## 为什么要引入齐次坐标

$$x_{2} = x_{1} + t_{x}$ $y_{2} = x_{2} + t_{y}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \end{bmatrix} = \begin{bmatrix} x_{1} \\ y_{1} \end{bmatrix} + \begin{bmatrix} t_{x} \\ t_{y} \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \end{bmatrix} = \begin{bmatrix} k_{x} && 0 \\ 0 && k_{y} \end{bmatrix} \begin{bmatrix} x_{1} \\ y_{1} \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \end{bmatrix} = \begin{bmatrix} cos\theta && sin\theta \\ -sin\theta && cons\theta \end{bmatrix} \begin{bmatrix} x_{1} \\ y_{1} \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \end{bmatrix} = \begin{bmatrix} 0.5 && 0 \\ 0 && 0.5 \end{bmatrix} \begin{bmatrix} cos45 && -sin45 \\ sin45 && cos45 \end{bmatrix} \begin{bmatrix} 2 && 0 \\ 0 && 2 \end{bmatrix} \begin{bmatrix} x_{1} \\ y_{1} \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \end{bmatrix} = \begin{bmatrix} x_{1} \\ y_{1} \end{bmatrix} + \begin{bmatrix} t_{x} \\ t_{y} \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \\ 1 \end{bmatrix} = \begin{bmatrix} 1 && 0 && t_{x} \\ 0 && 1 && t_{y} \\ 0 && 0 && 1 \\ \end{bmatrix} + \begin{bmatrix} x_{1} \\ y_{1} \\ 1 \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \\ 1 \end{bmatrix} = \begin{bmatrix} k_{x} && 0 && 0 \\ 0 && k_{y} && 0 \\ 0 && 0 && 1 \\ \end{bmatrix} + \begin{bmatrix} x_{1} \\ y_{1} \\ 1 \end{bmatrix}$$

$$\begin{bmatrix} x_{2} \\ y_{2} \\ 1 \end{bmatrix} = \begin{bmatrix} cos\theta && -sin\theta && 0 \\ sin\theta && cos\theta && 0 \\ 0 && 0 && 1 \\ \end{bmatrix} + \begin{bmatrix} x_{1} \\ y_{1} \\ 1 \end{bmatrix}$$

Over!

## Reordering on an Alpha processor

A very non-intuitive property of the Alpha processor is that it allows the following behavior:

Initially: p = & x, x = 1, y = 0

--------------------------------
y = 1         |
memoryBarrier |    i = *p
p = & y       |
--------------------------------
Can result in: i = 0


This behavior means that the reader needs to perform a memory barrier in lazy initialization idioms (e.g., Double-checked locking) and creates issues for synchronization-free immutable objects (e.g., ensuring. that other threads see the correct value for fields of a String object).

Kourosh Gharachorloo wrote a note explaining how it can actually happen on an Alpha multiprocessor:
The anomalous behavior is currently only possible on a 21264-based system. And obviously you have to be using one of our multiprocessor servers. Finally, the chances that you actually see it are very low, yet it is possible.

Here is what has to happen for this behavior to show up. Assume T1 runs on P1 and T2 on P2. P2 has to be caching location y with value 0. P1 does y=1 which causes an “invalidate y” to be sent to P2. This invalidate goes into the incoming “probe queue” of P2; as you will see, the problem arises because this invalidate could theoretically sit in the probe queue without doing an MB on P2. The invalidate is acknowledged right away at this point (i.e., you don’t wait for it to actually invalidate the copy in P2’s cache before sending the acknowledgment). Therefore, P1 can go through its MB. And it proceeds to do the write to p. Now P2 proceeds to read p. The reply for read p is allowed to bypass the probe queue on P2 on its incoming path (this allows replies/data to get back to the 21264 quickly without needing to wait for previous incoming probes to be serviced). Now, P2 can derefence P to read the old value of y that is sitting in its cache (the inval y in P2’s probe queue is still sitting there).

How does an MB on P2 fix this? The 21264 flushes its incoming probe queue (i.e., services any pending messages in there) at every MB. Hence, after the read of P, you do an MB which pulls in the inval to y for sure. And you can no longer see the old cached value for y.

Even though the above scenario is theoretically possible, the chances of observing a problem due to it are extremely minute. The reason is that even if you setup the caching properly, P2 will likely have ample opportunity to service the messages (i.e., inval) in its probe queue before it receives the data reply for “read p”. Nonetheless, if you get into a situation where you have placed many things in P2’s probe queue ahead of the inval to y, then it is possible that the reply to p comes back and bypasses this inval. It would be difficult for you to set up the scenario though and actually observe the anomaly.

