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

[Peter Lieven <pl@dlh.net>]

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?

peter