view Documentation/usb/ohci.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 23-Aug-2002
3 The "ohci-hcd" driver is a USB Host Controller Driver (HCD) that is derived
4 from the "usb-ohci" driver from the 2.4 kernel series. The "usb-ohci" code
5 was written primarily by Roman Weissgaerber <weissg@vienna.at> but with
6 contributions from many others (read its copyright/licencing header).
8 It supports the "Open Host Controller Interface" (OHCI), which standardizes
9 hardware register protocols used to talk to USB 1.1 host controllers. As
10 compared to the earlier "Universal Host Controller Interface" (UHCI) from
11 Intel, it pushes more intelligence into the hardware. USB 1.1 controllers
12 from vendors other than Intel and VIA generally use OHCI.
14 Changes since the 2.4 kernel include
16 - improved robustness; bugfixes; and less overhead
17 - supports the updated and simplified usbcore APIs
18 - interrupt transfers can be larger, and can be queued
19 - less code, by using the upper level "hcd" framework
20 - supports some non-PCI implementations of OHCI
21 - ... more
23 The "ohci-hcd" driver handles all USB 1.1 transfer types. Transfers of all
24 types can be queued. That was also true in "usb-ohci", except for interrupt
25 transfers. Previously, using periods of one frame would risk data loss due
26 to overhead in IRQ processing. When interrupt transfers are queued, those
27 risks can be minimized by making sure the hardware always has transfers to
28 work on while the OS is getting around to the relevant IRQ processing.
30 - David Brownell
31 <dbrownell@users.sourceforge.net>