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balloon: try harder to balloon up under memory pressure.

Currently if the balloon driver is unable to increase the guest's
reservation it assumes the failure was due to reaching its full
allocation, gives up on the ballooning operation and records the limit
it reached as the "hard limit". The driver will not try again until
the target is set again (even to the same value).

However it is possible that ballooning has in fact failed due to
memory pressure in the host and therefore it is desirable to keep
attempting to reach the target in case memory becomes available. The
most likely scenario is that some guests are ballooning down while
others are ballooning up and therefore there is temporary memory
pressure while things stabilise. You would not expect a well behaved
toolstack to ask a domain to balloon to more than its allocation nor
would you expect it to deliberately over-commit memory by setting
balloon targets which exceed the total host memory.

This patch drops the concept of a hard limit and causes the balloon
driver to retry increasing the reservation on a timer in the same
manner as when decreasing the reservation.

Also if we partially succeed in increasing the reservation
(i.e. receive less pages than we asked for) then we may as well keep
those pages rather than returning them to Xen.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Fri Jun 05 14:01:20 2009 +0100 (2009-06-05)
parents 831230e53067
line source
2 Linux I2O Support (c) Copyright 1999 Red Hat Software
3 and others.
5 This program is free software; you can redistribute it and/or
6 modify it under the terms of the GNU General Public License
7 as published by the Free Software Foundation; either version
8 2 of the License, or (at your option) any later version.
10 AUTHORS (so far)
12 Alan Cox, Building Number Three Ltd.
13 Core code, SCSI and Block OSMs
15 Steve Ralston, LSI Logic Corp.
16 Debugging SCSI and Block OSM
18 Deepak Saxena, Intel Corp.
19 Various core/block extensions
20 /proc interface, bug fixes
21 Ioctl interfaces for control
22 Debugging LAN OSM
24 Philip Rumpf
25 Fixed assorted dumb SMP locking bugs
27 Juha Sievanen, University of Helsinki Finland
28 LAN OSM code
29 /proc interface to LAN class
30 Bug fixes
31 Core code extensions
33 Auvo Häkkinen, University of Helsinki Finland
34 LAN OSM code
35 /Proc interface to LAN class
36 Bug fixes
37 Core code extensions
39 Taneli Vähäkangas, University of Helsinki Finland
40 Fixes to i2o_config
44 This work was made possible by
46 Red Hat Software
47 Funding for the Building #3 part of the project
49 Symbios Logic (Now LSI)
50 Host adapters, hints, known to work platforms when I hit
51 compatibility problems
53 BoxHill Corporation
54 Loan of initial FibreChannel disk array used for development work.
56 European Comission
57 Funding the work done by the University of Helsinki
59 SysKonnect
60 Loan of FDDI and Gigabit Ethernet cards
63 Loan of I2O motherboard