Assignment 6: ThreadPool

Due Date: Saturday, February 29th at 11:59PM

The ThreadPool part of this assignment was written by Jerry Cain. The Internet Archive part of the assignment was written by Ryan Eberhardt.

The Internet Archive maintains one of the largest archives of websites, books, movies, and more. Their Wayback Machine allows you to enter a website's address and see how it looked at various points in the past. (See how Stanford's website looked in 1996.)

In this assignment, you'll build the ThreadPool class, which will be used by a simplified version of the Internet Archive. A thread pool is a set of threads that are created when a ThreadPool class is instantiated in a program, and these threads are then used by the program to perform work in parallel. The benefit of using a thread pool over creating threads on-the-fly (as you did for the RSS reader from assignment 5) is to save thread creation time while your program runs. In other words, the threads are created once and can be re-used without the need to .join or recreate them. For the RSS reader, thread pools could have been used, and (for example) you would only need a thread pool of eight threads for the number of feeds. Instead of creating a thread for every feed, those eight threads would be shared, with the thread pool managing the waiting necessary when the pool ran out of available threads.

Because of the time constraints for this assignment, we have written the "crawl the web pages from the network" part of the assignment for you, and you are only responsible for writing the ThreadPool class itself. We have compiled the Internet Archive part of the assignment into a static library that already has the binary that expects a working version of the ThreadPool class. If you have time, you are welcome to also implement the web-crawling function as well, but it is not necessary, nor will it be graded (see archive.cc for details of how to use your version instead of ours).

Here is how the program works: given a website to archive, the program crawls through the network of pages it links to and resources it uses, archiving them to disk. Each page is scheduled to download via a thread from your thread pool.

The Finished Product

The program builds an archive executable. Given a seed website, this will begin downloading that website, any websites it links to, any websites those link to, and so on. Because downloading this network of interconnected websites could end up downloading a fair portion of the internet (depending on your seed website), you are restricting your downloads to a whitelist, so we only download from particular websites of interest.

To download Stanford's website, we can run the following. (The seed website is https://www.stanford.edu, and we are restricting to sites on the domain www.stanford.edu. You could use a wildcard and whitelist *.stanford.edu, but this ends up indexing a huge number of pages, and it takes a very long time to download. You can whitelist multiple different domains by using multiple w flags.)

./archive -w www.stanford.edu -d https://www.stanford.edu

This produces the following output:

[24-07-2018 08:07:00] Beginning download of https://www.stanford.edu
[24-07-2018 08:07:01] End download of https://www.stanford.edu (1.104319 seconds)
[24-07-2018 08:07:01] Skipping download of https://fonts.googleapis.com (not whitelisted, or blocked by robots.txt)
[24-07-2018 08:07:01] Skipping download of https://s.w.org (not whitelisted, or blocked by robots.txt)
[24-07-2018 08:07:01] Beginning download of https://www.stanford.edu/wp-json/
[24-07-2018 08:07:01] Beginning download of https://www.stanford.edu/wp-includes/wlwmanifest.xml
[24-07-2018 08:07:01] Beginning download of https://www.stanford.edu/xmlrpc.php?rsd
[24-07-2018 08:07:01] Beginning download of https://www.stanford.edu/wp-content/plugins/awesome-weather-pro/awesome-weather.css?ver=4.9.7
[24-07-2018 08:07:01] Skipping download of https://fonts.googleapis.com/css?family=Open+Sans%3A400%2C300&ver=4.9.7 (not whitelisted, or blocked by robots.txt)
[24-07-2018 08:07:13] Skipping download of https://www.stanford.edu/wp-content/plugins/awesome-weather-pro/js/js-cookie.js?ver=1.1 (already downloaded)
<many lines omitted...>
[24-07-2018 08:07:13] End download of https://www.stanford.edu/list/admin/#admin-finance (0.323541 seconds)
[24-07-2018 08:07:13] End download of https://www.stanford.edu/list/admin/#admin-research (0.318452 seconds)
[24-07-2018 08:07:13] End download of https://www.stanford.edu/list/admin/#admin-staff (0.314381 seconds)
[24-07-2018 08:07:13] End download of https://www.stanford.edu/list/admin/#admin-students (0.317646 seconds)
myth66.stanford.edu listening on port 9979...

