Re: [Xen-devel] Seamlessly sharing identical memory pages among domains

[Steven Hand <Steven.Hand@xxxxxxxxxxxx>]

> >> I'm not convinced there's that much sharing to be had between
> >> VMs
> 
> In the paper Carl Waldspurger reports some worthwhile sharing under
> certain real world workloads:
> 
>            | Total| Shared   |Reclaimed   |
> Guest Types|  MB  | MB |  %  | MB  |   %  |
> -----------+------+----+-----+-----+------+
> A)10 WinNT | 2048 | 880| 42.9| 673 | 32.9 |
> B) 9 Linux | 1846 | 539| 29.2| 345 | 18.7 |
> C) 5 Linux | 1658 | 165| 10.0| 120 | 7.2  |

Yes; and we may do some shared text support at some point. Not clear
how much additional benefit comes from the "scanning" they did; a 
simpler approach to detect read-only sharing is to explicitly check 
for resident versions of a certain text segment across VMs. Having
some "look-aside" in the block device interface (partic if also 
doing sharing at that level as per our current CoW implementation) 
might get considerable benefit for little cost. 


> >> Doing a proper shared cache is slightly trickier given the
> >> paravirtualised memory interface -- we'd have to introduce guests 
> >>to a new kind of write fault
> 
> Wouldn't it be possible for XEN to share the pages tranparently, without
> the guest OSes needing to know it?

Possibly. One challenge here is to maintain partitioning (hard guarantees) 
while providing sharing. Depending on how important isolation is to you, 
this is relatively less or more difficult. 

Something we'll be considering post OSDI anyway..


cheers,

S.

 


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