view Documentation/networking/6pack.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 This is the 6pack-mini-HOWTO, written by
3 Andreas Könsgen DG3KQ
4 Internet: ajk@iehk.rwth-aachen.de
5 AMPR-net: dg3kq@db0pra.ampr.org
6 AX.25: dg3kq@db0ach.#nrw.deu.eu
8 Last update: April 7, 1998
10 1. What is 6pack, and what are the advantages to KISS?
12 6pack is a transmission protocol for data exchange between the PC and
13 the TNC over a serial line. It can be used as an alternative to KISS.
15 6pack has two major advantages:
16 - The PC is given full control over the radio
17 channel. Special control data is exchanged between the PC and the TNC so
18 that the PC knows at any time if the TNC is receiving data, if a TNC
19 buffer underrun or overrun has occurred, if the PTT is
20 set and so on. This control data is processed at a higher priority than
21 normal data, so a data stream can be interrupted at any time to issue an
22 important event. This helps to improve the channel access and timing
23 algorithms as everything is computed in the PC. It would even be possible
24 to experiment with something completely different from the known CSMA and
25 DAMA channel access methods.
26 This kind of real-time control is especially important to supply several
27 TNCs that are connected between each other and the PC by a daisy chain
28 (however, this feature is not supported yet by the Linux 6pack driver).
30 - Each packet transferred over the serial line is supplied with a checksum,
31 so it is easy to detect errors due to problems on the serial line.
32 Received packets that are corrupt are not passed on to the AX.25 layer.
33 Damaged packets that the TNC has received from the PC are not transmitted.
35 More details about 6pack are described in the file 6pack.ps that is located
36 in the doc directory of the AX.25 utilities package.
38 2. Who has developed the 6pack protocol?
40 The 6pack protocol has been developed by Ekki Plicht DF4OR, Henning Rech
41 DF9IC and Gunter Jost DK7WJ. A driver for 6pack, written by Gunter Jost and
42 Matthias Welwarsky DG2FEF, comes along with the PC version of FlexNet.
43 They have also written a firmware for TNCs to perform the 6pack
44 protocol (see section 4 below).
46 3. Where can I get the latest version of 6pack for LinuX?
48 At the moment, the 6pack stuff can obtained via anonymous ftp from
49 db0bm.automation.fh-aachen.de. In the directory /incoming/dg3kq,
50 there is a file named 6pack.tgz.
52 4. Preparing the TNC for 6pack operation
54 To be able to use 6pack, a special firmware for the TNC is needed. The EPROM
55 of a newly bought TNC does not contain 6pack, so you will have to
56 program an EPROM yourself. The image file for 6pack EPROMs should be
57 available on any packet radio box where PC/FlexNet can be found. The name of
58 the file is 6pack.bin. This file is copyrighted and maintained by the FlexNet
59 team. It can be used under the terms of the license that comes along
60 with PC/FlexNet. Please do not ask me about the internals of this file as I
61 don't know anything about it. I used a textual description of the 6pack
62 protocol to program the Linux driver.
64 TNCs contain a 64kByte EPROM, the lower half of which is used for
65 the firmware/KISS. The upper half is either empty or is sometimes
66 programmed with software called TAPR. In the latter case, the TNC
67 is supplied with a DIP switch so you can easily change between the
68 two systems. When programming a new EPROM, one of the systems is replaced
69 by 6pack. It is useful to replace TAPR, as this software is rarely used
70 nowadays. If your TNC is not equipped with the switch mentioned above, you
71 can build in one yourself that switches over the highest address pin
72 of the EPROM between HIGH and LOW level. After having inserted the new EPROM
73 and switched to 6pack, apply power to the TNC for a first test. The connect
74 and the status LED are lit for about a second if the firmware initialises
75 the TNC correctly.
77 5. Building and installing the 6pack driver
79 The driver has been tested with kernel version 2.1.90. Use with older
80 kernels may lead to a compilation error because the interface to a kernel
81 function has been changed in the 2.1.8x kernels.
83 How to turn on 6pack support:
85 - In the linux kernel configuration program, select the code maturity level
86 options menu and turn on the prompting for development drivers.
88 - Select the amateur radio support menu and turn on the serial port 6pack
89 driver.
91 - Compile and install the kernel and the modules.
93 To use the driver, the kissattach program delivered with the AX.25 utilities
94 has to be modified.
96 - Do a cd to the directory that holds the kissattach sources. Edit the
97 kissattach.c file. At the top, insert the following lines:
99 #ifndef N_6PACK
100 #define N_6PACK (N_AX25+1)
101 #endif
103 Then find the line
105 int disc = N_AX25;
107 and replace N_AX25 by N_6PACK.
109 - Recompile kissattach. Rename it to spattach to avoid confusions.
111 Installing the driver:
113 - Do an insmod 6pack. Look at your /var/log/messages file to check if the
114 module has printed its initialization message.
116 - Do a spattach as you would launch kissattach when starting a KISS port.
117 Check if the kernel prints the message '6pack: TNC found'.
119 - From here, everything should work as if you were setting up a KISS port.
120 The only difference is that the network device that represents
121 the 6pack port is called sp instead of sl or ax. So, sp0 would be the
122 first 6pack port.
124 Although the driver has been tested on various platforms, I still declare it
125 ALPHA. BE CAREFUL! Sync your disks before insmoding the 6pack module
126 and spattaching. Watch out if your computer behaves strangely. Read section
127 6 of this file about known problems.
129 Note that the connect and status LEDs of the TNC are controlled in a
130 different way than they are when the TNC is used with PC/FlexNet. When using
131 FlexNet, the connect LED is on if there is a connection; the status LED is
132 on if there is data in the buffer of the PC's AX.25 engine that has to be
133 transmitted. Under Linux, the 6pack layer is beyond the AX.25 layer,
134 so the 6pack driver doesn't know anything about connects or data that
135 has not yet been transmitted. Therefore the LEDs are controlled
136 as they are in KISS mode: The connect LED is turned on if data is transferred
137 from the PC to the TNC over the serial line, the status LED if data is
138 sent to the PC.
140 6. Known problems
142 When testing the driver with 2.0.3x kernels and
143 operating with data rates on the radio channel of 9600 Baud or higher,
144 the driver may, on certain systems, sometimes print the message '6pack:
145 bad checksum', which is due to data loss if the other station sends two
146 or more subsequent packets. I have been told that this is due to a problem
147 with the serial driver of 2.0.3x kernels. I don't know yet if the problem
148 still exists with 2.1.x kernels, as I have heard that the serial driver
149 code has been changed with 2.1.x.
151 When shutting down the sp interface with ifconfig, the kernel crashes if
152 there is still an AX.25 connection left over which an IP connection was
153 running, even if that IP connection is already closed. The problem does not
154 occur when there is a bare AX.25 connection still running. I don't know if
155 this is a problem of the 6pack driver or something else in the kernel.
157 The driver has been tested as a module, not yet as a kernel-builtin driver.
159 The 6pack protocol supports daisy-chaining of TNCs in a token ring, which is
160 connected to one serial port of the PC. This feature is not implemented
161 and at least at the moment I won't be able to do it because I do not have
162 the opportunity to build a TNC daisy-chain and test it.
164 Some of the comments in the source code are inaccurate. They are left from
165 the SLIP/KISS driver, from which the 6pack driver has been derived.
166 I haven't modified or removed them yet -- sorry! The code itself needs
167 some cleaning and optimizing. This will be done in a later release.
169 If you encounter a bug or if you have a question or suggestion concerning the
170 driver, feel free to mail me, using the addresses given at the beginning of
171 this file.
173 Have fun!
175 Andreas