On Thu 09-03-17 15:09:04, Mel Gorman wrote: > On Wed, Mar 08, 2017 at 09:34:27PM +0530, Anshuman Khandual wrote: > > > Any comments, suggestions are welcome. > > > > Hello Vlastimil/Michal/Minchan/Mel/Dave, > > > > Apart from the comments from Naoya on a different thread posted by Zi > > Yan, I did not get any more review comments on this series. Could you > > please kindly have a look on the over all design and its benefits from > > page migration performance point of view and let me know your views. > > Thank you. > > > > I didn't look into the patches in detail except to get a general feel > for how it works and I'm not convinced that it's a good idea at all. > > I accept that memory bandwidth utilisation may be higher as a result but > consider the impact. THP migrations are relatively rare and when they > occur, it's in the context of a single thread. To parallelise the copy, > an allocation, kmap and workqueue invocation are required. There may be a > long delay before the workqueue item can start which may exceed the time > to do a single copy if the CPUs on a node are saturated. Furthermore, a > single thread can preempt operations of other unrelated threads and incur > CPU cache pollution and future misses on unrelated CPUs. It's compounded by > the fact that a high priority system workqueue is used to do the operation, > one that is used for CPU hotplug operations and rolling back when a netdevice > fails to be registered. It treats a hugepage copy as an essential operation > that can preempt all other work which is very questionable. > > The series leader has no details on a workload that is bottlenecked by > THP migrations and even if it is, the primary question should be *why* > THP migrations are so frequent and alleviating that instead of > preempting multiple CPUs to do the work. FWIW I very much agree here and the follow up reply. Making migration itself parallel is a hard task. You should start simple and optimize the current code first and each step accompany with numbers. Parallel migration should be the very last step - if it is needed at all of course. I am quite skeptical that a reasonable parallel load balancing is achievable without a large maintenance cost and/or predictable behavior. -- Michal Hocko SUSE Labs -- 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>
