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

[Peter Lieven <pl@dlh.net>]

On 28.02.2012 13:05, Stefan Hajnoczi wrote:
> On Tue, Feb 28, 2012 at 11:46 AM, Peter Lieven<pl@dlh.net>  wrote:
>> On 24.02.2012 08:23, Stefan Hajnoczi wrote:
>>> On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi<stefanha@gmail.com>
>>>   wrote:
>>>> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi<stefanha@gmail.com>
>>>>   wrote:
>>>>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com<pl@dlh.net>
>>>>>   wrote:
>>>>>> Stefan Hajnoczi<stefanha@gmail.com>    schrieb:
>>>>>>
>>>>>>> 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.
>>>>>>>
>>>>>> 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