Re: linux guests and ksm performance

[Peter Lieven <pl@dlh.net>]

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?

Peter