On Tue, Feb 28, 2012 at 11:46 AM, Peter Lieven <pl@xxxxxxx> wrote: > On 24.02.2012 08:23, Stefan Hajnoczi wrote: >> >> On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi<stefanha@xxxxxxxxx> >> wrote: >>> >>> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi<stefanha@xxxxxxxxx> >>> wrote: >>>> >>>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@xxxxxxxxx<pl@xxxxxxx> >>>> wrote: >>>>> >>>>> Stefan Hajnoczi<stefanha@xxxxxxxxx> schrieb: >>>>> >>>>>> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven<pl@xxxxxxx> wrote: >>>>>>> >>>>>>> However, in a virtual machine I have not observed the above slow down >>>>>> >>>>>> to >>>>>>> >>>>>>> that extend >>>>>>> while the benefit of zero after free in a virtualisation environment >>>>>> >>>>>> is >>>>>>> >>>>>>> obvious: >>>>>>> >>>>>>> 1) zero pages can easily be merged by ksm or other technique. >>>>>>> 2) zero (dup) pages are a lot faster to transfer in case of >>>>>> >>>>>> migration. >>>>>> >>>>>> The other approach is a memory page "discard" mechanism - which >>>>>> obviously requires more code changes than zeroing freed pages. >>>>>> >>>>>> The advantage is that we don't take the brute-force and CPU intensive >>>>>> approach of zeroing pages. It would be like a fine-grained ballooning >>>>>> feature. >>>>>> >>>>> I dont think that it is cpu intense. All user pages are zeroed anyway, >>>>> but at allocation time it shouldnt be a big difference in terms of cpu >>>>> power. >>>> >>>> It's easy to find a scenario where eagerly zeroing pages is wasteful. >>>> Imagine a process that uses all of physical memory. Once it >>>> terminates the system is going to run processes that only use a small >>>> set of pages. It's pointless zeroing all those pages if we're not >>>> going to use them anymore. >>> >>> Perhaps the middle path is to zero pages but do it after a grace >>> timeout. I wonder if this helps eliminate the 2-3% slowdown you >>> noticed when compiling. >> >> Gah, it's too early in the morning. I don't think this timer actually >> makes sense. > > > do you think it makes then sense to make a patchset/proposal to notice a > guest > kernel about the presense of ksm in the host and switch to zero after free? I think your idea is interesting - whether or not people are happy with it will depend on the performance impact. It seems reasonable to me. Stefan -- 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
