Re: [XenPPC] [RFC] 64mb Chunk Allocator

[Dan E Poff <poff@xxxxxxxxxx>]

On Thu, 2006-06-22 at 16:56 -0400, Michal Ostrowski
wrote:
>> How
is this expected to be integrated into existing memory allocators in Xen?


As I understand Jimi's approach, all
of physical memory would be divided into 64mb chunks. The 1st chunk,
starting at address 0x00000000, belongs
to the hypervisor. When a domain is started, at least 1 chunk is allocated,
with RMOR set to provide real address
0x00000000. A domain whose maxmem is 128mb would have 2 chunks and so on.
When the domain is destroyed, its chunk(s)
are freed.

While this approach seems straight-forward,
memory utilization could be better, especially considering chunk size.
On other hand, allowing multiple domains
access to a chunk leads to fragmentation issues raised by Hollis.

hollisb@xxxxxxxxxxxxxxxxxxxxxxx

06/23/2006 11:22 AM

To	Michal Ostrowski <mostrows@xxxxxxxxxxxxxx>
cc	Dan E Poff/Watson/IBM@IBMUS, xen-ppc-devel@xxxxxxxxxxxxxxxxxxx
Subject	Re: [XenPPC] [RFC] 64mb Chunk Allocator

On Thu, 2006-06-22 at 16:56 -0400, Michal Ostrowski
wrote:
> How is this expected to be integrated into existing memory allocators
in
> Xen?  Would the other allocators always allocate their own chunks
to
> work with and divide them up into smaller pieces or would other
> allocators compete with the chunk allocator?

Yeah, I think this is the key question.

Once you've given a chunk to the heap allocator, I don't think you can
ever expect it to be defragmented and given back. I worry about the
classic fragmentation problem:
"Create domain." -> -ENOMEM
"How much memory is free?" -> lots and lots

> Whereas the allocation strategies chosen by Linux encourage
> fragmentation and make memory hot-unplug difficult, this could be
an
> opportunity to introduce a generic large-granularity memory-management
> layer that makes it easier to deal with hot-plug and large pages.

That's an interesting point.

-- 
Hollis Blanchard
IBM Linux Technology Center


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