same when comparing high limit nr_pages reclaim with the current hard limit SWAP_CLUSTER_MAX reclaim, although system time is reduced with the high limit. High limit reclaim with SWAP_CLUSTER_MAX has better fault latency but it doesn't actually contain the workload - with 1G high and a 4G load, the consumption at the end of the run is 3.7G. So what I'm proposing works and is of equal quality from a THP POV. This change is complicated enough when we stick to the facts, let's not make up things based on gut feeling. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>