PA#4: Threading and Synchronization Solution

Introduction

In this programming assignment, you would increase the e ciency of your PA2. You may have noticed that PA2 takes a long time collect all the requests for just 1 person. Transferring les is even slower. You would incorporate multiple threads to make these operations faster. However, that must be done carefully so that we eliminate bottlenecks instead of needlessly using threads.


Threading for Faster Programs

In PA2, we have several bottlenecks. First, the communication channel is sequential for each request, which can take a random amount of processing time from the server side. As a result, one request being delayed would a ect all subsequent requests. Notice in the BIMDC/ directory that there are 15 les, one for each patient. If you are to collect data for say p of these patients, an e cient approach would be to use p threads from the client side, one for each patient. Each thread would create its own request channel with the server and then independently collect the responses for each person.

The server already processes each channel in a separate thread. As a result, you can get 15-fold speed over the sequential version assuming p = 15. Figure 1 describes this technique.

However, there is a big problem: even if there are adequate hardware resources (e.g., CPU cores), you cannot use more than 15-fold speedup. To avoid this roadblock, we have to make the number of threads some how indpendent of the number of patients.

The standard solution to this problem is using a bu er where the p patient threads would push requests. Now, there would be w threads, called worker threads, each communicating with the server independently of each other through a dedicated request channel. As a result, you can now decouple the speedup factor from the number of patients. Assuming that the communication between the client and the server is the main bottleneck, you can now acheive w times speedup, which is indpendent of p and potentially w >> p. Figure .2 demonstrates this technique.

For this technique to work, you need a special and more compilcated bu er between the patient threads and the worker threads. First, the bu er needs to be thread-safe; otherwise simultaneous access from the producers (i.e., patient threads) and consumers (i.e., worker threads) would lead to race condition. Second, the bu er must be made \bounded" so that the memory footprint of the bu er is under check and does not grow to in nity. In summary, the bu er must be protected against \race-conditions", \over ow" and \under ow".


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Over ow can happen when the patient threads are much faster than the worker threads, while under ow can happen under the opposite case. Use a BoundedBu er class to solve all these problems . See the class lectures to nd out more about BoundedBu er.

Client Requesting Data Points

The workers threads in this PA work as follows. Each worker thread pop a request from the request bu er (i.e., the bu er between the patient threads and the worker threads) that contains datamsg objects, sends it to the server, collects the response, and puts the response in the patient’s (the patient no is extracted from the data message request) histogram. There is a histogram per patient that keeps track of that patient’s statistics. Note that multiple worker threads would potentially update the same histogram leading to another race condition, which must be avoided by using mutual exclusion. Figure 3 shows the structure.

When requesting data messages, the program should take 4 command line arguments: n for number of data items (in range [1; 15K]), p for number of patients (range [1; 15]), w for number of worker threads (try between [50; 5000]), and b for bounded bu er size (range 1 to a few hundred). For instance, the following command is requesting 15K data items each 2 persons (i.e, patients 1 and 2) using 100 threads, and using a bounded bu er of capacity 50 requests.


prompt $>    ./client -n 15000 -p 2 -w 500 -b 50


Note that all these arguments are optional, meaning that they can be omitted, causing their default values being used.

Notice that there is a total of p + w threads in this design: p patient threads and w worker threads. All these threads must be running simultaneously for your program to work. You cannot just have a huge request bu er where all requests would t. Make sure to test your program using b = 1 indicating the bounded bu er size is 1 - your program should work perfectly ne (of course a bit slower) with this. Smaller b value along with high p; n increase concurrency and thus manifest race condition bugs that are not visible otherwise. Make sure to stress-test your program under these settings.

Client Requesting Files

You can also run the client for le requests. First, the client reqeusts the size of the le using a special filemsg (like PA2). After that, instead of sending the filemsg requests directly to the server, the client starts a thread that puts makes all the requests for this le and pushes those to the request bu er. The worker threads pop those requests from teh bu er, send those out to the server through their dedicated request channels, receives the response, and writes those responses into the appropriate locations of the le. The structure is shown in Figure 4. Note that while the progrm is running, there are w + 1 threads working simultaneously: 1 thread for making the requests and pushing them to the request bu er, the other w threads for the workers, who keep taking from the request bu er and sending those to the server.

Note that in this design, le chunks can be received out-of-order (earlier chunks arriving later or vice versa). You must make your worker threads robust enough so that they do


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not corrupt the le when they are writing to it simultaneously. There is a speci c mode for opening a le that would make this possible.

When requesting a ie, the client would take 3 command line arguments: w for the number of worker threads, m for maximum bu er size to keep the le content in memory, and f for le name. The rst two are optional and the last argument (i.e, \-f") is mandatory. The following example command asks to download the le \ le.bin" using 100 worker threads and using a bu er capacity of 256 bytes.


prompt $>    ./client -w 100 -f test.bin -m 256


Capacity m indicates the maximum number of bytes from the le that can be sent back from the server in each response. Note that the dataserver needs to know how this value, which would be passed from the client as a command line argument through the exec() function.

You should vary m and w and report the variation in runtime that you experience for les of di erent sizes. Make sure to test your program using boundary conditions (e.g., m = 1; w = 1, or m = 1; w = 5000).

Implementing the BoundedBu er

BoundedBuffer must use an STL queue of items to maintain the First-In-First-Out order. Each item in the queue must be type-agnostic and binary-data-capable, which means you cannot use std::string. Either std::vector<char> or some other variable-length data structure would be needed.

