Gamma Hawk: 4月 2014

用NoSQL解決資料庫不斷擴充的維護問題
NoSQL (Not Only SQL)資料庫是一個統稱的名詞，泛指哪些非關聯式資料庫的資料庫技術，包括了數十種不同類型的資料庫系統，因為這些資料庫大多沒有支援標準的SQL，例如知名的Google BigTable、Amazon S3的Dynamo資料庫，或是微軟Azure平臺儲存資料的方式，都屬於NoSQL資料庫的其中一種。

常見的NoSQL資料庫有4類:

圖學資料庫（Graph Database）
文件導向的資料庫（Document-Oriented Database）
Key-Value Store分散式資料庫
記憶體快取為主的資料庫

2014年4月22日星期二

About the L4Linux scheduler

Hi,

On Sun Sep 30, 2007 at 12:29:27 +0900, Lin wrote:

> I have some questions about the scheduler system on DROPS L4Linux.

> I would like your help or some hints.

> The version of my source is ...

> DROPS SVN revision : 205

> L4Linux SVN revision : 61

> Fiasco version : 1.2

> L4Linux version : 2.6.22

> In L4Linux system (on L4Env), there are two schedulers (in Fiasco

> and in Linux server), you know.

Yes, multiple schedulers may exist on the system.

> Some papers say any Linux user process consists of L4 task and the L4

> shceduler schedules all L4 thread.

> It is ok, but I can't understand the work of the Linux scheduler.

> In fact, a new thread is created in L4 when a process (has one thread)

> is created in Linux, and the active thread is changed when the context

> switch occurs in Linux server (made sure by using Fiasco JDB).

> I want to make this relation clear.

> In other words, what does the Linux scheduler do at these time.

In genernal, the Linux scheduler doesn't do anything special, it just

lives in its Linux world and schedules Linux processes. The Linux

scheduler doesn't know about L4.

> 1. Task creating

> ~~~~~~~~~~~~~~~~

> When "fork()" (or such functions) is called, the function:

> l4x_setup_next_exec (in "arch/l4/kernel/arch-i386/dispatch.c")

> may be called.

> This function seems push the argument function "ret_from_fork" to the

> stack of the new "task_struct" instance to execute it first when the

> task is scheduled.

Yes.

> Then, when "fork()" ends (when "do_fork()" returns), the new

> "task_struct" instance is created in Linux system.

do_fork and its helpers are allocating this new process, so when fork

returns the new process is already there.

> At this time, is the L4 thread (task ?) corresponds to that

> "task_struct" is not created yet ?

> (I assume it is created after the "task_struct" is scheduled.)

The corresponding L4 task is created when necessary, i.e. when this

task was actually scheduled and should run some user code.

> 2. Linux server's scheduling

> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> In L4Linux task, there are 7 threads and IRQ threads:

> (1)l4lx.rm (2)l4lx.sem (3)l4lx.main (4)l4lx.tamer

> (5)l4lx.cpu0 (6)l4lx.timer.i0 (7)l4lx.idler ...

> "l4lx.main" creates "l4lx.cpu0" (Linux server) and waits this end.

> "l4lx.cpu0" does many initialization in the function "start_kernel()",

> and then it reaches at the function:

> l4x_idle (in "arch/l4/kernel/dispatch.c").

> In this function, the Linux server creates the thread "l4lx.idler"

> and then enters the scheduling and IPC waiting loop.

> What does this function do ??

> (My expectation about it is written in 4.)

It's the idle thread. It is running when no other activity is in the

L4Linux system. It just sits there are waits for things to happen.

> And more, What is the thread "l4lx.idler" used for ??

It wakes up the idle thread when work is to be done.

> 3. Scheduled new Linux task

> ~~~~~~~~~~~~~~~~~~~~~~~~~~~

> The function "switch_to()" (depends on L4 arch) in Linux kernel only

> saves and restores the registers ESI, ..., ESP, and EIP.

> So, when all new created "task_struct"s are scheduled by the Linux

> server, they runs from "ret_from_fork" as the Linux server thread.

> (Is it right ?) ^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^

ret_from_fork is the first function a newly created process is

executing.

> "ret_from_fork" reaches at the function:

> l4x_user_dispatcher (in "arch/l4/kernel/dispatch.c").

> It calls "l4x_spawn_cpu_thread()" before its loop.

> I assume it creates the new L4 thread corresponds to the now scheduled

> "task_struct" in Linux server.

Yes.

> And, the loop in "l4x_user_dispatcher" seems to reply or wait page

> fault, exception and system call IPCs from any L4 thread runs as a

> Linux user task.

> What does this loop do in fact ??

It handles request from a user process, i.e. system calls, page faults,

exceptions etc. It receives the requests, handles them and replies

again.

> When the context switch occurs in Linux server for some reasons,

> the active L4 thread is also switched almost at the same time.

> (I see it by using "ps" on Linux and the thread list of JDB on L4.)

> I want to know the mechanism of this synchronization.

Only one user process is running at a time, i.e. when the user

dispatcher is waiting for an incoming message, a user process is

running. If the user dispatcher is not waiting, no user process is

running.

> 4. Summary

> ~~~~~~~~~~

> My main quiestions is ...

> (1) What is "l4lx.idler" created for ? (in 2)

> (2) What does the function "l4x_idle" do ? (in 2)

Hopefully answered.

> (3) Do all Linux user tasks ("task_struct" instance) only execute

> "l4x_user_dispatcher" (from "ret_from_fork") as the Linux

> server ? (in 1 and 3)

Yes, all user processes run in the user-dispatcher loop.

> (4) What does the function "l4x_user_dispatcher" do ?

