[vbox-dev] Docs on how page fusion works?

Howard Su howard0su at gmail.com
Wed May 2 06:11:10 GMT 2012

On Wed, Apr 18, 2012 at 2:39 AM, Ramshankar <
ramshankar.venkataraman at oracle.com> wrote:

> Our current page fusion logic involves knowledge from within the guest
> as to what can be fused. Instead of pegging through the entire guest
> memory using some-sort of daemon/service and maintaining hashes and
> last-touch times of pages and comparing them, our guest additions
> (currently page fusion implemented only for Windows guests) gives hints
> about which pages are most likely candidates for fusion.
> This saves a lot of time than sweeping the memory but it also means we
> will not be squeezing out every last bit. We made this trade-off
> decision because we felt this is a good approach for the fulfilling the
> objective.
I read a project http://code.google.com/p/uksm which is improving KSM's
scan speed and result. It is called UKSM.
in a type of hardware, Interl Core 2 9300, KSM scans page at 260M/s. UKSM
can hit 477MB/s - 923M/s, even 627- 2446MB/s in the pages which doesn't
contain duplications. I think the speed is a big factor here to do the
tradeoff. full memory scan can avoid the duplication of anony memory pages.
will this change your decision about the tradeoff?

> A daemon on the guest runs which locates common system files/dlls/ro
> kernel memory etc. paged-in on the guest and reports the physical pages
> that can be deduplicated. We don't scan the guest memory actively
> looking for fusion candidates. If the guest touches the pages for write
> access that'll be marked as no longer a candidate. Because of
> contextually knowledge from within the guest, VirtualBox's page fusion
> identifies only long term fusion candidates that are very unlikely to be
> touched often.
> That's just the broad overview.
> Regards,
> Ram
> On 04/16/12 05:23 PM, Alexey Eromenko wrote:
> >>
> >> What kind if obstacles would I face if I tried to implemented the
> >> same behavior (Scan processes) for Linux guests? I plan on scanning
> every
> >> process then check the memory maps from /proc/<pid>/maps. If the
> permissions
> >> are set to r only or rx then I'll register the pages with the host. This
> >> wouldn't cover the process it self, but a major portion of
> >> the wasted memory.
> >>
> >> Sounds simple (everything does these days) and I plan to work it.
> However, I
> >> just need an expert to give me the go-ahead since this is all new to me.
> >
> > I think before undertaking such a massive effort, it pays off to
> > compare existing (Open-Source) technologies: Linux KSM vs. VBox
> > PageFusion.
> >
> > Why ?
> > KSM *avoids* the need of developing guest-side drivers altogether.
> > With KSM all mem dedup logic is done host-side-only, so all legacy and
> > future OSes work out-of-the-box.
> > VBox PageFusion requires GuestAdditions, which means developing and
> > testing (!) drivers for lots of guest OSes and OS versions.
> > Developing KSM-equivalent for VBox may pay off better than extending
> > VBox PageFusion to Linux guests (this will require writing new Linux
> > kernel drivers).
> > KSM itself is Linux-host-only, so cannot be used directly. (While VBox
> > PageFusion is Windows-guest-only ATM)
> > KSM-like system will require only host-side development and testing.
> >
> > What needs to be considered ? (KSM-like approach vs. VBox PageFusion
> approach)
> > 1. performance - how much CPU usage does it takes ?
> > 2. speed convergence [related to 1.] - how much time does it take to
> > find 1 GiB of RAM and dedup it ?
> > 3. efficiency - how many pages were actually shared ?
> > 4. any other advantages/disadvantages of both approaches.
> >
> > Disclaimer: I have NOT tested either solution. Just my 2 cents.
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