view Documentation/IRQ.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 What is an IRQ?
3 An IRQ is an interrupt request from a device.
4 Currently they can come in over a pin, or over a packet.
5 Several devices may be connected to the same pin thus
6 sharing an IRQ.
8 An IRQ number is a kernel identifier used to talk about a hardware
9 interrupt source. Typically this is an index into the global irq_desc
10 array, but except for what linux/interrupt.h implements the details
11 are architecture specific.
13 An IRQ number is an enumeration of the possible interrupt sources on a
14 machine. Typically what is enumerated is the number of input pins on
15 all of the interrupt controller in the system. In the case of ISA
16 what is enumerated are the 16 input pins on the two i8259 interrupt
17 controllers.
19 Architectures can assign additional meaning to the IRQ numbers, and
20 are encouraged to in the case where there is any manual configuration
21 of the hardware involved. The ISA IRQs are a classic example of
22 assigning this kind of additional meaning.