The above addresses how current Alpha’s may violate what you have shown. Future Alpha’s can violate it due to other optimizations. One interesting optimization is value prediction.

Over!

## Configuring Bonding Manually via Sysfs

Configuring Bonding Manually via Sysfs
------------------------------------------

Starting with version 3.0.0, Channel Bonding may be configured
via the sysfs interface.  This interface allows dynamic configuration
of all bonds in the system without unloading the module.  It also
allows for adding and removing bonds at runtime.  Ifenslave is no
longer required, though it is still supported.

Use of the sysfs interface allows you to use multiple bonds
with different configurations without having to reload the module.
It also allows you to use multiple, differently configured bonds when
bonding is compiled into the kernel.

You must have the sysfs filesystem mounted to configure
bonding this way.  The examples in this document assume that you
are using the standard mount point for sysfs, e.g. /sys.  If your
sysfs filesystem is mounted elsewhere, you will need to adjust the
example paths accordingly.

Creating and Destroying Bonds
-----------------------------
To add a new bond foo:
# echo +foo > /sys/class/net/bonding_masters

To remove an existing bond bar:
# echo -bar > /sys/class/net/bonding_masters

To show all existing bonds:
# cat /sys/class/net/bonding_masters

NOTE: due to 4K size limitation of sysfs files, this list may be
truncated if you have more than a few hundred bonds.  This is unlikely
to occur under normal operating conditions.

--------------------------
Interfaces may be enslaved to a bond using the file
/sys/class/net//bonding/slaves.  The semantics for this file
are the same as for the bonding_masters file.

To enslave interface eth0 to bond bond0:
# ifconfig bond0 up
# echo +eth0 > /sys/class/net/bond0/bonding/slaves

To free slave eth0 from bond bond0:
# echo -eth0 > /sys/class/net/bond0/bonding/slaves

When an interface is enslaved to a bond, symlinks between the
two are created in the sysfs filesystem.  In this case, you would get
/sys/class/net/bond0/slave_eth0 pointing to /sys/class/net/eth0, and
/sys/class/net/eth0/master pointing to /sys/class/net/bond0.

This means that you can tell quickly whether or not an
interface is enslaved by looking for the master symlink.  Thus:
# echo -eth0 > /sys/class/net/eth0/master/bonding/slaves
will free eth0 from whatever bond it is enslaved to, regardless of
the name of the bond interface.

Changing a Bond's Configuration
-------------------------------
Each bond may be configured individually by manipulating the
files located in /sys/class/net//bonding

The names of these files correspond directly with the command-
line parameters described elsewhere in this file, and, with the
exception of arp_ip_target, they accept the same values.  To see the
current setting, simply cat the appropriate file.

A few examples will be given here; for specific usage
guidelines for each parameter, see the appropriate section in this
document.

To configure bond0 for balance-alb mode:
# ifconfig bond0 down
# echo 6 > /sys/class/net/bond0/bonding/mode
- or -
# echo balance-alb > /sys/class/net/bond0/bonding/mode
NOTE: The bond interface must be down before the mode can be
changed.

To enable MII monitoring on bond0 with a 1 second interval:
# echo 1000 > /sys/class/net/bond0/bonding/miimon
NOTE: If ARP monitoring is enabled, it will disabled when MII
monitoring is enabled, and vice-versa.

# echo +192.168.0.100 > /sys/class/net/bond0/bonding/arp_ip_target
# echo +192.168.0.101 > /sys/class/net/bond0/bonding/arp_ip_target
NOTE:  up to 16 target addresses may be specified.

To remove an ARP target:
# echo -192.168.0.100 > /sys/class/net/bond0/bonding/arp_ip_target

To configure the interval between learning packet transmits:
# echo 12 > /sys/class/net/bond0/bonding/lp_interval
NOTE: the lp_inteval is the number of seconds between instances where
the bonding driver sends learning packets to each slaves peer switch.  The
default interval is 1 second.

Example Configuration
---------------------
We begin with the same example that is shown in section 3.3,
executed with sysfs, and without using ifenslave.

