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  • Thread Synchronisation 101

    - by taspeotis
    Previously I've written some very simple multithreaded code, and I've always been aware that at any time there could be a context switch right in the middle of what I'm doing, so I've always guarded access the shared variables through a CCriticalSection class that enters the critical section on construction and leaves it on destruction. I know this is fairly aggressive and I enter and leave critical sections quite frequently and sometimes egregiously (e.g. at the start of a function when I could put the CCriticalSection inside a tighter code block) but my code doesn't crash and it runs fast enough. At work my multithreaded code needs to be a tighter, only locking/synchronising at the lowest level needed. At work I was trying to debug some multithreaded code, and I came across this: EnterCriticalSection(&m_Crit4); m_bSomeVariable = true; LeaveCriticalSection(&m_Crit4); Now, m_bSomeVariable is a Win32 BOOL (not volatile), which as far as I know is defined to be an int, and on x86 reading and writing these values is a single instruction, and since context switches occur on an instruction boundary then there's no need for synchronising this operation with a critical section. I did some more research online to see whether this operation did not need synchronisation, and I came up with two scenarios it did: The CPU implements out of order execution or the second thread is running on a different core and the updated value is not written into RAM for the other core to see; and The int is not 4-byte aligned. I believe number 1 can be solved using the "volatile" keyword. In VS2005 and later the C++ compiler surrounds access to this variable using memory barriers, ensuring that the variable is always completely written/read to the main system memory before using it. Number 2 I cannot verify, I don't know why the byte alignment would make a difference. I don't know the x86 instruction set, but does mov need to be given a 4-byte aligned address? If not do you need to use a combination of instructions? That would introduce the problem. So... QUESTION 1: Does using the "volatile" keyword (implicity using memory barriers and hinting to the compiler not to optimise this code) absolve a programmer from the need to synchronise a 4-byte/8-byte on x86/x64 variable between read/write operations? QUESTION 2: Is there the explicit requirement that the variable be 4-byte/8-byte aligned? I did some more digging into our code and the variables defined in the class: class CExample { private: CRITICAL_SECTION m_Crit1; // Protects variable a CRITICAL_SECTION m_Crit2; // Protects variable b CRITICAL_SECTION m_Crit3; // Protects variable c CRITICAL_SECTION m_Crit4; // Protects variable d // ... }; Now, to me this seems excessive. I thought critical sections synchronised threads between a process, so if you've got one you can enter it and no other thread in that process can execute. There is no need for a critical section for each variable you want to protect, if you're in a critical section then nothing else can interrupt you. I think the only thing that can change the variables from outside a critical section is if the process shares a memory page with another process (can you do that?) and the other process starts to change the values. Mutexes would also help here, named mutexes are shared across processes, or only processes of the same name? QUESTION 3: Is my analysis of critical sections correct, and should this code be rewritten to use mutexes? I have had a look at other synchronisation objects (semaphores and spinlocks), are they better suited here? QUESTION 4: Where are critical sections/mutexes/semaphores/spinlocks best suited? That is, which synchronisation problem should they be applied to. Is there a vast performance penalty for choosing one over the other? And while we're on it, I read that spinlocks should not be used in a single-core multithreaded environment, only a multi-core multithreaded environment. So, QUESTION 5: Is this wrong, or if not, why is it right? Thanks in advance for any responses :)

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  • threaded serial port IOException when writing

    - by John McDonald
