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#  FRAMEBUFFER RESOLUTION SETTINGS
#     +————————————————-+
#          | 640×480    800×600    1024×768   1280×1024
#      —-+——————————————–
#      256 | 0x301=769  0x303=771  0x305=773   0x307=775
#      32K | 0x310=784  0x313=787  0x316=790   0x319=793
#      64K | 0x311=785  0x314=788  0x317=791   0x31A=794
#      16M | 0x312=786  0x315=789  0x318=792   0x31B=795
#     +————————————————-+

ubuntu 9.10 使用了最新的grub2，启动参数好像有不少变动，虚拟机的分辨率调整：

方法1，还是原来的vga=788

编辑 /etc/default/grub ，中的GRUB_CMDLINE_LINUX=”vga=788″

保存以后运行update-grub

但是这样子会显示

vga=788 is deprecated and asks me to use “set gfxpayload=800x600x16;800×600” before the linux line.

意思就是vga参数已经是建议不要使用了，要用另外一种方法：

方法2：

编辑/boot/grub/grub.cfg

找到引导linux那几行

增加 set gfxpayload=800x600x16，注意不要带分号，如下：

### BEGIN /etc/grub.d/10_linux ###
menuentry “Ubuntu, Linux 2.6.31-14-generic-pae” {
        recordfail=1
        if [ -n ${have_grubenv} ]; then save_env recordfail; fi
        set quiet=1
        insmod ext2
        set root=(hd0,1)
        search –no-floppy –fs-uuid –set 9a441a57-5a71-4800-b46d-2e4c1cec6dee
        set gfxpayload=800x600x16
        linux   /boot/vmlinuz-2.6.31-14-generic-pae root=UUID=9a441a57-5a71-4800-b46d-2e4c1cec6dee ro   quiet splash
        initrd  /boot/initrd.img-2.6.31-14-generic-pae
}

一个常见的错误是，命令行双引号中使用%时，未加反斜线\，例如：

# 错误的例子:
1 2 3 4 5 touch ~/error_`date "+%Y%m%d"`.txt

在守护进程发出的电子邮件中会见到错误信息：

/bin/sh: unexpected EOF while looking for `'''''''

# 正确的例子:
1 2 3 4 5 touch ~/right_$(date +\%Y\%m\%d).txt

# 使用单引号也可以解决问题：
1 2 3 4 5 touch ~/error_$(date '+%Y%m%d').txt

# 使用单引号就不用加反斜线了。这个例子会产生这样一个文件 ~/error_\2006\04\03.txt
1 2 3 4 5 touch ~/error_$(date '+\%Y\%m\%d').txt

下例是另一个常见错误：

# Prepare for the daylight savings time shift
59 1 1-7 4 0 /root/shift_my_times.sh

初看似要在四月的第一个星期日早晨1时59分运行shift_my_times.sh，但是这样设置不对。

与其他域不同，第三和第四个域之间执行的是“或”操作。所以这个程序会在4月1日至7日以及4月余下的每一个星期日执行。

这个例子可以重写如下：

# Prepare for the daylight savings time shift
59 1 1-7 4 * test `date +\%w` = 0 && /root/shift_my_times.sh

另一个常见错误是对分钟设置的误用。下例欲一个程两个小时运行一次：

# adds date to a log file
* 0,2,4,6,8,10,12,14,16,18,20,22 * * * date >> /var/log/date.log

而上述设置会使该程序在偶数小时内的每一分钟执行一次。正确的设置是：

# runs the date command every even hour at the top of the hour
0 0,2,4,6,8,10,12,14,16,18,20,22 * * * date >> /var/log/date.log

# an even better way
0 */2 * * * date >> /var/log/date.log

除了调整tcpip.sys 的并发连接数，还需要修改windows并发连接数上限，默认大概只有5k左右

最重要的两个注册表键值 TcpNumConnections（TCP连接上限），MaxUserPort（能使用的端口数，默认5000）

还有其他的一些调整参数，具体如下：

Configure the max limit for concurrent TCP connections

There is a parameter that limits the maximum number of connections that TCP may have open simultaneously.

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
TcpNumConnections = 0x00fffffe (Default = 16,777,214)

Note a 16 Million connection limit sounds very promising, but there are other parameters (See below), which keeps us from ever reaching this limit.

When a client makes a connect() call to make a connection to a server, then the client invisible/implicit bind the socket to a local dynamic (anonymous, ephemeral, short-lived) port number. The default range for dynamic ports in Windows is 1024 to 5000, thus giving 3977 outbound concurrent connections for each IP Address. It is possible to change the upper limit with this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxUserPort = 5000 (Default = 5000, Max = 65534)

Note it is possible to reserve port numbers so they aren’t used as dynamic ports in case one have a certain application that needs them. This is done by using the ReservedPorts (Q812873) setting.

