Re: KVM architecture docs

Avi Kivity
2008-03-04T15:08:49Z

Javier Guerra wrote:
> On 3/4/08, Avi Kivity <avi@qumranet.com> wrote:
>
>> apply to kvm guests. With mmu notifiers, the trend will grow even stronger.
>>
>
> could you (or anybody) elaborate on that? the mmu-related threads show
> lots of progress, but it's way (way) out of my league.
>
> AFAICT, it's about the infrastructure to later write drivers (virtio?)
> to DMA-heavy hardware (IB, RDMA, etc). am i wrong? or is it
> something more complete (like a ready to use driver)?
>
>

mmu notifiers provide a way for the core Linux memory management code to
propagate changes in how Linux views a process' memory map to external
memory management units that are also interested in that memory map.
These changes include things like swapping, page migration, changes to
memory protection, defragmentation, and copy-on-write. In this context,
kvm appears as a dma capable memory controller, like RDMA NICs or GPUs.

For kvm, this is important as it allows all those features to be used
transparently with guests.

- swapping allows you to overcommit memory
- page migration allows optimization of memory placement within the host
in response to changing workloads
- defragmentation will allow (if/when it is merged into Linux) more
widespread use of large pages, which improve performance
- copy-on-write allows sharing identical pages of memory among guests,
increasing guest density

--
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