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From: | Peter Lieven |
Subject: | Re: [Qemu-devel] linux guests and ksm performance |
Date: | Tue, 28 Feb 2012 13:16:45 +0100 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.23) Gecko/20110921 Thunderbird/3.1.15 |
On 28.02.2012 13:05, Stefan Hajnoczi wrote:
On Tue, Feb 28, 2012 at 11:46 AM, Peter Lieven<address@hidden> wrote:On 24.02.2012 08:23, Stefan Hajnoczi wrote:On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi<address@hidden> wrote:On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi<address@hidden> wrote:On Thu, Feb 23, 2012 at 7:08 PM, address@hidden<address@hidden> wrote:Stefan Hajnoczi<address@hidden> schrieb:On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven<address@hidden> wrote:However, in a virtual machine I have not observed the above slow downtothat extend while the benefit of zero after free in a virtualisation environmentisobvious: 1) zero pages can easily be merged by ksm or other technique. 2) zero (dup) pages are a lot faster to transfer in case ofmigration. 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.
could you support/help me in implementing and publishing this approach? Peter
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