BoundedBuffer class should need 2 synchronization primitives: a mutex and two condition variables. You should not need any other data structures or types. You are not allowed to use other primitives either.

Condition variable is an excellent way for asynchronously waiting for certain event/con-dition. We have seen in class the alternative of such asynchronous wait and noti cation is some busy-spin wait which is either ine cient or wasteful in terms of CPU cycles. POSIX provides pthread cond t type that can waited upon and signaled by one or more threads. In PA4, we can use this type to guard both over ow and under ow from happening.


You will need one condition variable for guarding over ow, and another one for guarding under ow. Each producer thread (i.e., request threads) waits for the bu er to get out of over ow (i.e., bu er size is less then the maximum) encoded in condition # 1 and then pushes an item. It also noti es the consumer threads (i.e., worker threads) by signaling condition # 2 that data is now available. This wakes up all waiting consumer threads (if any) one at a time. The logic for the consumer threads is similar with the di erence that consumer threads wait on condition # 2 and signals condition # 1. Please check the man pages and the L11 slide set for reference.









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Assignment

Given Code

The given source package includes the les from PA2 (i.e., dataserver.cpp, client.cpp, common.h/.cpp, and FIFOreqchannel.cpp/.h). In addition, it now includes Histogram.h/cpp and HistogramCollection.h both fully implemented for you. The Histogram class encapsu-lates histogram management functionality. Note that you need to create a Histogram object for each patient, resulting in p (i.e., p 2 [1; 15]) Histograms. All these histograms are added to the HistogramCollection object which maintains the list and provides a nice print() function to output the histogram values together. Finally, the package contains a template for the BoundedBuffer class (.h/.cpp) that you have to ll out and use that properly in the client.cpp le.

Your Task

Your code must also incorporate the following modi cations compared to PA2:

    • Your client program should accept all the command line arguments: n, p, w, b, m, f. Based on whether the f argument was provided, the client chooses to request data or a le. All the other arguments are optional.

    • Start all threads (e.g., p patient threads and w worker threads) and wait for the threads to nish. Time your program under di erent setting and collect runtime for each setting. You need to wait for all threads

    • For data requests, your client program should call HistogramCollection::print() function at the end. If you program is working correctly, the nal output should show n for each person.

    • Your program should include a functional BoundedBuffer class that is thread-safe and guarded against over- ow and under- ow.

    • The dataserver should accept another argument m for bu er capacity which should be passed along from the client.

Bonus

You have the opportunity to gain bonus credit for this Programming Assignment. To gain this bonus credit, you must implement a real-time histogram display for the requests being processed.

Write a signal-handler function that clears the terminal window (system(\clear") is an easy way to do this) and then displays the output of either the HistogramCollection::print() function or show how much of the le have been received so far in the form of percentage or fraction.

In main, register your signal-handler function as the handler for SIGALRM (man 2 sigaction). Then, set up a timer to raise SIGALRM at 2-second intervals (man 2 timer

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create, man 2 timer settime), so that your handler is invoked and displays the current patient response totals and frequency counts approximately every 2 seconds. To do this, you will need to make sure that your signal handler is given all the necessary parameters when it catches a signal (man 7 sigevent). When all requests have been processed, stop the timer (man 2 timer delete).

If you have succeeded, the result should look like a histogram table that stays in one place in the terminal while its component values are updated to re ect the execution of the underlying program. You can use global variables for the bonus part.

Note that this is an example of asynchronous/real-time programming where the program performs certain operations based on the clock instead of in synchrnous manner. Such technique is useful when a program itself is busy doing its main work, while the asynchronous part is in charge of dealing with real-time events (e.g., printing something every few seconds, wrap-up computation after a deadline expires).

Report

    1. Data Requests: Make two graphs for the performance of your client program with varying numbers of worker threads and varying size of request bu er (i.e. di erent values of w and b) for n = 15K. Discuss how performance changes (or fails to change) with each of them, and o er explanations for both. Do we see linear scaling on any of the parameters?

    2. File Request: Make two graphs for the performance of your client program with varying numbers of worker threads and varying bu er capacity (i.e. di erent values of w and m). Discuss how performance changes (or fails to change) with each of them, and o er explanations for both. Do we see a linear scaling? Why or why not?


What to Turn In

    • The full solution directory including all cpp/h  les and a makefile

    • Completed report

Rubric

    1. BoundedBu er class (20 pts)

        ◦ Your program cannot have a Semaphore class. Having one would result in 20 lost points

    2. Cleaning up  fo  les and all dynamically allocated objects (10 pts)

    3. Correct counts in the histogram (25 pts) for data requests

    4. File Reqeusts (25 pts) with multiple threads

        ◦ Binary  les (15 pts)


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        ◦ Text  les (10 pts)

    5. No global variables are allowed (except for the bonus part). (5 points)

    6. Report (15 pts)

        ◦ Should show plots of runtime under varying n; b; w; m.

        ◦ Having w < 500 will result in 5 lost points. You must nd a platform that supports 500 threads or more.

        ◦ Requiring b = 3 n will result in 15 lost points, because that indicates that you did not need any over ow or under ow checks.

    7. Bonus: using timers to display the counts (10 pts)

        ◦ If your implementation uses a separate thread instead of a signal handler, you only get 5 bonus pts












































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Figure 1: First try to scaling PA2 performance - one thread per patient.










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Figure 2: Second try with a bu er - number of worker threads w is now independent of number of patients p.









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Figure 3: Structure of PA4 for data requests.









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Figure 4: Structure of PA4 for    le requests.









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