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Open Source and information security mailing list archives
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Date: Tue, 11 Nov 2008 22:08:15 +0200 From: Izik Eidus <ieidus@...hat.com> To: Andrew Morton <akpm@...ux-foundation.org> CC: avi@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org, aarcange@...hat.com, chrisw@...hat.com Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux Izik Eidus wrote: > Andrew Morton wrote: >> On Tue, 11 Nov 2008 21:18:23 +0200 >> Izik Eidus <ieidus@...hat.com> wrote: >> >> >>>> hm. >>>> >>>> There has been the occasional discussion about idenfifying all-zeroes >>>> pages and scavenging them, repointing them at the zero page. Could >>>> this infrastructure be used for that? (And how much would we gain >>>> from >>>> it?) >>>> >>>> [I'm looking for reasons why this is more than a >>>> muck-up-the-vm-for-kvm >>>> thing here ;) ] >>>> >> >> ^^ this? >> >> >>> KSM is separate driver , it doesn't change anything in the VM but >>> adding two helper functions. >>> >> >> What, you mean I should actually read the code? Oh well, OK. >> > Andrea i think what is happening here is my fault Sorry, meant to write here Andrew :-) > i will try to give here much more information about KSM: > first the bad things: > KSM shared pages are right now (we have patch that can change it but > we want to wait with it) unswappable > this mean that the entire memory of the guest is swappable but the > pages that are shared are not. > (when the pages are splited back by COW they become anonymous again > with the help of do_wp_page() > the reason that the pages are not swappable is beacuse the way the > Linux Rmap is working, this not allow us to create nonlinear anonymous > pages > (we dont want to use nonlinear vma for kvm, as it will make swapping > for kvm very slow) > the reason that ksm pages need to have nonlinear reverse mapping is > that for one guest identical page can be found in whole diffrent > offset than other guest have it > (this is from the userspace VM point of view) > > the rest is quite simple: > it is walking over the entire guest memory (or only some of it) and > scan for identical pages using hash table > it merge the pages into one single write protected page > > numbers for ksm is something that i have just for desktops and just > the numbers i gave you > what is do know is: > big overcommit like 300% is possible just when you take into account > that some of the guest memory will be free > we are sharing mostly the DLLs/ KERNEL / ZERO pages, for the DLLS and > KERNEL PAGEs this pages likely will never break > but ZERO pages will be break when windows will allocate them and will > come back when windows will free the memory. > (i wouldnt suggest 300% overcommit for servers workload, beacuse you > can end up swapping in that case, > but for desktops after runing in production and passed some seiroes qa > tress tests it seems like 300% is a real number that can be use) > > i just ran test on two fedora 8 guests and got that results (using > GNOME in both of them) > 9959 root 15 0 730m 537m 281m S 8 3.4 0:44.28 > kvm > > 9956 root 15 0 730m 537m 246m S 4 3.4 0:41.43 kvm > as you can see the physical sharing was 281mb and 246mb (kernel pages > are counted as shared) > there is small lie in this numbers beacuse pages that was shared > across two guests and was splited by writing from guest number 1 will > still have 1 refernce count to it > and will still be kernel page (untill the other guest (num 2) will > write to it as well) > > > anyway i am willing to make much better testing or everything that > needed for this patchs to be merged. > (just tell me what and i will do it) > > beside that you should know that patch 4 is not a must, it is just > nice optimization... > > thanks. > > -- > To unsubscribe from this list: send the line "unsubscribe > linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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