On Thu 30-05-19 11:17:48, Minchan Kim wrote: > On Wed, May 29, 2019 at 12:33:52PM +0200, Michal Hocko wrote: > > On Wed 29-05-19 03:08:32, Daniel Colascione wrote: > > > On Mon, May 27, 2019 at 12:49 AM Minchan Kim <minchan@xxxxxxxxxx> 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. > > First time, I didn't think about atomicity about address range race > because MADV_COLD/PAGEOUT is not critical for the race. > However you raised the atomicity issue because people would extend > hints to destructive ones easily. I agree with that and that's why > we discussed how to guarantee the race and Daniel comes up with good idea. Just for the clarification, I didn't really mean atomicity but rather a _consistency_ (essentially time to check to time to use consistency). > - vma configuration seq number via process_getinfo(2). > > We discussed the race issue without _read_ workloads/requests because > it's common sense that people might extend the syscall later. > > Here is same. For current workload, we don't need to support vector > for perfomance point of view based on my experiment. However, it's > rather limited experiment. Some configuration might have 10000+ vmas > or really slow CPU. > > Furthermore, I want to have vector support due to atomicity issue > if it's really the one we should consider. > With vector support of the API and vma configuration sequence number > from Daniel, we could support address ranges operations's atomicity. I am not sure what do you mean here. Perform all ranges atomicaly wrt. other address space modifications? If yes I am not sure we want that semantic because it can cause really long stalls for other operations but that is a discussion on its own and I would rather focus on a simple interface first. > However, since we don't introduce vector at this moment, we need to > introduce *another syscall* later to be able to handle multile ranges > all at once atomically if it's okay. Agreed. > Other thought: > Maybe we could extend address range batch syscall covers other MM > syscall like mmap/munmap/madvise/mprotect and so on because there > are multiple users that would benefit from this general batching > mechanism. Again a discussion on its own ;) -- Michal Hocko SUSE Labs