    Hi, I'm trying to write a small application that simply reads data from a socket, extracts some information (two integers) from the data and sends the extracted information off on a serial port. The idea is that it should start and just keep going. In short, it works, but not for long. After a consistently short period I start to receive IOExceptions and socket receive buffer is swamped. The thread framework has been taken from the MSDN serial port example. The delay in send(), readThread.Join(), is an effort to delay read() in order to allow serial port interrupt processing a chance to occur, but I think I've misinterpreted the join function. I either need to sync the processes more effectively or throw some data away as it comes in off the socket, which would be fine. The integer data is controlling a pan tilt unit and I'm sure four times a second would be acceptable, but not sure on how to best acheive either, any ideas would be greatly appreciated, cheers. using System; using System.Collections.Generic; using System.Text; using System.IO.Ports; using System.Threading; using System.Net; using System.Net.Sockets; using System.IO; namespace ConsoleApplication1 { class Program { static bool _continue; static SerialPort _serialPort; static Thread readThread; static Thread sendThread; static String sendString; static Socket s; static int byteCount; static Byte[] bytesReceived; // synchronise send and receive threads static bool dataReceived; const int FIONREAD = 0x4004667F; static void Main(string[] args) { dataReceived = false; readThread = new Thread(Read); sendThread = new Thread(Send); bytesReceived = new Byte[16384]; // Create a new SerialPort object with default settings. _serialPort = new SerialPort("COM4", 38400, Parity.None, 8, StopBits.One); // Set the read/write timeouts _serialPort.WriteTimeout = 500; _serialPort.Open(); string moveMode = "CV "; _serialPort.WriteLine(moveMode); s = null; IPHostEntry hostEntry = Dns.GetHostEntry("localhost"); foreach (IPAddress address in hostEntry.AddressList) { IPEndPoint ipe = new IPEndPoint(address, 10001); Socket tempSocket = new Socket(ipe.AddressFamily, SocketType.Stream, ProtocolType.Tcp); tempSocket.Connect(ipe); if (tempSocket.Connected) { s = tempSocket; s.ReceiveBufferSize = 16384; break; } else { continue; } } readThread.Start(); sendThread.Start(); while (_continue) { Thread.Sleep(10); ;// Console.WriteLine("main..."); } readThread.Join(); _serialPort.Close(); s.Close(); } public static void Read() { while (_continue) { try { //Console.WriteLine("Read"); if (!dataReceived) { byte[] outValue = BitConverter.GetBytes(0); // Check how many bytes have been received. s.IOControl(FIONREAD, null, outValue); uint bytesAvailable = BitConverter.ToUInt32(outValue, 0); if (bytesAvailable > 0) { Console.WriteLine("Read thread..." + bytesAvailable); byteCount = s.Receive(bytesReceived); string str = Encoding.ASCII.GetString(bytesReceived); //str = Encoding::UTF8->GetString( bytesReceived ); string[] split = str.Split(new Char[] { '\t', '\r', '\n' }); string filteredX = (split.GetValue(7)).ToString(); string filteredY = (split.GetValue(8)).ToString(); string[] AzSplit = filteredX.Split(new Char[] { '.' }); filteredX = (AzSplit.GetValue(0)).ToString(); string[] ElSplit = filteredY.Split(new Char[] { '.' }); filteredY = (ElSplit.GetValue(0)).ToString(); // scale values int x = (int)(Convert.ToInt32(filteredX) * 1.9); string scaledAz = x.ToString(); int y = (int)(Convert.ToInt32(filteredY) * 1.9); string scaledEl = y.ToString(); String moveAz = "PS" + scaledAz + " "; String moveEl = "TS" + scaledEl + " "; sendString = moveAz + moveEl; dataReceived = true; } } } catch (TimeoutException) {Console.WriteLine("timeout exception");} catch (NullReferenceException) {Console.WriteLine("Read NULL reference exception");} } } public static void Send() { while (_continue) { try { if (dataReceived) { // sleep Read() thread to allow serial port interrupt processing readThread.Join(100); // send command to PTU dataReceived = false; Console.WriteLine(sendString); _serialPort.WriteLine(sendString); } } catch (TimeoutException) { Console.WriteLine("Timeout exception"); } catch (IOException) { Console.WriteLine("IOException exception"); } catch (NullReferenceException) { Console.WriteLine("Send NULL reference exception"); } } } } }

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  • Condition Variable in Shared Memory - is this code POSIX-conformant?

