Re: KSM with Debian GNU/Linux

[Michael Tokarev <mjt@tls.msk.ru>]

09.08.2010 18:27, Daniel Bareiro wrote:
> On Monday, 09 August 2010 14:27:12 +0400,
> Michael Tokarev wrote:
>
>>> I have understood that in addition to support from the kernel and
>>> qemu-kvm, there must be support in libc. So in testing I was doing on
>>> Lenny, it did not work using libc from Debian repositories. Now that
>>> squeeze is frozen, anyone knows if it will support KSM?
>
>> KSM is working in qemu-kvm packages in bpo, for quite long
>> time already.  It has nothing to do with libc, and now it
>> does not even require recent-enough kernel headers to compile.
>> What's needed is recent-enough kernel that actually implements
>> the feature.  The kernel in Lenny does not KSM support, but
>> the one in bpo has.  Also note that Lenny's kernel is too old
>> for kvm.
>
> I'm using Linux 2.6.32.3 compiled by myself with the kernel.org source
> code with support for KSM:
>
> # cat /boot/config-2.6.32.3-dgb | grep KSM
> CONFIG_KSM=y
>
> I'm also using qemu-kvm 0.12.1.2 compiled by myself with the source code
> of SourceForge.

Note that this one has numerous bugs, some of which involves
data corruption.  JFYI.

> These are the checks that I was doing in the VMHost:
>
> # ll /sys/kernel/mm/ksm/
> total 0
> -r--r--r-- 1 root root 4096 ago  9 06:28 full_scans
> -rw-r--r-- 1 root root 4096 ago  9 06:28 max_kernel_pages
> -r--r--r-- 1 root root 4096 ago  9 06:28 pages_shared
> -r--r--r-- 1 root root 4096 ago  9 06:28 pages_sharing
> -rw-r--r-- 1 root root 4096 ago  9 06:28 pages_to_scan
> -r--r--r-- 1 root root 4096 ago  9 06:28 pages_unshared
> -r--r--r-- 1 root root 4096 ago  9 06:28 pages_volatile
> -rw-r--r-- 1 root root 4096 ago  9 06:27 run
> -rw-r--r-- 1 root root 4096 ago  9 06:28 sleep_millisecs
>
>
> # cat /sys/kernel/mm/ksm/max_kernel_pages
> 253500
>
> But KSM seems not work:
>
> # cat /sys/kernel/mm/ksm/pages_sharing
> 0

Did you actually enable it in the kernel?  Did you read
the ksm docs shipped with the kernel?

> Should I have some additional consideration when compiling qemu-kvm?

Please note what Nikola Ciprich said.  If you don't have
kernel headers that defines MADV_MERGEABLE, you'll have
to define it when compiling kvm.

Note again that debian qemu-kvm package (you're asking
about debian after all) includes support for KSM.  It
also includes a small patch to #define that symbol.

/mjt