view Documentation/time_interpolators.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 Time Interpolators
2 ------------------
4 Time interpolators are a base of time calculation between timer ticks and
5 allow an accurate determination of time down to the accuracy of the time
6 source in nanoseconds.
8 The architecture specific code typically provides gettimeofday and
9 settimeofday under Linux. The time interpolator provides both if an arch
10 defines CONFIG_TIME_INTERPOLATION. The arch still must set up timer tick
11 operations and call the necessary functions to advance the clock.
13 With the time interpolator a standardized interface exists for time
14 interpolation between ticks. The provided logic is highly scalable
15 and has been tested in SMP situations of up to 512 CPUs.
17 If CONFIG_TIME_INTERPOLATION is defined then the architecture specific code
18 (or the device drivers - like HPET) may register time interpolators.
19 These are typically defined in the following way:
21 static struct time_interpolator my_interpolator {
22 .frequency = MY_FREQUENCY,
23 .source = TIME_SOURCE_MMIO32,
24 .shift = 8, /* scaling for higher accuracy */
25 .drift = -1, /* Unknown drift */
26 .jitter = 0 /* time source is stable */
27 };
29 void time_init(void)
30 {
31 ....
32 /* Initialization of the timer *.
33 my_interpolator.address = &my_timer;
34 register_time_interpolator(&my_interpolator);
35 ....
36 }
38 For more details see include/linux/timex.h and kernel/timer.c.
40 Christoph Lameter <christoph@lameter.com>, October 31, 2004