    - by GrahamS
    We've been trying to use a mutex and condition variable to synchronise access to named shared memory on a LynuxWorks LynxOS-SE system (POSIX-conformant). One shared memory block is called "/sync" and contains the mutex and condition variable, the other is "/data" and contains the actual data we are syncing access to. We're seeing failures from pthread_cond_signal() if both processes don't perform the mmap() calls in exactly the same order, or if one process mmaps in some other piece of shared memory before it mmaps the sync memory. This example code is about as short as I can make it: #include <sys/types.h> #include <sys/stat.h> #include <sys/mman.h> #include <sys/file.h> #include <stdlib.h> #include <pthread.h> #include <errno.h> #include <iostream> #include <string> using namespace std; static const string shm_name_sync("/sync"); static const string shm_name_data("/data"); struct shared_memory_sync { pthread_mutex_t mutex; pthread_cond_t condition; }; struct shared_memory_data { int a; int b; }; //Create 2 shared memory objects // - sync contains 2 shared synchronisation objects (mutex and condition) // - data not important void create() { // Create and map 'sync' shared memory int fd_sync = shm_open(shm_name_sync.c_str(), O_CREAT|O_RDWR, S_IRUSR|S_IWUSR); ftruncate(fd_sync, sizeof(shared_memory_sync)); void* addr_sync = mmap(0, sizeof(shared_memory_sync), PROT_READ|PROT_WRITE, MAP_SHARED, fd_sync, 0); shared_memory_sync* p_sync = static_cast<shared_memory_sync*> (addr_sync); // init the cond and mutex pthread_condattr_t cond_attr; pthread_condattr_init(&cond_attr); pthread_condattr_setpshared(&cond_attr, PTHREAD_PROCESS_SHARED); pthread_cond_init(&(p_sync->condition), &cond_attr); pthread_condattr_destroy(&cond_attr); pthread_mutexattr_t m_attr; pthread_mutexattr_init(&m_attr); pthread_mutexattr_setpshared(&m_attr, PTHREAD_PROCESS_SHARED); pthread_mutex_init(&(p_sync->mutex), &m_attr); pthread_mutexattr_destroy(&m_attr); // Create the 'data' shared memory int fd_data = shm_open(shm_name_data.c_str(), O_CREAT|O_RDWR, S_IRUSR|S_IWUSR); ftruncate(fd_data, sizeof(shared_memory_data)); void* addr_data = mmap(0, sizeof(shared_memory_data), PROT_READ|PROT_WRITE, MAP_SHARED, fd_data, 0); shared_memory_data* p_data = static_cast<shared_memory_data*> (addr_data); // Run the second process while it sleeps here. sleep(10); int res = pthread_cond_signal(&(p_sync->condition)); assert(res==0); // <--- !!!THIS ASSERT WILL FAIL ON LYNXOS!!! munmap(addr_sync, sizeof(shared_memory_sync)); shm_unlink(shm_name_sync.c_str()); munmap(addr_data, sizeof(shared_memory_data)); shm_unlink(shm_name_data.c_str()); } //Open the same 2 shared memory objects but in reverse order // - data // - sync void open() { sleep(2); int fd_data = shm_open(shm_name_data.c_str(), O_RDWR, S_IRUSR|S_IWUSR); void* addr_data = mmap(0, sizeof(shared_memory_data), PROT_READ|PROT_WRITE, MAP_SHARED, fd_data, 0); shared_memory_data* p_data = static_cast<shared_memory_data*> (addr_data); int fd_sync = shm_open(shm_name_sync.c_str(), O_RDWR, S_IRUSR|S_IWUSR); void* addr_sync = mmap(0, sizeof(shared_memory_sync), PROT_READ|PROT_WRITE, MAP_SHARED, fd_sync, 0); shared_memory_sync* p_sync = static_cast<shared_memory_sync*> (addr_sync); // Wait on the condvar pthread_mutex_lock(&(p_sync->mutex)); pthread_cond_wait(&(p_sync->condition), &(p_sync->mutex)); pthread_mutex_unlock(&(p_sync->mutex)); munmap(addr_sync, sizeof(shared_memory_sync)); munmap(addr_data, sizeof(shared_memory_data)); } int main(int argc, char** argv) { if(argc>1) { open(); } else { create(); } return (0); } Run this program with no args, then another copy with args, and the first one will fail at the assert checking the pthread_cond_signal(). But change the open() function to mmap() the "/sync" memory first and it will all work fine. This seems like a major bug in LynxOS but LynuxWorks claim that using mutex and condition variable in this way is not covered by the POSIX standard, so they are not interested. Can anyone determine if this code does violate POSIX? Or does anyone have any convincing documentation that it is POSIX compliant?