To make a simple bond of two e100 devices (presumed to be eth0
and eth1), and have it persist across reboots, edit the appropriate
file (/etc/init.d/boot.local or /etc/rc.d/rc.local), and add the
following:

modprobe bonding
modprobe e100
echo balance-alb > /sys/class/net/bond0/bonding/mode
ifconfig bond0 192.168.1.1 netmask 255.255.255.0 up
echo 100 > /sys/class/net/bond0/bonding/miimon
echo +eth0 > /sys/class/net/bond0/bonding/slaves
echo +eth1 > /sys/class/net/bond0/bonding/slaves

To add a second bond, with two e1000 interfaces in
active-backup mode, using ARP monitoring, add the following lines to

modprobe e1000
echo +bond1 > /sys/class/net/bonding_masters
echo active-backup > /sys/class/net/bond1/bonding/mode
ifconfig bond1 192.168.2.1 netmask 255.255.255.0 up
echo +192.168.2.100 /sys/class/net/bond1/bonding/arp_ip_target
echo 2000 > /sys/class/net/bond1/bonding/arp_interval
echo +eth2 > /sys/class/net/bond1/bonding/slaves
echo +eth3 > /sys/class/net/bond1/bonding/slaves


## System V AMD64 ABI calling conventions

The calling convention of the System V AMD64 ABI is followed on Solaris, Linux, FreeBSD, Mac OS X, and other UNIX-like or POSIX-compliant operating systems. The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9, while XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6 and XMM7 are used for floating point arguments. For system calls, R10 is used instead of RCX. As in the Microsoft x64 calling convention, additional arguments are passed on the stack and the return value is stored in RAX.

Registers RBP, RBX, and R12-R15 are callee-save registers; all others must be saved by the caller if they wish to preserve their values.

Unlike the Microsoft calling convention, a shadow space is not provided; on function entry, the return address is adjacent to the seventh integer argument on the stack.

Over!

## How to connect to a WPA/WPA2 WiFi network using Linux command line

This is a step-to-step guide for connecting to a WPA/WPA2 WiFi network via the Linux command line interface. The tools are:
1. wpa_supplicant
2. iw
3. ip
4. ping

iw is the basic tool for WiFi network-related tasks, such as finding the WiFi device name, and scanning access points. wpa_supplicant is the wireless tool for connecting to a WPA/WPA2 network. ip is used for enabling/disabling devices, and finding out general network interface information.

The steps for connecting to a WPA/WPA2 network are:

1. Find out the wireless device name.

    $/sbin/iw dev phy#0 Interface wlan0 ifindex 3 type managed  The above output showed that the system has 1 physical WiFi card, designated as phy#0. The device name is wlan0. The type specifies the operation mode of the wireless device. managed means the device is a WiFi station or client that connects to an access point. 2. Check that the wireless device is up. $ ip link show wlan0
3: wlan0: (BROADCAST,MULTICAST) mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000


Look for the word “UP” inside the brackets in the first line of the output.

In the above example, wlan0 is not UP. Execute the following command to bring it up:

    $sudo ip link set wlan0 up [sudo] password for peter:  Note: you need root privilege for the above operation. If you run the show link command again, you can tell that wlan0 is now UP. $ ip link show wlan0
3: wlan0: (NO-CARRIER,BROADCAST,MULTICAST,UP) mtu 1500 qdisc mq state DOWN mode DEFAULT qlen 1000


3. Check the connection status.

    $/sbin/iw wlan0 link Not connected.  The above output shows that you are not connected to any network. 4. Scan to find out what WiFi network(s) are detected $ sudo /sbin/iw wlan0 scan
BSS 00:14:d1:9c:1f:c8 (on wlan0)
... sniped ...
freq: 2412
SSID: gorilla
RSN:	 * Version: 1
* Group cipher: CCMP
* Pairwise ciphers: CCMP
* Authentication suites: PSK
* Capabilities: (0x0000)
... sniped ...


The 2 important pieces of information from the above are the SSID and the security protocol (WPA/WPA2 vs WEP). The SSID from the above example is gorilla. The security protocol is RSN, also commonly referred to as WPA2. The security protocol is important because it determines what tool you use to connect to the network.

5. Connect to WPA/WPA2 WiFi network.
This is a 2 step process. First, you generate a configuration file for wpa_supplicant that contains the pre-shared key (“passphrase”) for the WiFi network.

    $sudo -s [sudo] password for peter:$ wpa_passphrase gorilla >> /etc/wpa_supplicant.conf
...type in the passphrase and hit enter...


wpa_passphrase takes the SSID as the single argument. You must type in the passphrase for the WiFi network gorilla after you run the command. Using that information, wpa_passphrase will output the necessary configuration statements to the standard output. Those statements are appended to the wpa_supplicant configuration file located at /etc/wpa_supplicant.conf.

Note: you need root privilege to write to /etc/wpa_supplicant.conf.

    $cat /etc/wpa_supplicant.conf # reading passphrase from stdin network={ ssid="gorilla" #psk="testtest" psk=4dfe1c985520d26a13e932bf0acb1d4580461dd854ed79ad1a88ec221a802061 }  The second step is to run wpa_supplicant with the new configuration file. $ sudo wpa_supplicant -B -D wext -i wlan0 -c /etc/wpa_supplicant.conf
-B means run wpa_supplicant in the background.
-D specifies the wireless driver. wext is the generic driver.
-c specifies the path for the configuration file.


