view Documentation/networking/dccp.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 DCCP protocol
2 ============
4 Last updated: 10 November 2005
6 Contents
7 ========
9 - Introduction
10 - Missing features
11 - Socket options
12 - Notes
14 Introduction
15 ============
17 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
18 based protocol designed to solve issues present in UDP and TCP particularly
19 for real time and multimedia traffic.
21 It has a base protocol and pluggable congestion control IDs (CCIDs).
23 It is at draft RFC status and the homepage for DCCP as a protocol is at:
24 http://www.icir.org/kohler/dcp/
26 Missing features
27 ================
29 The DCCP implementation does not currently have all the features that are in
30 the draft RFC.
32 In particular the following are missing:
33 - CCID2 support
34 - feature negotiation
36 When testing against other implementations it appears that elapsed time
37 options are not coded compliant to the specification.
39 Socket options
40 ==============
42 DCCP_SOCKOPT_PACKET_SIZE is used for CCID3 to set default packet size for
43 calculations.
45 DCCP_SOCKOPT_SERVICE sets the service. This is compulsory as per the
46 specification. If you don't set it you will get EPROTO.
48 Notes
49 =====
51 SELinux does not yet have support for DCCP. You will need to turn it off or
52 else you will get EACCES.
54 DCCP does not travel through NAT successfully at present. This is because
55 the checksum covers the psuedo-header as per TCP and UDP. It should be
56 relatively trivial to add Linux NAT support for DCCP.