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  • CodePlex Daily Summary for Saturday, April 14, 2012

    CodePlex Daily Summary for Saturday, April 14, 2012Popular ReleasesJson.NET: Json.NET 4.5 Release 3: Change - DefaultContractResolver.IgnoreSerializableAttribute is now true by default Fix - Fixed MaxDepth on JsonReader recursively throwing an error Fix - Fixed SerializationBinder.BindToName not being called with full assembly namesVisual Studio Team Foundation Server Branching and Merging Guide: v2 - For Visual Studio 11: Welcome to the BETA of the Branching and Merging Guide preview As this is a BETA release and the quality bar for the final Release has not been achieved, we value your candid feedback and recommend that you do not use or deploy these BETA artifacts in a production environment. Quality-Bar Details Documentation has been reviewed by Visual Studio ALM Rangers Documentation has not been through an independent technical review Documentation has been reviewed by the quality and recording te...Media Companion: MC 3.435b Release: This release should be the last beta for 3.4xx. A handful of problems have been sorted out since last weeks release. If there are no major problems this time, it will upgraded to 3.500 Stable at the end of the week! General The .NET Framework has been modified to use the Client profile, as provided by normal Windows updates; no longer is there a requirement to download and install the Full profile! mc_com.exe has been worked on to mimic proper Media Companion output (a big thanks to vbat99...THE NVL Maker: The NVL Maker Ver 3.12: SIM??????,TRA??????,ZIP????。 ????????????????,??????~(??????????????????) ??????? simpatch1440x900 trapatch1440x900 ?????1400x900??1440x900,?????????????Data.xp3。 ???? ?????3.12?EXE????????????????, ??????????????,??Tool/krkrconf.exe,??Editor.exe, ???????????????「??????」。 ?????Editor.exe??????。 ???? ???? http://etale.us/gameupload/THE_NVL_Maker_ver3.12_sim.zip ???? http://www.mediafire.com/?je51683g22bz8vo ??Infinite Creation?? http://bbs.etale.us/forum.php ?????? ???? 3.12 ??? ???、????...SnmpMessenger: 0.1.1.1: Project Description SnmpMessenger, a messenger. Using the SNMP protocol to exchange messages. It's developed in C#. SnmpMessenger For .Net 4.0, Mono 2.8. Support SNMP V1, V2, V3. Features Send get, set and other requests and get the response. Send and receive traps. Handle requests and return the response. Note This library is compliant with the Common Language Specification(CLS). The latest version is 0.1.1.1. It is only a messenger, does not involve VACM. Any problems, Please mailto: wa...Python Tools for Visual Studio: 1.1.1: We’re pleased to announce the release of Python Tools for Visual Studio 1.1.1. Python Tools for Visual Studio (PTVS) is an open-source plug-in for Visual Studio which supports programming with the Python language. PTVS supports a broad range of features including: • Supports CPython and IronPython • Python editor with advanced member and signature intellisense • Code navigation: “Find all refs”, goto definition, and object browser • Local and remote debugging • Profiling with multiple view...Supporting Guidance and Whitepapers: v1 - Team Foundation Service Whitepapers: Welcome to the BETA release of the Team Foundation Service Whitepapers preview As this is a BETA release and the quality bar for the final Release has not been achieved, we value your candid feedback and recommend that you do not use or deploy these BETA artifacts in a production environment. Quality-Bar Details Documentation has been reviewed by Visual Studio ALM Rangers Documentation has been through an independent technical review All critical bugs have been resolved Known Issue...Microsoft .NET Gadgeteer: .NET Gadgeteer Core 2.42.550 (BETA): Microsoft .NET Gadgeteer Core RELEASE NOTES Version 2.42.550 11 April 2012 BETA VERSION WARNING: This is a beta version! Please note: - API changes may be made before the next version (2.42.600) - The designer will not show modules/mainboards for NETMF 4.2 until you get upgraded libraries from the module/mainboard vendors - Install NETMF 4.2 (see link below) to use the new features of this release That warning aside, this version should continue to sup...LINQ to Twitter: LINQ to Twitter Beta v2.0.24: Supports .NET 3.5, .NET 4.0, Silverlight 4.0, Windows Phone 7.1, and Client Profile. 