Re: [Xen-devel] [PATCH 0/4] fix freemem loop

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2015-03-05 at 21:08 -0700, Mike Latimer wrote:
> On Thursday, March 05, 2015 05:49:35 PM Ian Campbell wrote:
> > On Tue, 2015-03-03 at 11:08 +0000, Stefano Stabellini wrote:
> > > Hi all,
> > > 
> > > this patch series fixes the freemem loop on machines with very large
> > > amount of memory, where the current wait time is not enough.
> > > 
> > > In order to be able to handle arbitrarly large amount of ram, we
> > > implement in libxl_wait_for_memory_target a policy of waiting until dom0
> > > is making progress.
> > 
> > What is the impact of the libxl API change made here on other callers,
> > in particular libvirt?

I should have CCd Jim when I asked the question. Now done.

> This change will have one interesting effect on libvirt. Currently, 
> libxlDomainFreeMem loops 3 times, then returns 0 (if no errors are 
> encountered). This means that domain creation starts before dom0 finishes 
> ballooning (unlike xl's previous behavior, which would fail).
> 
> With this change, domain creation through virsh will wait (in 
> libxl_wait_for_memory_target) until dom0 finishes ballooning. This should 
> result in an increase in the speed of the domain starting, as there will not 
> be memory contention between the two processes.
> 
> The libvirt side calls libxl_wait_for_memory_target with a 1 second timeout - 
> which doesn't leave a huge amount of room for slow memory allocation. This 
> timeout, as well as the logic in general, should be changed to match the new 
> xl behavior (IMO). I expect this to really only matter when dealing with 
> large 
> domains.

For libvirt we also need to consider what happens when a libvirt which
is modified along these lines uses an older libxl (I believe back to 4.2
is supported).

Ian.


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