Jeremy Fitzhardinge wrote:
> Rik van Riel wrote:
>
>> Jeremy Fitzhardinge wrote:
>>
>>> Rik van Riel wrote:
>>>
>>>> Jeremy Fitzhardinge wrote:
>>>>
>>>>
>>>>> That said, people have been looking at tracking block IO to work
>>>>> out when it might be useful to try and share pages between guests
>>>>> under Xen.
>>>>>
>>>> Tracking block IO seems like a bass-ackwards way to figure
>>>> out what the contents of a memory page are.
>>>>
>>> Well, they're research projects, so nobody said that they're
>>> necessarily useful results ;). I think the rationale is that, in
>>> general, there aren't all that many sharable pages, and asize from
>>> zero-pages, the bulk of them are the result of IO.
>>>
>> I'll give you a hint: Windows zeroes out freed pages.
>>
>
> Right: "aside from zero-pages". If you exclude zero-pages from your
> count of shared pages, the amount of sharing drops a lot.
>
>
>> It should also be possible to hook up arch_free_page() so
>> freed pages in Linux guests become sharable.
>>
>> Furthermore, every guest with the same OS version will be
>> running the same system daemons, same glibc, etc. This
>> means sharable pages from not just disk IO (probably from
>> different disks anyway),
>>
>
> Why? If you're starting a bunch of cookie-cutter guests, then you're
> probably starting them from the same template image or COW block
> devices. (Also, if you're wearing the cost of physical IO anyway, then
> additional cost of hashing is relatively small.)
>
>
>> but also in the BSS and possibly
>> even on the heap.
>>
>
> Well, maybe. Modern systems generally randomize memory layouts, so even
> if they're semantically the same, the pointers will all have different
> values.
>
> Other research into "sharing" mostly-similar pages is more promising for
> that kind of case.
>
>
>> Eventually. It starts out with hashing the first 128 (IIRC)
>> bytes of page content and comparing the hashes. If that
>> matches, it will do content comparison.
>>
The algorithm was changed quite a bit. Izik is planning to resubmit it
any day now.
>> Content comparison is done in the background on the host.
>> I suspect (but have not checked) that it is somehow hooked
>> up to the page reclaim code on the host.
>>
>
> Yeah, that's the straightforward approach; there's about a research
> project/year doing a Xen implementation, but they never seem to get very
> good results aside from very artificial test conditions.
>
Actually we got really good results by using ksm along with kvm, running
large
amount of windows virtual machines. We can achieve over commit ratio
of up to 400% of the host ram for VMs doing M$ office load.
-dor
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