100% Twitter API coverage. Also available via NuGet.Kendo UI ASP.NET Sample Applications: Sample Applications (2012-04-11): Sample application(s) demonstrating the use of Kendo UI in ASP.NET applications.SCCM Client Actions Tool: SCCM Client Actions Tool v1.12: SCCM Client Actions Tool v1.12 is the latest version. It comes with following changes since last version: Improved WMI date conversion to be aware of timezone differences and DST. Fixed new version check. The tool is downloadable as a ZIP file that contains four files: ClientActionsTool.hta – The tool itself. Cmdkey.exe – command line tool for managing cached credentials. This is needed for alternate credentials feature when running the HTA on Windows XP. Cmdkey.exe is natively availab...Dual Browsing: Dual Browser: Please note the following: I setup the address bar temporarily to only accepts http:// .com addresses. Just type in the name of the website excluding: http://, www., and .com; (Ex: for www.youtube.com just type: youtube then click OK). The page splitter can be grabbed by holding down your left mouse button and move left or right. By right clicking on the page background, you can choose to refresh, go back a page and so on. Demo video: http://youtu.be/L7NTFVM3JUYCslaGenFork: Rules sample v.1.1.0: On projects for CSLA v.4.2.2, added 5 new Business Rules: - DependencyFrom - RequiredWhenCanWrite - RequiredWhenIsNotNew - RequiredWhenNew - StopIfNotFieldExists Added new projects for CSLA v.4.3.10 with 6 new Business Rules: - DependencyFrom - FieldExists - RequiredWhenCanWrite - RequiredWhenIsNotNew - RequiredWhenNew - StopIfNotFieldExists Following CSLA convention, SL stands for Silverligth 5 and SL4 stands for Silverlight 4. NOTE - Although the projects for CSLA v.4.1.0 still exist, thi...Multiwfn: Multiwfn 2.3.3: Multiwfn 2.3.3Liberty: v3.2.0.1 Release 9th April 2012: Change Log-Fixed -Reach Fixed a bug where the object editor did not work on non-English operating systemsCommonData - Common Functions for ASP.NET projects: CommonData 0.3L: Common Data has been updated to the latest NUnit (2.6.0) The demo project has been updated with an example on how to correctly compare a floating point value.ASP.Net MVC Dynamic JS/CSS Script Compression Framework: Initial Stable: Initial Stable Version Contains Source for Compression Library and example for usage in web application.Path Copy Copy: 10.1: This release addresses the following work items: 11357 11358 11359 This release is a recommended upgrade, especially for users who didn't install the 10.0.1 version.ExtAspNet: ExtAspNet v3.1.3: ExtAspNet - ?? ExtJS ??? ASP.NET 2.0 ???,????? AJAX ?????????? ExtAspNet ????? ExtJS ??? ASP.NET 2.0 ???,????? AJAX ??????????。 ExtAspNet ??????? JavaScript,?? CSS,?? UpdatePanel,?? ViewState,?? WebServices ???????。 ??????: IE 7.0, Firefox 3.6, Chrome 3.0, Opera 10.5, Safari 3.0+ ????:Apache License 2.0 (Apache) ??:http://extasp.net/ ??:http://bbs.extasp.net/ ??:http://extaspnet.codeplex.com/ ??:http://sanshi.cnblogs.com/ ????: +2012-04-08 v3.1.3 -??Language="zh_TW"?JS???BUG(??)。 +?D...Coding4Fun Tools: Coding4Fun.Phone.Toolkit v1.5.5: New Controls ChatBubble ChatBubbleTextBox OpacityToggleButton New Stuff TimeSpan languages added: RU, SK, CS Expose the physics math from TimeSpanPicker Image Stretch now on buttons Bug Fixes Layout fix so RoundToggleButton and RoundButton are exactly the same Fix for ColorPicker when set via code behind ToastPrompt bug fix with OnNavigatedTo Toast now adjusts its layout if the SIP is up Fixed some issues with Expression Blend supportNew ProjectsArkadia Operating System: This operating system is based on Cosmos Project and C# Programming Langage.Copy Microsoft Online User Attributes from one domain user to another: CopyMSOLAttributes copies MSOL-specific user attributes from source user to target user (legacyExchangeDN, mail, msExchMailboxGuid, proxyAddresses, targetAddress). This assists in migrating to Office 365 in a multi-domain, multi-forest environment.DevChat: DevChat is a small secure chat for dev groups or teams inside of an organisation that wish to control their information.Dynamic UI Framework: Dynamic UI FrameworkEjemploAndroid: Prueba Trabajar con eclipse y codeplexElfos vs Orcos: videojuego xna elfos vs orcosFacebook Suite Rules Orchard module: Part of the Facebook Suite Orchard module that provides various rules for the Rules engine to interact with Facebook.FakeMail: FakeMail makes testing of email enabled applications easier for DEVs and QAs. You no longer need to have multiple "real" email addresses to test and validate registration