[Xen-devel] Re: Tmem vs order>0 allocation, workaround RFC

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 12/02/2010 17:24, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> I just had an idea for a workaround that might be low enough
> impact to get in for 4.0 and allow tmem to be enabled by
> default.  I think it will not eliminate the fragmentation
> problem entirely, but would greatly reduce the probability
> of it causing problems for domain creation/migration when tmem
> is enabled, and possibly for the other memory utilization
> features as well.
> 
> Simply, avail_heap_pages would fail if total_avail_pages
> is less than 1%(?) of the total memory on the system AND
> the request is order==0.  Essentially, this is reserving
> a "zone" for order>0 allocations.

I don't see how that necessarily works. Pages can be allocated in order>0
chunks and freed order==0, so even that last 1% can get fragmented. For
example, guests get their memory allocated in 2MB chunks where possible; but
their balloon drivers may then free arbitrary 4kB pages within those chunks.

 -- Keir



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