Re: KSM with Debian GNU/Linux

[Daniel Bareiro <daniel-listas@gmx.net>]

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Hi, Michael.

On Monday, 09 August 2010 19:28:23 +0400,
Michael Tokarev wrote:

>> 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.

Thanks for the observation.

>> 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?

Yes, I forgot to mention that I had enabled it:

# cat /sys/kernel/mm/ksm/run
1

>> 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.

Keeping the kernel I had compiled and installing the qemu-kvm package in
Backports, now KSM is working:

# cat /sys/kernel/mm/ksm/pages_sharing
181406


This indicates that this was a problem in compiling qemu-kvm since the
kernel is still exactly the same. Now, according to what I was seeing,
headers of my kernel defines MADV_MERGEABLE:

# cat /usr/src/linux-2.6.32.3/debian/linux-headers-2.6.32.3-dgb/usr/src/linux-headers-2.6.32.3-dgb/include/asm-generic/mman-common.h | grep MERGEABLE
#define MADV_MERGEABLE   12             /* KSM may merge identical pages */
#define MADV_UNMERGEABLE 13             /* KSM may not merge identical pages */


Would it be that perhaps in my compilation of qemu-kvm, ksm was not
working because I would have to indicate the path
/usr/src/linux-2.6.32.3/debian/...?


Thanks for your reply.


Regards,
Daniel
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