and notification features. Written in C# this ASP.Net MVC application uses RavenDB Embedded as a document store and hosts a custom SMTP server. Configure FakeMail and update your existing email-enabled application's SMTP server settings are you are ready to go. FakeMail mail server will accept email sent to ANY addre...FIM Object Visualizer: The FIM Object Visualizer is a tool to display and document configurable FIM objects such as Synchronization Rules, Workflows and Management Policy Rules. FXIB: ThisGoogle Places Autocomplete API for WP7: The Google Places Autocomplete API for WP7 is a project for WP7 developers to use when implementing an autocomplete box in their application. This project provides an easy non-blocking way to get fast results from the Google Places Autocomplete API. hook send/recv function with CreateRemoteThread: this sample is hooking send/recv function with CreateRemoteThread api.Infinity Music: Entertainment center, music player, youtube player, Internet radio, Facebook, Twitter ... All in one application ...! Centro de entretenimiento, reproductor de música, reproductor videos you tube, radio por internet, Facebook, Twitter... Todo en una misma aplicación...! Centre de divertissement, lecteur de musique, lecteur YouTube, la radio sur Internet, Facebook, Twitter ... Tout dans une seule application ...!Library Guard: Library Guard helps you maintain your media library(primarily audio) by correcting tags, maintaining location of your files, etc.MVVM Source Control Monitor: An exercise in MVVM with Wpf to create a useful and unobtrusive source control notification tool that lives in the system tray, and can also be viewed in a window. This is meant to provide a 'real world' application to provide examples of MVVM implementation without understanding any other frameworks that can blur the lines about what MVVM really is (it's a pattern, folks). The application will use as little 3rd party code as possible (Wpf Toolkit, some other goodies) that are all unrel...Orchard Calendar: Module provides calendar capabilities in Orchard. This is accomplished by a new calendar content part and content type along with new calendar layout for Projector module. PaidRanks: PaidRanks makes it easier for minecraft admins to mangae user rights and rewards donators. You'll no longer have to manually change nick names again. It's developed in Java 7.Perritos Project: Practice of the subject projectPython Pygame Sprites Example: This project example uses Python and Pygame to create a game environment. This code requires major refactoring and there is no warranty. Reversi.NET: Reversi is a board game involving abstract strategy and played by two players on a board with 8 rows and 8 columns and a set of distinct pieces for each side.SharePoint Social Tag Counters: The SharePoint Social Tag Counter project takes the social features from SharePoint 2010 to another level. It allows you to immediately show your SharePoint visitors how popular the content is with the help of Social "I like it" and "Tag" counters.Shop systems1: dddddddddddSource Block - Data Access Components: Source Block - Data Access Components Contains two components. 1. DBHandler 2. DBSchemaHandler DBHandler : Pure ADO.Net based Data access layer DBSchemaHandler : Pure ADO.Net based Database schema handlerSource Block - Domain Driven Development Framework: A framework which promotes domain pattern based development. This promotes patterns such as : Repository, Unit of work, Dependency injection and Inversion of control. Built on MS E.F 4.2 using code first approach. Includes features such as code generationSuperSocket Proxy Server: A .NET proxy server based on SuperSocketTestC-Q: This project is on working with KDB database and fetching data from open market data providers. This will do rigorous analysis. Don't try to find any code till Oct 2012.tofinish: tofinishUse National Geographic Photo of the Day as Wallpaper: I used to watch National Geographic Photo of the day everyday. http://photography.nationalgeographic.com/photography/photo-of-the-day So I thought it might be helpful for people like me if a software synchronise the wallpaper with national geographic photo of the day. It also archives photo of the day along with publication date and photo title.VAMP: Projet industriel MBDS 2012?????: ???????Model???????: ????:??Model??????? ???:??? ??:?????????python?SDK: ??python??????SDK。????

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