Problem set 6: Pong

This problem set will teach you some useful strategies for synchronization and networking using a game called pong.

About Pong

Pong, one of the earliest arcade video games, features a ball that moves diagonally on a rectangular playing field.

In CS 61, a pong board is a rectangular grid of cells with one or more diagonally-moving balls. Each cell can contain at most one ball. Balls bounce off the edges of the playing field and off each other. Board cells can be normal or sticky; when a ball hits a sticky cell, it stops moving until dislodged by another ball.

The logic for pong boards and balls is implemented in pongboard.hh.

Getting started

Merge our problem set 6 code into your repository with git pull handout master. This will merge our code with your previous work. If you have any “conflicts” from problem set 5, resolve them before continuing further. Run git push to save your work.

Please use an explicit merge to create your repository. If you copy code by hand, our automated scripts will have trouble analyzing your code, and it’ll be harder for you to incorporate our updates.

You may also create a new cs61-psets repository for this assignment. Tell us if you do.

Once you have merged, edit the pset6/serverinfo.h file so that PONG_USER is defined to a unique string that only you know.

Note. We currently do not plan to implement authentication on the pong server. This means that, if you learn another person’s string, you can mess with their heads. Please do not abuse this system. Aggressive abuse will be reported to The Authorities.

Type make, and then run the ./pong61 program. It will print out a URL. Visit that URL in your browser. You should see a bouncing ball in a rectangular field:

Pong sample

Goals

Your goal for this problem set is to:

Two test programs are provided, one for each goal. The ./pong61 program tests network robustness (and some synchronization), while the ./simpong61 program tests thread safety. These goals can be tackled independently and in either order.

Thread safety: simpong61

The simpong61 program simulates a Pong board with multiple balls and sticky cells. Each ball runs in its own thread.

simpong61 takes the following arguments:

Try the command ./simpong61 -d0.1 -p0.1. Your terminal should fill up with board pictures like this:

....................................................................................................
...................._....................................................O......O........O..........
..........O.........................................................................................
................................................................................................._..
..........................................................._........................................
........................................._..........................................................
....................................................................................................
................................_...............................................OO..................
........O..........................................O................................................
......................O.............................................................................
.............................................O......................................................
....................................................................................................
....................................................................................................
....................................................O...............................................
......_.............................................................................................
............................................................................................O.......
................................O...................................................................
....................................................................................................
.............O..................................................................._..................
....................................................................................................
....................................................................................................
..................O.......................................O....................._...................
...................................................................O................................
.................................................O..................................................
...............................................O....................................................
....................................................................................................
.........................................................._.........................................
..O.........O................................................_............._........................
..............................................O..............................O......................
...........O........................................................................................
......................_.............................................................................

You should be able to see the balls bounce off the board edges and even off each other.

Now try the same command without delay: ./simpong61. We get an almost immediate assertion failure!

Or try the command with delay and thread sanitization: make TSAN=1 simpong61 && ./simpong61 -d0.1. The thread sanitizer is very unhappy and reports many “data races.”

The handout code is not thread-safe; you must make it thread-safe. Given any combination of arguments not including -p:

The signal handler that supports -p is inherently thread-unsafe. You don’t need to make it safe, and don’t use -p when the thread sanitizer is on.

Hints

Our solution code is in pong_ball::move (in pongboard.hh) and ball_thread (in simpong61.cc), with initialization code elsewhere (we added members and/or constructor code to pong_board and/or pong_ball). But you may change anything.

It is possible to avoid all data races with two or three lines of code. Do that first. Fine-grained parallelism and blocking require more work.

Think carefully about your fine-grained parallelism strategy. If you’re not careful, your code will deadlock. Consider reducing the amount of parallelism you support in favor of ease of programming.

You may assume that cell types do not change as the board evolves, so there are no data races involving pong_cell::type_.

The C++ std::mutex and friends have some restrictions that may surprise you. For example, you cannot have a std::vector of mutex objects; instead, you must create a plain old dynamically-allocated array using new std::mutex[SIZE].

Use a command such as ./simpong61 -b24 -s18 -d0.1 to test for blocking. If your laptop fan starts running, your code is polling rather than blocking. (Or use Linux top or Mac OS X top -o cpu in a terminal window; a polling simpong61 thread will appear at the top of the list.)

