BALLOON(4) | Device Drivers Manual (xen) | BALLOON(4) |
balloon
—
balloon* at xenbus?
balloon
driver supports the memory ballooning
operations offered in Xen environments. It allows shrinking or extending a
domain's available memory by passing pages between different domains. At any
time, the total memory available to a domain is called the ``reservation''.
Pages are moved via the use of the balloon, a reserved quantity of memory available to all domains that can be freely deflated (or inflated) at a domain's will. Deflating balloon means that pages are moved out from it, and bound to domain's virtual memory. Respectively, inflating balloon indicates that pages are moved out of domain's memory and pushed inside balloon. This is similar to a dynamic allocation of wired physical memory, except that the pages are not available to domain anymore.
Any domain is free to request memory from
balloon
up to the maximum value set by the host's
administrator through the mem-max
command of
xm(1). Alternatively, the
host's administrator is free to request to a particular domain to give some
memory back. This command requires the targeted domain's cooperation and
requires balloon
support within it. This can be done
through the mem-set
command of
xm(1). Alternatively, one can
control the ballooning directly by writing under the
“memory/target” node inside Xenstore. This entry controls the
target memory reservation of a given domain, indicated in kilobytes
(KiB).
An interface to control balloon
is also
available through sysctl(8)
under “machdep.xen.balloon” (all values being in
kilobytes):
balloon
driver. Any request that would require
domain to reduce its reservation below this threshold will be refused by
the driver. This can be used by a domain's administrator to control the
number of memory pages that will be kept available to domain.mem-max
command of
xm(1).balloon
failed to reach the target reservation.
This is typically due to a target set too low; the kernel prevented memory
exhaustion by refusing further allocation.balloon
will limit
reservation up to the maximum value it can handle.EPERM
]Carl A. Waldspurger, Memory Resource Management in VMware ESX Server, Proceedings of the 5th Symposium on Operating Systems Design and Implementation, USENIX Association, http://www.usenix.org/events/osdi02/tech/full_papers/waldspurger/waldspurger.pdf, December 9-11, 2002.
balloon
driver first appeared in
NetBSD 6.0.
balloon
driver was written by
Cherry G. Mathew
<cherry@NetBSD.org>
and Jean-Yves Migeon
<jym@NetBSD.org>.
balloon
can be empty and cannot be
collapsed further, domain may not have enough free memory pages (due to memory
fragmentation, memory exhaustion, ...) so it cannot give enough back to
balloon
.
Currently, the virtual memory sub-system of NetBSD is not capable of ``hot-plugging'' new memory pages into place. This means that increasing a domain's memory reservation above its initial maximum value is pointless, as new memory pages cannot be consumed by the memory management sub-system.
Over expanding balloon
generates high
kernel memory pressure. While the driver tries to stay as conservative as
possible to avoid crashes, a very low memory reservation will lead to
unwanted swap or even panic
().
balloon
but this
may not be the case for other operating systems.
July 30, 2011 | NetBSD 9.4 |