Use the iw command to verify that you are indeed connected to the SSID.

    $/sbin/iw wlan0 link Connected to 00:14:d1:9c:1f:c8 (on wlan0) SSID: gorilla freq: 2412 RX: 63825 bytes (471 packets) TX: 1344 bytes (12 packets) signal: -27 dBm tx bitrate: 6.5 MBit/s MCS 0 bss flags: short-slot-time dtim period: 0 beacon int: 100  6. Obtain IP address by DHCP $ sudo dhclient wlan0


Use the ip command to verify the IP address assigned by DHCP. The IP address is 192.168.1.113 from below.

    $ip addr show wlan0 3: wlan0: mtu 1500 qdisc mq state UP qlen 1000 link/ether 74:e5:43:a1:ce:65 brd ff:ff:ff:ff:ff:ff inet 192.168.1.113/24 brd 192.168.1.255 scope global wlan0 inet6 fe80::76e5:43ff:fea1:ce65/64 scope link valid_lft forever preferred_lft forever  7. Add default routing rule. The last configuration step is to make sure that you have the proper routing rules. $ ip route show
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.113


The above routing table contains only 1 rule which redirects all traffic destined for the local subnet (192.168.1.x) to the wlan0 interface. You may want to add a default routing rule to pass all other traffic through wlan0 as well.

    $sudo ip route add default via 192.168.1.254 dev wlan0$ ip route show
default via 192.168.1.254 dev wlan0
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.113


8. ping external ip address to test connectivity

    \$ ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_req=1 ttl=48 time=135 ms
64 bytes from 8.8.8.8: icmp_req=2 ttl=48 time=135 ms
64 bytes from 8.8.8.8: icmp_req=3 ttl=48 time=134 ms
^C
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2000ms
rtt min/avg/max/mdev = 134.575/134.972/135.241/0.414 ms


The above series of steps is a very verbose explanation of how to connect a WPA/WPA2 WiFi network. Some steps can be skipped as you connect to the same access point for a second time. For instance, you already know the WiFi device name, and the configuration file is already set up for the network. The process needs to be tailored according to your situation.

From: http://linuxcommando.blogspot.com/2013/10/how-to-connect-to-wpawpa2-wifi-network.html
Over!

## How to disable auto suspend when I close laptop lid?

Edit /etc/systemd/logind.conf and make sure you have,

HandleLidSwitch=ignore


which will make it ignore the lid being closed. (You may need to also undo the other changes you’ve made).

Full details over at the archlinux Wiki.

The man page for logind.conf also has the relevant information,

   HandlePowerKey=, HandleSuspendKey=, HandleHibernateKey=,
HandleLidSwitch=
Controls whether logind shall handle the system power and sleep
keys and the lid switch to trigger actions such as system power-off
or suspend. Can be one of ignore, poweroff, reboot, halt, kexec,
suspend, hibernate, hybrid-sleep and lock. If ignore logind will
never handle these keys. If lock all running sessions will be
screen locked. Otherwise the specified action will be taken in the
respective event. Only input devices with the power-switch udev tag
will be watched for key/lid switch events.  HandlePowerKey=
defaults to poweroff.  HandleSuspendKey= and HandleLidSwitch=
default to suspend.  HandleHibernateKey= defaults to hibernate.


Over!

## GCC 预定义宏: Endian

#if (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
// little endian
#elif (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
// big endian
#elif (__BYTE_ORDER__ == __ORDER_PDP_ENDIAN__)
// pdp endian, The bytes in 16-bit words are laid out in
// a little-endian fashion, whereas the 16-bit subwords of
// a 32-bit quantity are laid out in big-endian fashion.
#endif


Over!

## Fix backlight problem in Arch Linux

Sometimes, ACPI does not work well due to different motherboard implementations and ACPI quirks. This includes some laptops with dual graphics (e.g. Nvidia/Radeon dedicated GPU with Intel/AMD integrated GPU) – some examples of laptops with these problems are Dell Studio, Dell XPS 14/15/17 and some Lenovo series’ laptops. Kamal Mostafa, a kernel developer made patches for solving this problem – these are included after Linux 3.1 . Additionally, on Nvidia-optimus laptops, the kernel parameter nomodeset can interfere with the ability to adjust the backlight. You can try adding the following kernel parameters in your bootloader(grub, syslinux…) to adjust ACPI model:

video.use_native_backlight=1


Note: This kernel setting was added in Linux 3.13.

From: https://wiki.archlinux.org/index.php/backlight
Over!