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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 6 Mar 2015, Ian Campbell wrote:
> 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).

In the libvirt case even if libvirt calls libxl_wait_for_memory_target
with just 1 second timeout, it is likely going to end up waiting longer
than that due to the change in behaviour of the function.

What I am saying is that even if we don't modify libvirt, we are going to
get a more reliable and more stable behaviour than what we have now.

That said, I agree that it would be good to improve the freemem loop in
libvirt in a similar way to what we are doing with xl.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel