.\" $NetBSD: balloon.4,v 1.11 2014/03/18 18:20:40 riastradh Exp $ .\" .\" Copyright (c) 2011 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" This code is derived from software contributed to The NetBSD Foundation .\" by Jean-Yves Migeon . .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED .\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS .\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS .\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN .\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE .\" POSSIBILITY OF SUCH DAMAGE. .\" .Dd July 30, 2011 .Dt BALLOON 4 xen .Os .Sh NAME .Nm balloon .Nd Xen memory balloon driver .Sh SYNOPSIS .Cd "balloon* at xenbus?" .Sh DESCRIPTION The .Nm driver supports the memory ballooning operations offered in .Tn 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''. .Pp 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. .Pp Any domain is free to request memory from .Nm up to the maximum value set by the host's administrator through the .Ic mem-max command of .Xr 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 .Nm balloon support within it. This can be done through the .Ic mem-set command of .Xr xm 1 . Alternatively, one can control the ballooning directly by writing under the .Dq memory/target node inside Xenstore. This entry controls the target memory reservation of a given domain, indicated in kilobytes (KiB). .Pp An interface to control .Nm is also available through .Xr sysctl 8 under .Dq machdep.xen.balloon (all values being in kilobytes): .Bl -tag -width xxxxxxx .It current (read-only) The current memory reservation of the domain. .It min (read-write) The minimum reservation value acceptable by the domain's .Nm 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. .It max (read-only) The maximum reservation accessible to a domain. Its value can only be changed by the dom0's administrator, through the .Ic mem-max command of .Xr xm 1 . .It target (read-write) The target reservation of the domain. This entry serves the same purpose as the .Dq 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. .El .Sh DIAGNOSTICS .Bl -diag .It "WARNING: balloon could not reach target %zu (current %zu)" .Nm 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. .It "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. .It "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, .Nm will limit reservation up to the maximum value it can handle. .El .Sh ERRORS When setting the minimum threshold or target reservation entries through .Dq machdep.xen.balloon , the following errors can be returned: .Bl -tag -width Er .It Bq Er EPERM The value passed is beyond limits. The new value is either too low .Po Dq min is below driver's safeguard value, or .Dq target is below minimum value .Pc , or too high .Po Dq target is above maximum value .Pc . .El .Sh SEE ALSO .Xr xm 1 , .Xr xenbus 4 , .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 HISTORY The .Nm driver first appeared in .Nx 6.0 . .Sh AUTHORS .An -nosplit The .Nm driver was written by .An Cherry G. Mathew Aq Mt cherry@NetBSD.org and .An Jean-Yves Migeon Aq Mt jym@NetBSD.org . .Sh BUGS There are a number of reasons why a domain may not attain the targeted memory reservation: .Nm 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 .Nm . .Pp Currently, the virtual memory sub-system of .Nx 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. .Pp Over expanding .Nm 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 .Fn panic . .Sh 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 .Nx kernel zeroes all pages before relinquishing them to .Nm but this may not be the case for other operating systems.