Re: [Qemu-devel] linux guests and ksm performance

[Stefan Hajnoczi <stefanha@gmail.com>]

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