Note Vista changes the default range from 1024-5000 to 49152-65535, which can be controlled with the dynamicport setting using netsh. More Info MS KB929851.

More Info The Cable Guy – Ephemeral, Reserved, and Blocked Port Behavior
More Info MS KB Q196271
More Info MS KB Q319502
More Info MS KB Q319504
More Info MS KB Q328476
More Info MS KB Q836429

For each connection a TCP Control Block (TCB – Data structure using 0.5 KB pagepool and 0.5 KB non-pagepool) is maintained. The TCBs are pre-allocated and stored in a table, to avoid spending time on allocating/deallocating the TCBs every time connections are created/closed. The TCB Table enables reuse/caching of TCBs and improves memory management, but the static size limits how many connections TCP can support simultaneously (Active + TIME_WAIT). Configure the size of the TCB Table with this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxFreeTcbs = 2000 (Default = RAM dependent, but usual Pro = 1000, Srv=2000)

To make lookups in the TCB table faster a hash table has been made, which is optimized for finding a certain active connection. If the hash table is too small compared to the total amount of active connections, then extra CPU time is required to find a connection. Configure the size of the hash table with this DWORD registry key (Is allocated from pagepool memory):

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
MaxHashTableSize = 512 (Default = 512, Range = 64-65536)

Note Microsoft recommends for a multiprocessor environment, that the value should not be higher than the maximum amount of concurrent connections (MaxFreeTcbs), also if multiprocessor then it might be interesting to look at the registry-key NumTcbTablePartitions (Recommended value CPU-count multiplied by 4).

More Info MS KB Q151418
More Info MS KB Q224585

If having allocated a 1000 TCBs then it doesn’t mean that one will be able to have a 1000 active connections. Especially if the application is quickly opening and closing connections, because after a connection is “closed” it enters the state TIME_WAIT, and will continue to occupy the port number for 4 minutes (2*Maximum Segment Live, MSL) before it is actually removed. This behavior is specified in RFC 793, and prevents attempts to reconnect to the same party, before the old socket is recognized as closed at both sides. It is possible to change how long a socket should be in TIME_WAIT state before it can be re-used freely:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
TcpTimedWaitDelay = 120 (Default = 240 secs, Range = 30-300)

More Info MS KB Q137984
More Info MS KB Q149532
More Info MS KB Q832954

Note with Win2k the reuse of sockets have been changed, so when reaching the limit of more than 1000 connections in TIME-WAIT state, then it starts to mark sockets that have been in TIME_WAIT state for more than 60 secs as free. It is possible to configure this limit:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
MaxFreeTWTcbs = 1000 (Default = 1000 sockets)

Note with Win2k3 SP1 the reuse of sockets have been changed, so when it has to re-use sockets in TIME_WAIT state, then it checks whether the other party is different from the old socket. Eliminating the need to fiddle with (TcpTimedWaitDelay) and (MaxFreeTWTcbs) any more.

If using an application protocol that doesn’t implement timeout checking, but relies on the TCPIP timeout checking without specifying how often it should be done, then it is possible to get connections that “never” closes, if the remote host disconnects without closing the connection properly. The TCPIP timeout checking is by default done every 2 hour, by sending a keep alive packet. It is possible to change how often TCPIP should check the connections (Affects all TCPIP connections):

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
KeepAliveTime = 1800000 (Default = 7,200,000 milisecs)

More Info MS KB Q140325

When data is sent/received the data is copied back and forth to non-paged pool memory for buffering. If there are many connections receiving/sending data, then it is possible to exhaust the non-paged pool memory. The max size of the non-paged pool buffer allocated for each connection is controlled by MaxBufferredReceiveBytes or TCPIP Receive Window depending on which is smallest. More Info MS KB Q296265

Note if using the Professional/Home edition of Windows then it is very likely that it is crippled (By Microsoft) not to handle many concurrent TCP connections. Ex. Microsoft have officially stated that the backlog limit is 5 (200 when Server), so the Professional edition is not able to accept() more than 5 new connections concurrently. More Info MS KB Q127144

Note even if having optimized Windows to handle many concurrent connections, then connections might still be refused when reaching a certain limit, in case a NAT-Router/Firewall is placed infront of it, which is unable to handle so many concurrent connections.

Note if having activated SYN-Attack-Protection (Enabled by default in Win2k3 SP1) or installed WinXP SP2, a limit is introduced on how many connection attempts (half-open) one can make simultaneously (XP SP2 & Vista = 10; Vista SP2 = no limit). This will keep worms like blaster and sasser from spreading too fast, but it will also limit other applications that creates many new connections simultaneously (Like P2P).

EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts

More Info www.LvlLord.de

Windows Vista SP2 removes the limit again, but it can be enabled with the following DWORD registry setting:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters]
EnableConnectionRateLimiting = 1

More Info MS KB 969710

Related No more than 10 connections to a remote computer