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. -- Mel Gorman SUSE Labs
