view Documentation/block/switching-sched.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 As of the Linux 2.6.10 kernel, it is now possible to change the
2 IO scheduler for a given block device on the fly (thus making it possible,
3 for instance, to set the CFQ scheduler for the system default, but
4 set a specific device to use the anticipatory or noop schedulers - which
5 can improve that device's throughput).
7 To set a specific scheduler, simply do this:
9 echo SCHEDNAME > /sys/block/DEV/queue/scheduler
11 where SCHEDNAME is the name of a defined IO scheduler, and DEV is the
12 device name (hda, hdb, sga, or whatever you happen to have).
14 The list of defined schedulers can be found by simply doing
15 a "cat /sys/block/DEV/queue/scheduler" - the list of valid names
16 will be displayed, with the currently selected scheduler in brackets:
18 # cat /sys/block/hda/queue/scheduler
19 noop anticipatory deadline [cfq]
20 # echo anticipatory > /sys/block/hda/queue/scheduler
21 # cat /sys/block/hda/queue/scheduler
22 noop [anticipatory] deadline cfq