On Mon 21-09-20 07:56:33, Christoph Hellwig wrote: > On Mon, Aug 31, 2020 at 05:06:33PM -0700, Minchan Kim wrote: > > There is usecase that System Management Software(SMS) want to give a > > memory hint like MADV_[COLD|PAGEEOUT] to other processes and in the > > case of Android, it is the ActivityManagerService. > > > > The information required to make the reclaim decision is not known to > > the app. Instead, it is known to the centralized userspace > > daemon(ActivityManagerService), and that daemon must be able to > > initiate reclaim on its own without any app involvement. > > > > To solve the issue, this patch introduces a new syscall process_madvise(2). > > It uses pidfd of an external process to give the hint. It also supports > > vector address range because Android app has thousands of vmas due to > > zygote so it's totally waste of CPU and power if we should call the > > syscall one by one for each vma.(With testing 2000-vma syscall vs > > 1-vector syscall, it showed 15% performance improvement. I think it > > would be bigger in real practice because the testing ran very cache > > friendly environment). > > I'm really not sure this syscall is a good idea. If you want central > control you should implement an IPC mechanisms that allows your > supervisor daemon to tell the application to perform the madvice > instead of forcing the behavior on it. Even though I am not entirely happy about the interface [1]. As it seems I am in minority in my concern I backed off and decided to not block this work because I do not see the problem with the functionality itself. And I find it very useful for userspace driven memory management people are asking for a long time. This functionality shouldn't be much different from the standard memory reclaim. It has some limitations (e.g. it can only handle mapped memory) but allows to pro-actively swap out or reclaim disk based memory based on a specific knowlege of the workload. Kernel is not able to do the same. [1] http://lkml.kernel.org/r/20200117115225.GV19428@xxxxxxxxxxxxxx -- Michal Hocko SUSE Labs
