On Thu, Feb 23, 2012 at 7:08 PM, peter.lie...@gmail.com <p...@dlh.net> wrote: > > > > > Stefan Hajnoczi <stefa...@gmail.com> schrieb: > >>On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <p...@dlh.net> 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. Stefan