After crawling a ton of www.stanford.edu pages, this launches a server; the last line tells you where to connect. (It will be different for you.) If you connect to http://myth66.stanford.edu:9979 in a web browser while leaving archive running, you will see the following:

A web browser displaying the homepage of the archive program, which your program creates. It displays the title 'CS 110 Archive` and a search box where you can enter a URL and click 'Go!' to view it.

If you enter https://www.stanford.edu and click "Go!", you can see Stanford's homepage, in all its glory:

A web browser displaying an archived copy of the Stanford homepage, loaded by our archive program.

We can click around on the links, and everything works, as long as we only click links pointing to www.stanford.edu sites (the domain we whitelisted). Even if Stanford's website goes offline, this archive can still continue serving it as if nothing had ever happened.

Program Usage

You may want to play around with ./samples/archive_soln before you begin, just to get a sense of how to test the program.

Important note: archive saves downloaded files in the indexed-documents/ directory. Running archive several times will add to this database without clearing it. This allows you to crawl several different websites and have them all be accessible from your archive web server, but it might not be what you want in testing.

If you want to start fresh, run make filefree to clear the indexed-documents directory. You can also run archive with the -m flag to make it memory-only (it won't read from disk or persist downloads on disk).

Running The Program Off-Campus

As shown above, this assignment features a web server that shows the pages your program has archived. We're running this server on ports 2000-65535, but unfortunately, for security reasons, the myth machines only allow access to these ports from on-campus computers.

You have several options if you want to run the program off-campus:

  1. You can run archive with the web server disabled. When you run archive, add the -nim flag. archive won't run the web server, but will spit out a list of downloaded content that you can check for correctness. This is the easiest option of this list, but you miss out on the cool factor of being able to use your archive from your browser.

  2. Use an SSH proxy. SSH has a feature that allows us to send traffic to an SSH server, and it will forward that traffic to a web server. If we SSH into a Stanford computer, we can then use that computer to forward web requests to your archive server.

    • Launch archive. Let's say our server is listening on myth55 port 9979.
    • Open an extra terminal window and ssh -L 9979:myth55.stanford.edu:9979 rice.stanford.edu. Leave this running on the side. (Make sure this is running in a separate terminal window from the one you're using to connect to myth, and make sure it continues running while you try to use archive with your browser.) You should replace myth55 with whatever server archive is running on.
    • In your browser, go to http://localhost:9979.

    This method might be a little annoying if you have a bad network or frequently sleep your computer (logging into rice requires 2 factor authentication), but it is probably the easiest way to be able to use your browser with archive.

    By the way, you may be wondering, why not use SSH into myth and have myth forward the traffic to archive -- why use rice? That would definitely be more familiar and straightforward, but unfortunately, myth has SSH tunneling disabled.

  3. Connect to the campus network using a VPN; instructions are here. If you feel like taking the time to install the VPN client and get everything set up, then this will probably be the easiest option in the long run.

  4. Use the terminal-based lynx browser. Open up another terminal window, SSH into myth, and then run something like this:

    lynx http://myth66.stanford.edu:9979/https://www.stanford.edu

    If you don't care much for the interactivity of lynx, you can ask your server for a single page by using curl:

    curl http://myth66.stanford.edu:9979/https://www.stanford.edu

    This simply downloads the page from myth and prints it out on your terminal window. If you get anything back, your archive is probably working correctly. (Errors in the page output probably come from the crawling framework that we've written for you.)

A Note On robots.txt

If you look at the sample output of some of your program runs, you'll notice that some downloads may be skipped. For example, if a site is not in the whiltelist, it will be skipped. However, another reason a site may not be downloaded, even if it is in the specified whitelist, is because of robots.txt. This file is used by website administrators to tell crawlers (like us) not to download particular parts of the website. You can see it by going to /robots.txt on any website. For example, we tried using archive to download the Linux man pages, but ran into issues. When we checked https://linux.die.net/robots.txt, we saw that everything was disallowed for our crawler.

