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SubjectRe: [PATCH 0/4] ksm - dynamic page sharing driver for linux v2
Nikola Ciprich wrote:
> Hi Izik,
> Is there some user documentation available? (apart from RTFS?:))
> I've compiled kernel with v2 of Your patches, loaded ksm module,
> did echo 1 > /proc/sys/kernel/mm/ksm/run, but I think it didn't do
> anything, at least no pages were collected..
> Could You advise me a bit?
> thanks a lot in advance...
> I can't wait to try it on our hosts runing 50-60 KVMs :)
> BR
> nik
You need the userspace / kvm patchs that i posted together with V1 about
1-2 weeks ago...
What you should do is this:
Patch Linus kernel git with the ksm patchs (V2) (like you just did)
This patchs can be found at:

Then patch Avi kernel git with the kvm patchs that were sent togather
with V1
Patchs can be found at:

and then Avi git userspace with this patchs:

Now, after you finish patching the kernel, load the kvm modules from avi
git, and then using patched userspace
you can start using ksm:

set up the speed: (just number, you can change them to make it take less
or more cpu)
echo 400 > /sys/kernel/mm/ksm/pages_to_scan
echo 10000 > /sys/kernel/mm/ksm/sleep

echo 1 > /sys/kernel/mm/ksm/run

Dont raise all the VMS at once, beacuse then KSM wont be able to catch
with the memory allocation...
Raise few VMS, see that their memory get shared and your host free
memory grow, then raise more VMS and so on...