> How to switch contexts in Linux and in L4 almost at the same

> time ? (in 3)

Context switching in L4 is done by the kernel (Fiasco). Also note that

the L4Linux server itself runs in an L4 thread and thus Fiasco is doing

context switches when switching from a user process (L4 thread) to the

L4Linux server (another L4 thread). This basically happens when the IPCs

are sent. The context switch in the other direction happens when the

L4Linux server replies the user process.

The context switches in the Linux server happen inside the schedule()

call, the L4 side is not affected by this, as it basically only means

that the Linux server switches to another stack.

> My expectation about Linux scheduling system is the following:

> - When there is no need to reschedule Linux user processes, Linux

> server is in IPC sleep and waiting the wakeup IPC in "l4x_idle".

Yes.

> - If Linux server gets IPC for some reasons, it wakes up and check

> the IPC sender. Then, if the sender is a thread corresponds to

> certain Linux task, Linux server wakes up this Linux task and

> reschedules by calling "schedule()".

I don't quite understand what you want to say. If the Linux server gets

an IPC it has to handle this request (Pagefault, syscall...). If the

Linux system wants to reschedule, schedule() is called so that another

context may be selected.

> - The waked up task will be active as Linux server. This context

> may execute only "l4x_user_dispatcher" which replies and wait IPCs

> and handles page faults and exceptions based on information of the

> received IPC.

Basically yes, but the kernel-side context of the user process will be

running.

> - When the L4 thread of the Linux task exits, that Linux task in

> Linux (task_struct instance) also is freed. If there is no

> schedulable task in Linux at that time, the original context

> (executes "l4x_idle") is re-scheduled.

There's always one process active in Linux (init). Idle is actually

process 0 so this would be 'running' then.

Adam

Adam [EMAIL PROTECTED]

Lackorzynski http://os.inf.tu-dresden.de/~adam/

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L4 Fiasco's scheduling in Multi-Core

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Hi,

> Suppose we have 2 cores (core 0 and core 1) and we have 2 tasks

> (task A and task B). And I assign all threads of task A to run in

> core 0 and all threads of task B to run in core 1. How L4 Fiasco's

> scheduler manages this? Will the tasks run together or they have to

> switch from one to another?

>> From what I understand, the tasks don't run together regardless

>> which core

> they are located, is it correct?

Fiasco's scheduler is only concerned with the threads you are running.

In your example, the threads of task A will be scheduled independently

from the threads of task B because they run on different cores. Each

core maintains a separate run queue. The scheduler schedules these

threads round-robin by their priority. If all your threads have the

same prio, CPU time will be distributed fairly among these threads.

Note that tasks in the L4 context are address spaces, i.e. what other

OSes would call processes. Do not confuse them with the term task as

it is used in real-time systems.

Kind regards,

Bjoern

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2014年4月15日星期二

C++ 強制轉型的介紹

http://blog.sasajun.net/2009/09/19/c-plus-transfer-type/

2014年4月10日星期四

C++ 繼承物件之 (virtual) 函數呼叫

下面程式輸出之結果為:
print from A
print from B
print from A

前兩行為 constructor 輸出，根據最後一行之輸出可以得出下列結論，當一個物件B被parent物件A 的指標指定時 (A* ptrA = &objB;)，使用 (ptrA->y();) 會呼叫物件A的function (y();)，若想呼叫物件B的function (y();)，要在class A的定義中將 (void y();) 宣告成 (virtual void y();)。

所以，衍生物件呼叫哪一個function是由當下的變數所決定而不是只看宣告。
=================================
#include <iostream>

using namespace std;

class A
{
public:
A();
~A();
void y();
//virtual void y();
void print();

private:

};

2014年4月7日星期一

Using L4Linux on Fiasco.OC - How?

Hi Andreas,

On Mon Aug 09, 2010 at 15:37:42 +0200, Andreas Speier wrote:

> I just started porting some Projects running on L4Linux on the "old"

> Fiasco to Fiasco.OC with the current L4Linux version.

> Starting with the "hello" example of the documentation was no problem,

> but I could not find out how things have to be configured when using

> L4Linux.

> Which dependencies must be regarded to make it run properly?

> Is there any documentation I could use, or can anyone publish some

> example documentation and configuration to run L4Linux on Fiasco.OC?

The full snapshot (the big one) contains a simple L4Linux config without

any connection to anything else. There L4Linux is started as any other

program.

For example to get a L4Linux with a framebuffer you can look at the

example in conf/examples/x86-fb.cfg and exchange the start of

ex_fb_spectrum with vmlinuz.

To give host devices to L4Linux, make sure to enable the virtual PCI bus

option in L4Linux. Then add a new virtual io bus in the config (in the

start script and in the io config) and plug the devices you want there

(e.g. dev => wrap(hw-root.match("PCI/CC_02")); to get all PCI network

cards). Give this io-bus to L4Linux in the config script via

'vbus = io_busses.l4linux' in the caps listing of L4Linux.

HTH,

Adam

Adam adam at os.inf.tu-dresden.de

Lackorzynski http://os.inf.tu-dresden.de/~adam/

http://os.inf.tu-dresden.de/pipermail/l4-hackers/2010/004456.html

2014年4月3日星期四

Space Saving Ideas for ACM SIG LaTex Template Users

http://www.sheridanprinting.com/typedept/latex-users.htm

2014年4月28日 星期一

2014年4月25日 星期五

PROBLEM DESCRIPTION

SOLUTION

2014年4月24日 星期四

2014年4月22日 星期二

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2014年4月7日 星期一

2014年4月3日 星期四

2014年4月28日星期一

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2014年4月24日星期四

2014年4月22日星期二

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2014年4月7日星期一

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