On 01/01/2014 02:53 AM, Marcelo Tosatti wrote: > On Tue, Dec 31, 2013 at 08:06:51PM +0800, Xiao Guangrong wrote: >> >> On Dec 31, 2013, at 4:23 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >> >>> On Tue, Dec 17, 2013 at 01:59:04PM +0800, Xiao Guangrong wrote: >>>> >>>> CCed KVM guys. >>>> >>>> On 05/10/2013 01:11 PM, Stefan Hajnoczi wrote: >>>>> On Fri, May 10, 2013 at 4:28 AM, wenchao <wenchaolinux@xxxxxxxxx> wrote: >>>>>> 于 2013-5-9 22:13, Mel Gorman 写道: >>>>>> >>>>>>> On Thu, May 09, 2013 at 05:50:05PM +0800, wenchaolinux@xxxxxxxxx wrote: >>>>>>>> >>>>>>>> From: Wenchao Xia <wenchaolinux@xxxxxxxxx> >>>>>>>> >>>>>>>> This serial try to enable mremap syscall to cow some private memory >>>>>>>> region, >>>>>>>> just like what fork() did. As a result, user space application would got >>>>>>>> a >>>>>>>> mirror of those region, and it can be used as a snapshot for further >>>>>>>> processing. >>>>>>>> >>>>>>> >>>>>>> What not just fork()? Even if the application was threaded it should be >>>>>>> managable to handle fork just for processing the private memory region >>>>>>> in question. I'm having trouble figuring out what sort of application >>>>>>> would require an interface like this. >>>>>>> >>>>>> It have some troubles: parent - child communication, sometimes >>>>>> page copy. >>>>>> I'd like to snapshot qemu guest's RAM, currently solution is: >>>>>> 1) fork() >>>>>> 2) pipe guest RAM data from child to parent. >>>>>> 3) parent write down the contents. >>>>>> >>>>>> To avoid complex communication for data control, and file content >>>>>> protecting, So let parent instead of child handling the data with >>>>>> a pipe, but this brings additional copy(). I think an explicit API >>>>>> cow mapping an memory region inside one process, could avoid it, >>>>>> and faster and cow less pages, also make user space code nicer. >>>>> >>>>> A new Linux-specific API is not portable and not available on existing >>>>> hosts. Since QEMU supports non-Linux host operating systems the >>>>> fork() approach is preferable. >>>>> >>>>> If you're worried about the memory copy - which should be benchmarked >>>>> - then vmsplice(2) can be used in the child process and splice(2) can >>>>> be used in the parent. It probably doesn't help though since QEMU >>>>> scans RAM pages to find all-zero pages before sending them over the >>>>> socket, and at that point the memory copy might not make much >>>>> difference. >>>>> >>>>> Perhaps other applications can use this new flag better, but for QEMU >>>>> I think fork()'s portability is more important than the convenience of >>>>> accessing the CoW pages in the same process. >>>> >>>> Yup, I agree with you that the new syscall sometimes is not a good solution. >>>> >>>> Currently, we're working on live-update[1] that will be enabled on Qemu firstly, >>>> this feature let the guest run on the new Qemu binary smoothly without >>>> restart, it's good for us to do security-update. >>>> >>>> In this case, we need to move the guest memory on old qemu instance to the >>>> new one, fork() can not help because we need to exec() a new instance, after >>>> that all memory mapping will be destroyed. >>>> >>>> We tried to enable SPLICE_F_MOVE[2] for vmsplice() to move the memory without >>>> memory-copy but the performance isn't so good as we expected: it's due to >>>> some limitations: the page-size, lock, message-size limitation on pipe, etc. >>>> Of course, we will continue to improve this, but wenchao's patch seems a new >>>> direction for us. >>>> >>>> To coordinate with your fork() approach, maybe we can introduce a new flag >>>> for VMA, something like: VM_KEEP_ONEXEC, to tell exec() to do not destroy >>>> this VMA. How about this or you guy have new idea? Really appreciate for your >>>> suggestion. >>>> >>>> [1] http://marc.info/?l=qemu-devel&m=138597598700844&w=2 >>>> [2] https://lkml.org/lkml/2013/10/25/285 >>> >>> Hi, >>> >> >> Hi Marcelo, >> >> >>> What is the purpose of snapshotting guest RAM here, in the context of >>> local migration? >> >> RAM-shapshotting and local-migration are on the different ways. >> Why i asked for your guy’s suggestion here is beacuse i thought >> they need do a same thing that moves memory from one process >> to another in a efficient way. Your idea? :) > > Another possibility is to use memory that is not anonymous for guest > RAM, such as hugetlbfs or tmpfs. > > IIRC ksm and thp have limitations wrt tmpfs. Yes, KSM and THP are what we're concerning about. > > Still curious about RAM snapshotting. Wen Chao, could you please tell it more? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
