[Xen-devel] domain creation vs querying free memory (xend and xl)

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

I was asked a question that seems like it should be obvious
but it doesn't seem to be, at least in xm-land.  I'll look
into it further, as well as for xl, but I thought I'd ask
first to see if there is a known answer or if this is a known
problem:

Suppose that xm/xl create is issued on a large-memory
domain (PV or HVM or, future, PVH).  It takes awhile
for this domain to launch and during at least part of this
time, the toolset hasn't yet requested all of the
required memory from the hypervisor to complete the
launch of the domain...  or perhaps the toolset has,
but the hypervisor is slow about calling the long sequence
of page allocations (e.g. maybe because it is zeroing
each page?).

Then it is desired to launch a second large-memory domain.
The tools can query Xen to see if there is sufficient RAM
and there is, because the first launch has not yet
allocated all the RAM assigned to it.

But the second domain launch fails, possibly after
several minutes because, actually, there isn't enough
physical RAM for both.

Does this make sense?  Should the tools "reserve"
maxmem as a "transaction" and/or ensure that "xm/xl
free" calls account for the entire requested amount
of RAM?  Or maybe xl _does_ work this way?

Thanks for any comments or discussion!

Dan

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