On Wed 29-05-19 03:08:32, Daniel Colascione wrote: > On Mon, May 27, 2019 at 12:49 AM Minchan Kim <minc...@kernel.org> wrote: > > > > On Tue, May 21, 2019 at 12:37:26PM +0200, Michal Hocko wrote: > > > On Tue 21-05-19 19:26:13, Minchan Kim wrote: > > > > On Tue, May 21, 2019 at 08:24:21AM +0200, Michal Hocko wrote: > > > > > On Tue 21-05-19 11:48:20, Minchan Kim wrote: > > > > > > On Mon, May 20, 2019 at 11:22:58AM +0200, Michal Hocko wrote: > > > > > > > [Cc linux-api] > > > > > > > > > > > > > > On Mon 20-05-19 12:52:53, Minchan Kim wrote: > > > > > > > > Currently, process_madvise syscall works for only one address > > > > > > > > range > > > > > > > > so user should call the syscall several times to give hints to > > > > > > > > multiple address range. > > > > > > > > > > > > > > Is that a problem? How big of a problem? Any numbers? > > > > > > > > > > > > We easily have 2000+ vma so it's not trivial overhead. I will come > > > > > > up > > > > > > with number in the description at respin. > > > > > > > > > > Does this really have to be a fast operation? I would expect the > > > > > monitor > > > > > is by no means a fast path. The system call overhead is not what it > > > > > used > > > > > to be, sigh, but still for something that is not a hot path it should > > > > > be > > > > > tolerable, especially when the whole operation is quite expensive on > > > > > its > > > > > own (wrt. the syscall entry/exit). > > > > > > > > What's different with process_vm_[readv|writev] and vmsplice? > > > > If the range needed to be covered is a lot, vector operation makes > > > > senese > > > > to me. > > > > > > I am not saying that the vector API is wrong. All I am trying to say is > > > that the benefit is not really clear so far. If you want to push it > > > through then you should better get some supporting data. > > > > I measured 1000 madvise syscall vs. a vector range syscall with 1000 > > ranges on ARM64 mordern device. Even though I saw 15% improvement but > > absoluate gain is just 1ms so I don't think it's worth to support. > > I will drop vector support at next revision. > > Please do keep the vector support. Absolute timing is misleading, > since in a tight loop, you're not going to contend on mmap_sem. We've > seen tons of improvements in things like camera start come from > coalescing mprotect calls, with the gains coming from taking and > releasing various locks a lot less often and bouncing around less on > the contended lock paths. Raw throughput doesn't tell the whole story, > especially on mobile.
This will always be a double edge sword. Taking a lock for longer can improve a throughput of a single call but it would make a latency for anybody contending on the lock much worse. Besides that, please do not overcomplicate the thing from the early beginning please. Let's start with a simple and well defined remote madvise alternative first and build a vector API on top with some numbers based on _real_ workloads. -- Michal Hocko SUSE Labs