(You can check pages_shared for the number of pages that have been
shared, you can run top as well)
> On Sat, Apr 04, 2009 at 05:35:18PM +0300, Izik Eidus wrote:
>> From v1 to v2:
>> 1)Fixed security issue found by Chris Wright:
>> Ksm was checking if page is a shared page by running !PageAnon.
>> Beacuse that Ksm scan only anonymous memory, all !PageAnons
>> inside ksm data strctures are shared page, however there might
>> be a case for do_wp_page() when the VM_SHARED is used where
>> do_wp_page() would instead of copying the page into new anonymos
>> page, would reuse the page, it was fixed by adding check for the
>> dirty_bit of the virtual addresses pointing into the shared page.
>> I was not finding any VM code tha would clear the dirty bit from
>> this virtual address (due to the fact that we allocate the page
>> using page_alloc() - kernel allocated pages), ~but i still want
>> confirmation about this from the vm guys - thanks.~
>> 2)Moved to sysfs to control ksm:
>> It was requested as a better way to control the ksm scanning
>> thread than ioctls.
>> the sysfs api:
>> dir: /sys/kernel/mm/ksm/
>> kernel_pages_allocated - information about how many kernel pages
>> ksm have allocated, this pages are not swappable, and each page
>> like that is used by ksm to share pages with identical content
>> pages_shared - how many pages were shared by ksm
>> run - set to 1 when you want ksm to run, 0 when no
>> max_kernel_pages - set the maximum amount of kernel pages
>> to be allocated by ksm, set 0 for unlimited.
>> pages_to_scan - how many pages to scan before ksm will sleep
>> sleep - how much usecs ksm will sleep.
>> 3)Add sysfs paramater to control the maximum kernel pages to be by
>> ksm.
>> 4)Add statistics about how much pages are really shared.
>> One issue still to be discussed:
>> There was a suggestion to use madvice(SHAREABLE) instead of using
>> ioctls to register memory that need to be scanned by ksm.
>> Such change is outside the area of ksm.c and would required adding
>> new madvice api, and change some parts of the vm and the kernel
>> code, so first thing to do, is realized if we really want this.
>> I dont know any other open issues.
>> Thanks.
>> This is from the first post:
>> (The kvm part, togather with the kvm-userspace part, was post with V1
>> before about a week, whoever want to test ksm may download the
>> patch from lkml archive)
>> KSM is a linux driver that allows dynamicly sharing identical memory
>> pages between one or more processes.
>> Unlike tradtional page sharing that is made at the allocation of the
>> memory, ksm do it dynamicly after the memory was created.
>> Memory is periodically scanned; identical pages are identified and
>> merged.
>> The sharing is unnoticeable by the process that use this memory.
>> (the shared pages are marked as readonly, and in case of write
>> do_wp_page() take care to create new copy of the page)
>> To find identical pages ksm use algorithm that is split into three
>> primery levels:
>> 1) Ksm will start scan the memory and will calculate checksum for each
>> page that is registred to be scanned.
>> (In the first round of the scanning, ksm would only calculate
>> this checksum for all the pages)
>> 2) Ksm will go again on the whole memory and will recalculate the
>> checmsum of the pages, pages that are found to have the same
>> checksum value, would be considered "pages that are most likely
>> wont changed"
>> Ksm will insert this pages into sorted by page content RB-tree that
>> is called "unstable tree", the reason that this tree is called
>> unstable is due to the fact that the page contents might changed
>> while they are still inside the tree, and therefore the tree would
>> become corrupted.
>> Due to this problem ksm take two more steps in addition to the
>> checksum calculation:
>> a) Ksm will throw and recreate the entire unstable tree each round
>> of memory scanning - so if we have corruption, it will be fixed
>> when we will rebuild the tree.
>> b) Ksm is using RB-tree, that its balancing is made by the node color
>> and not by the content, so even if the page get corrupted, it still
>> would take the same amount of time to search on it.
>> 3) In addition to the unstable tree, ksm hold another tree that is called
>> "stable tree" - this tree is RB-tree that is sorted by the pages
>> content and all its pages are write protected, and therefore it cant get
>> corrupted.
>> Each time ksm will find two identcial pages using the unstable tree,
>> it will create new write-protected shared page, and this page will be
>> inserted into the stable tree, and would be saved there, the
>> stable tree, unlike the unstable tree, is never throwen away, so each
>> page that we find would be saved inside it.
>> Taking into account the three levels that described above, the algorithm
>> work like that:
>> search primary tree (sorted by entire page contents, pages write protected)
>> - if match found, merge
>> - if no match found...
>> - search secondary tree (sorted by entire page contents, pages not write
>> protected)
>> - if match found, merge
>> - remove from secondary tree and insert merged page into primary tree
>> - if no match found...
>> - checksum
>> - if checksum hasn't changed
>> - insert into secondary tree
>> - if it has, store updated checksum (note: first time this page
>> is handled it won't have a checksum, so checksum will appear
>> as "changed", so it takes two passes w/ no other matches to
>> get into secondary tree)
>> - do not insert into any tree, will see it again on next pass
>> The basic idea of this algorithm, is that even if the unstable tree doesnt
>> promise to us to find two identical pages in the first round, we would
>> probably find them in the second or the third or the tenth round,
>> then after we have found this two identical pages only once, we will insert
>> them into the stable tree, and then they would be protected there forever.
>> So the all idea of the unstable tree, is just to build the stable tree and
>> then we will find the identical pages using it.
>> The current implemantion can be improved alot:
>> we dont have to calculate exspensive checksum, we can just use the host
>> dirty bit.
>> currently we dont support shared pages swapping (other pages that are not
>> shared can be swapped (all the pages that we didnt find to be identical
>> to other pages...).
>> Walking on the tree, we keep call to get_user_pages(), we can optimized it
>> by saving the pfn, and using mmu notifiers to know when the virtual address
>> mapping was changed.
>> We currently scan just programs that were registred to be used by ksm, we
>> would later want to add the abilaty to tell ksm to scan PIDS (so you can
>> scan closed binary applications as well).
>> Right now ksm scanning is made by just one thread, multiple scanners
>> support might would be needed.
>> This driver is very useful for KVM as in cases of runing multiple guests
>> operation system of the same type.
>> (For desktop work loads we have achived more than x2 memory overcommit
>> (more like x3))
>> This driver have found users other than KVM, for example CERN,
>> Fons Rademakers:
>> "on many-core machines we run one large detector simulation program per core.
>> These simulation programs are identical but run each in their own process and
>> need about 2 - 2.5 GB RAM.
>> We typically buy machines with 2GB RAM per core and so have a problem to run
>> one of these programs per core.
>> Of the 2 - 2.5 GB about 700MB is identical data in the form of magnetic field
>> maps, detector geometry, etc.
>> Currently people have been trying to start one program, initialize the geometry
>> and field maps and then fork it N times, to have the data shared.
>> With KSM this would be done automatically by the system so it sounded extremely
>> attractive when Andrea presented it."
>> I am sending another seires of patchs for kvm kernel and kvm-userspace
>> that would allow users of kvm to test ksm with it.
>> The kvm patchs would apply to Avi git tree.
>> Izik Eidus (4):
>> MMU_NOTIFIERS: add set_pte_at_notify()
>> add page_wrprotect(): write protecting page.
>> add replace_page(): change the page pte is pointing to.
>> add ksm kernel shared memory driver.
>> include/linux/ksm.h | 48 ++
>> include/linux/miscdevice.h | 1 +
>> include/linux/mm.h | 5 +
>> include/linux/mmu_notifier.h | 34 +
>> include/linux/rmap.h | 11 +
>> mm/Kconfig | 6 +
>> mm/Makefile | 1 +
>> mm/ksm.c | 1668 ++++++++++++++++++++++++++++++++++++++++++
>> mm/memory.c | 90 +++-
>> mm/mmu_notifier.c | 20 +
>> mm/rmap.c | 139 ++++
>> 11 files changed, 2021 insertions(+), 2 deletions(-)
>> create mode 100644 include/linux/ksm.h
>> create mode 100644 mm/ksm.c
>> --
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