Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux
From: Izik Eidus
Date: Tue Nov 11 2008 - 15:09:02 EST
Izik Eidus wrote:
Andrew Morton wrote:
On Tue, 11 Nov 2008 21:18:23 +0200
Izik Eidus <ieidus@xxxxxxxxxx> 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.
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