ia64/linux-2.6.18-xen.hg

view Documentation/smart-config.txt @ 897:329ea0ccb344

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
children
line source
1 Smart CONFIG_* Dependencies
2 1 August 1999
4 Michael Chastain <mec@shout.net>
5 Werner Almesberger <almesber@lrc.di.epfl.ch>
6 Martin von Loewis <martin@mira.isdn.cs.tu-berlin.de>
8 Here is the problem:
10 Suppose that drivers/net/foo.c has the following lines:
12 #include <linux/config.h>
14 ...
16 #ifdef CONFIG_FOO_AUTOFROB
17 /* Code for auto-frobbing */
18 #else
19 /* Manual frobbing only */
20 #endif
22 ...
24 #ifdef CONFIG_FOO_MODEL_TWO
25 /* Code for model two */
26 #endif
28 Now suppose the user (the person building kernels) reconfigures the
29 kernel to change some unrelated setting. This will regenerate the
30 file include/linux/autoconf.h, which will cause include/linux/config.h
31 to be out of date, which will cause drivers/net/foo.c to be recompiled.
33 Most kernel sources, perhaps 80% of them, have at least one CONFIG_*
34 dependency somewhere. So changing _any_ CONFIG_* setting requires
35 almost _all_ of the kernel to be recompiled.
37 Here is the solution:
39 We've made the dependency generator, mkdep.c, smarter. Instead of
40 generating this dependency:
42 drivers/net/foo.c: include/linux/config.h
44 It now generates these dependencies:
46 drivers/net/foo.c: \
47 include/config/foo/autofrob.h \
48 include/config/foo/model/two.h
50 So drivers/net/foo.c depends only on the CONFIG_* lines that
51 it actually uses.
53 A new program, split-include.c, runs at the beginning of
54 compilation (make bzImage or make zImage). split-include reads
55 include/linux/autoconf.h and updates the include/config/ tree,
56 writing one file per option. It updates only the files for options
57 that have changed.
59 Flag Dependencies
61 Martin Von Loewis contributed another feature to this patch:
62 'flag dependencies'. The idea is that a .o file depends on
63 the compilation flags used to build it. The file foo.o has
64 its flags stored in .flags.foo.o.
66 Suppose the user changes the foo driver from resident to modular.
67 'make' will notice that the current foo.o was not compiled with
68 -DMODULE and will recompile foo.c.
70 All .o files made from C source have flag dependencies. So do .o
71 files made with ld, and .a files made with ar. However, .o files
72 made from assembly source do not have flag dependencies (nobody
73 needs this yet, but it would be good to fix).
75 Per-source-file Flags
77 Flag dependencies also work with per-source-file flags.
78 You can specify compilation flags for individual source files
79 like this:
81 CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
83 This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
84 and several other Makefiles.
86 Credit
88 Werner Almesberger had the original idea and wrote the first
89 version of this patch.
91 Michael Chastain picked it up and continued development. He is
92 now the principal author and maintainer. Please report any bugs
93 to him.
95 Martin von Loewis wrote flag dependencies, with some modifications
96 by Michael Chastain.
98 Thanks to all of the beta testers.