.\"	$NetBSD: balloon.4,v 1.4 2011/01/14 10:01:03 jruoho Exp $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Cherry G. Mathew <cherry@zyx.in> and
.\" Jean-Yves Migeon <jym@NetBSD.org>
.\"
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.Dd January 14, 2011
.Dt BALLOON 4 xen
.Os
.Sh NAME
.Nm balloon
.Nd Xen hypervisor dynamic memory resize helper
.Sh SYNOPSIS
.Cd "options XEN_BALLOON"
.Sh DESCRIPTION
The Xen
.Nm
driver can be enabled to allow the Xen hypervisor to dynamically
resize the amount of RAM a
.Nx
VM is allowed to use.
This allows the hypervisor to manage RAM more
efficiently, similar in spirit to how the
.Nx
kernel would manage RAM wrt processes, with
.Xr uvm 9 .
.Pp
The
.Nm
driver negotiates domain VM memory allocation change requests from
the Xen hypervisor, by reserving "pinned" memory for itself from the
.Nx
.Xr uvm 9
subsystem.
.Pp
There are two methods to notify the
.Nm
driver of memory allocation change requests.
.Bl -tag -width "Xr sysctl 3" -offset indent
.It Xr xenbus 4
In the xenbus method, the change notification originates outside the
VM, presumable from the Hypervisor, and is communicated over the
xenbus interface.
.It Xr sysctl 3
In the
.Xr sysctl 3
method, the change notification originates from within the VM.
Two
.Xr sysctl 7
nodes are provided:
.Bl -ohang -width kern.xen.balloon.current
.It Va kern.xen.balloon.current
This node is read-only and returns the number of memory pages
currently allocated to the VM.
.It Va kern.xen.balloon.target
This node can be written to, and sets a new target memory reservation,
in pages.
Note that targets set this way from within the VM do not
reflect in the hypervisor.
This is therefore a less preferred way to use the
.Nm
.El
.El
.Pp
In either case, once a new allocation target is set, the balloon
driver thread asynchronously attempts to reach this target in discrete
steps.
Every step, if it fails, is tried more than once.
When the target is reached, the thread goes back to sleep, waiting
for a new incoming target.
If it is not reached, the target is updated to
reflect the value after the last successful step.
.Sh EXAMPLES
.Bd -literal
   # # Have a look at the current reservations.
   # sysctl -a|grep balloon
   kern.xen.balloon.current = 32512
   kern.xen.balloon.target = 32512
   #
   # # Change the current target to 30000 kernel pages.
   # sysctl -w kern.xen.balloon.target=30000
   kern.xen.balloon.target: 32512 -\*[Gt] 30000
   #
.Ed
.Sh SEE ALSO
.Xr uvm 9
.Rs
.%A Carl A. Waldspurger
.%T "Memory Resource Management in VMware ESX Server"
.%I USENIX Association
.%B Proceedings of the 5th Symposium on \
Operating Systems Design and Implementation
.%D December 9-11, 2002
.%U http://www.usenix.org/events/osdi02/tech/full_papers/waldspurger/waldspurger.pdf
.Re
.\"
.Sh AUTHORS
.An Cherry G. Mathew Aq cherry@zyx.in
.An Jean-Yves Migeon Aq jym@NetBSD.org
.Sh CAVEATS
VMs can be inadvertently
.No crashed/ Ns Fn panic Ns ed
by over expanding the
balloon allocation, creating extreme OS memory pressure.
.Sh BUGS
.Fn xenmem_get_maxreservation
is currently hard-coded to return nkmempages.
This driver is still regarded as EXPERIMENTAL.