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

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Sat, 2006-06-24 at 16:31 -0400, Dan E Poff wrote:
> 
> 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. 

Again, I see no reason why an ordinary domain would need more than one
chunk.

> 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. 

I'm not sure what you mean by this. Fragmentation will exist for all
chunks used by the heap allocator, for example. If you're planning to
allow the smaller allocators access to only a single chunk, then that
would prevent fragmentation elsewhere but would limit hypervisor
operations.

I would be interested to see allocator usage statistics from a
fully-running system (especially with stressed virtual IO devices). If
it's a relatively fixed amount of memory then I'm OK with restricting
the page/heap allocators to a fixed number of chunks. How to determine
that number is another story though...

> 
> 
> 
> 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
> 
> 
-- 
Hollis Blanchard
IBM Linux Technology Center


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