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

[Avi Kivity <avi@redhat.com>]

On 02/28/2012 03:20 PM, Peter Lieven wrote:
> On 28.02.2012 14:16, Avi Kivity wrote:
>> On 02/24/2012 08:41 AM, Stefan Hajnoczi wrote:
>>>> 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.
>> In the long term, we will use them, except if the guest is completely
>> idle.
>>
>> The scenario in which zeroing is expensive is when the page is refilled
>> through DMA.  In that case the zeroing was wasted.  This is a pretty
>> common scenario in pagecache intensive workloads.
>>
> Avi, what do you think of the proposal to give the guest vm a hint
> that the host is running ksm? In that case the administrator
> has already chosen that saving physical memory is more important
> than performance to him?

It makes some sense.  Perhaps through the balloon device, a flag that
indicates that voluntary ballooning will be gratefully accepted.

-- 
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