BALLOON(4)

NAME
balloon - Xen memory balloon driver

SYNOPSIS
balloon* at xenbus?

DESCRIPTION
The 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):

current
    (read-only) The current memory reservation of the domain.

min (read-write) The minimum reservation value acceptable by the domain's 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.

max (read-only) The maximum reservation accessible to a domain. Its value can only be changed by the dom0's administrator, through the mem-max command of xm(1).

target
    (read-write) The target reservation of the domain. This entry serves the same purpose as the “memory/target” entry in Xenstore. This controls the targeted number of pages that the domain should have. Note that this is only a target, and may not be achieved for a variety of reasons.

DIAGNOSTICS

WARNING: balloon could not reach target %zu (current %zu) 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.

increase reservation incomplete: was %zu, returned %d The hypervisor only gave a partial set of memory pages to domain. This happens when host's memory consumption is high, and hypervisor is unable to give enough free pages back to domain.

memory 'hot-plug' unsupported - clipping reservation %zu => %zu pages. An attempt was made by domain to get more memory than initially obtained during boot. As physical memory pages cannot be added to memory management sub-system dynamically, balloon will limit reservation up to the maximum value it can handle.

ERRORS
When setting the minimum threshold or target reservation entries through “machdep.xen.balloon”, the following errors can be returned:

[EPERM]
    The value passed is beyond limits. The new value is either too low (“min” is below driver's safeguard value, or “target” is below minimum value), or too high (“target” is above maximum value).

SEE ALSO
xm(1), xenbus(4), uvm(9) Carl A. Waldspurger, Memory Resource Management in VMware ESX Server, USENIX Association, Proceedings of the 5th Symposium on Operating Systems Design and Implementation, December 9-11, 2002, http://www.usenix.org/events/osdi02/tech/full_papers/waldspurger/waldspurger.pdf.

HISTORY
The balloon driver first appeared in NetBSD 6.0.

AUTHORS
-nosplit The balloon driver was written by Cherry G. Mathew <cherry@NetBSD.org> and Jean-Yves Migeon <jym@NetBSD.org>.

BUGS
There are a number of reasons why a domain may not attain the targeted memory reservation: 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().

SECURITY CONSIDERATIONS
Ballooning involves moving pages between different domains. This includes their content, which can lead to information leak. If you are running domains of different sensitivities on the same host, consider disabling the use of ballooning altogether. The NetBSD kernel zeroes all pages before relinquishing them to balloon but this may not be the case for other operating systems.