Getting Started

Clone the repository that we've set up for you by typing:

git clone /usr/class/cs110/repos/assign6/$USER assign6

Compile often, test incrementally and almost as often as you compile, run ./tools/sanitycheck a bunch to test your work, and run ./tools/submit when you're done. As you've seen in past assignments, you can play with a sample application by invoking ./samples/archive_soln.

Because you are building the ThreadPool class and need to debug it, you should make judicious use of the tpcustomtest.cc tests. You can add to these tests yourself to test the functionality of your ThreadPool class. There are four tests in the program already, and to add another test, you update the buildMap function with a flag describing the test and pointing to a function that runs the test. To run, make the assignment and type (for example):

$ ./tpcustomtest --single-thread-no-wait
This is a test.

Files

You can ignore the first five files in this list -- they are used to build the downloading feature.
Start paying attention here

Assignment Roadmap

You have one thing to do for this assignment: create a rock-solid ThreadPool class. Think back to farm from Assignment 3. You created several worker processes, then distributed work to each worker as it finished its previous work. There were only eight child processes ever created, but each child factored several numbers when there was a lot of work to be done.

An easier approach would have been as follows: every time you had a number to factor, you could fork to launch a worker to factor that number. You could have implemented it such that a maximum of 8 workers were running at a time (on an 8-core machine), and some might argue that this would be just as good as your implementation, since this avoids contention for hardware resources just like your implementation did. However, this approach is definitely worse, because even if it has the same number of processes running at a time, it creates many more processes over the entire execution of the program. Creating processes is relatively expensive, so if we can reuse processes to do work, we should do so.

Because the archive program downloads many files (thousands, for big websites), the code has the potential to use many, many threads. We don't want to create thousands of individual threads, especially because we are going to limit the number of threads hammering the website we are trying to download. So, instead of creating those threads, we are going to have a fixed number of threads and then just re-use them with the thread pool.

The archive program limits itself to sixteen threads by setting up a sixteen-thread thread pool:

ThreadPool tp(16);

It does this at the beginning of the program. Then, work is added to the pool (using the tp.schedule() function) one of the threads wakes up to do the work. Concretely, we can add functions to the thread pool's queue, and one of the threads will wake up, execute the function we added, and then go back to sleep. (Note: specifically, thunks are added to the queue. Thunks are just functions that take no parameters and return no values.)

The thread pool concept is practically inescapable in software engineering; you'll see it in database libraries, web servers, some graphics rendering engines, and more. The code we're implementing is quite general-purpose, and the concepts involved will serve you well.

Using The ThreadPool Class

The ThreadPool class has the following interface:

class ThreadPool {
public:
    ThreadPool(size_t numThreads);
    void schedule(const std::function<void(void)>& thunk);
    void wait();
    ~ThreadPool();
};

A simple program can use this pool to execute 10 function calls across 4 threads:

static const size_t kNumThreads = 4;
static const size_t kNumFunctions = 10;
int main(int argc, char *argv[]) {
    ThreadPool pool(kNumThreads);
    for (size_t id = 0; id < kNumFunctions; id++) {
        pool.schedule([id] {
            cout << oslock << "Thread (ID: " << id << ") has started." << endl << osunlock;
            size_t sleepTime = (id % 3) * 10;
            sleep_for(sleepTime);
            cout << oslock << "Thread (ID: " << id << ") has finished." << endl << osunlock;
        });
    }

    pool.wait();
    cout << "All done!" << endl;
    return 0;
}

Implementation

You can test your ThreadPool using tptest.c and tpcustomtest.c (which compile to tptest and tpcustomtest). If you use dynamic memory allocation, make sure that you do not leak any memory. (You shouldn't need to.)

Can I implement ThreadPool without a dispatcher thread?

Yes, it's quite possible to implement a scheme where workers are notified of incoming work, and then they pull work off the queue without the dispatcher specifically handing the work to them. However, we want you to implement ThreadPool with a dispatcher thread. It's better practice with thread communication/synchronization, and the dispatcher thread is essential to implementing more capable ThreadPools (such as a ThreadPool with lazy initialization, where the worker threads aren't created unless they're actually needed).

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