view Documentation/leds-class.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
line source
1 LED handling under Linux
2 ========================
4 If you're reading this and thinking about keyboard leds, these are
5 handled by the input subsystem and the led class is *not* needed.
7 In its simplest form, the LED class just allows control of LEDs from
8 userspace. LEDs appear in /sys/class/leds/. The brightness file will
9 set the brightness of the LED (taking a value 0-255). Most LEDs don't
10 have hardware brightness support so will just be turned on for non-zero
11 brightness settings.
13 The class also introduces the optional concept of an LED trigger. A trigger
14 is a kernel based source of led events. Triggers can either be simple or
15 complex. A simple trigger isn't configurable and is designed to slot into
16 existing subsystems with minimal additional code. Examples are the ide-disk,
17 nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
18 optimises away.
20 Complex triggers whilst available to all LEDs have LED specific
21 parameters and work on a per LED basis. The timer trigger is an example.
23 You can change triggers in a similar manner to the way an IO scheduler
24 is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
25 parameters can appear in /sys/class/leds/<device> once a given trigger is
26 selected.
29 Design Philosophy
30 =================
32 The underlying design philosophy is simplicity. LEDs are simple devices
33 and the aim is to keep a small amount of code giving as much functionality
34 as possible. Please keep this in mind when suggesting enhancements.
37 LED Device Naming
38 =================
40 Is currently of the form:
42 "devicename:colour"
44 There have been calls for LED properties such as colour to be exported as
45 individual led class attributes. As a solution which doesn't incur as much
46 overhead, I suggest these become part of the device name. The naming scheme
47 above leaves scope for further attributes should they be needed.
50 Known Issues
51 ============
53 The LED Trigger core cannot be a module as the simple trigger functions
54 would cause nightmare dependency issues. I see this as a minor issue
55 compared to the benefits the simple trigger functionality brings. The
56 rest of the LED subsystem can be modular.
58 Some leds can be programmed to flash in hardware. As this isn't a generic
59 LED device property, this should be exported as a device specific sysfs
60 attribute rather than part of the class if this functionality is required.
63 Future Development
64 ==================
66 At the moment, a trigger can't be created specifically for a single LED.
67 There are a number of cases where a trigger might only be mappable to a
68 particular LED (ACPI?). The addition of triggers provided by the LED driver
69 should cover this option and be possible to add without breaking the
70 current interface.