Our full solutions require less than 20 lines of code.

Networking: pong61

pong61 works by sending HTTP messages to a Web server we run. HTTP is the application protocol on which the Web is built. Read more about HTTP before you continue. When it starts up, pong61 makes a single request to a URL like this:

http://cs61.seas.harvard.edu:6168/PONG_USER/reset

This tells the server to reset the pong board’s state. The server clears the board and returns a simple response containing the board’s width and height:

18 23

Then pong61 makes many requests to URLs like this:

http://cs61.seas.harvard.edu:6168/PONG_USER/move?x=XPOS&y=YPOS&style=on

This request causes a new ball to appear at position (XPOS,YPOS). The server responds with a numeric code and explanation. If everything goes well, it says:

0 OK

If there’s a problem, the numeric code is negative, as in:

-1 x and y parameters must be numbers

After each request, pong61 waits 0.1 seconds before making the next request.

Our handout code runs each HTTP request in its own thread. The main thread spins to wait for each thread to complete before going on to the next. This works just fine on Phase 0. To do the problem set, you must change the code so it works on the other phases too, and introduce synchronization along the way. Use the web page’s phase buttons to change phases.

Phase 1: Loss

In Phase 1, the server starts to lose messages. It will occasionally go offline for a short period. During that time, every move request is rejected by closing down the connection. The http_connection::receive_response_headers() function sets conn->cstate to cstate_broken and conn->status_code to -1 when this happens, but the pong thread ignores this problem and continues as if everything was fine. That position in the pong trail never gets filled in. Our server shows this mistake by drawing black marks in the spaces.

Your job in this phase is to detect lost messages and retry. When the server drops a connection, your code should close that connection (to free its resources) and make a new connection attempt at the same position. It shouldn’t move to the next position until the server responds for the current position.

However, you must be careful not to bombard the server while it is offline. The server will notice this and explode. Instead, you must implement a form of exponential backoff. This is a simple, powerful concept.

Exponential backoff is awesome because it responds to short outages quickly, but imposes only logarithmic overhead (i.e., the number of messages sent during the outage is logarithmic in the length of the outage). It’s ubiquitous: Ethernet is built on it, and the next time your Gmail goes offline, check out the numbers that appear after “Not connected. Trying again in...”.

Hint: Implement one phase at a time, always thinking how you could accomplish the task in the simplest correct way. Avoid overengineering! Our solution set implements all phases, without race conditions, in less than 60 lines of code.

Slow networks: The pong61 server may report spurious errors if you are doing the problem set remotely or over a slow network. Use the grading server to check for errors if you think your errors are spurious.

Phase 2: Delay

In Phase 2, the server delays its responses. It will send you the full header for its response, but delay the body. Since the handout loop waits for the body before sending the next request, the pong ball will move extremely slowly in Phase 2. Too slowly, in fact: the server enforces a minimum ball speed, and when your code is slower than that speed, you’ll see some black marks on the display.

You might think solving this problem would be easy: just close the connection before the response arrives. But the server is too clever for this. If you close a connection prematurely, it explodes.

To support delay, your pong61 must handle multiple concurrent connections to the server. Now, the main thread may need to spawn a new thread before the response arrives! But watch out. If you leak connections, the server will explode.

Your Phase 2 code must also work in Phase 1. We suggest you make Phase 2 work first on its own, then go back and make Phase 1 work again.

Phase 3: Utilization and Synchronization

So far, your pong61 client opens a new network connection for every ball. This is wasteful and slow and in Phase 3 the server will not allow it. You should instead reuse valid HTTP connections for new ball positions.

An http_connection object is available for reuse if and only if conn->cstate == cstate_idle. This means that the server sent a complete response and is waiting for another request.

Reusing connections would be really easy—except that in Phase 3 the server also drops some connections (as in Phase 1) and delays some connections (as in Phase 2). Your pong61 client must handle it all, and you must use synchronization primitives correctly.

The key function you’ll need to add is a connection table of available connections. This can be a linked list, an array, a std::list or std::deque, or whatever you’d like. When a connection reaches state cstate_idle, add it to the table. When you need to call a new RPC, check the table first, and use that connection if one exists.

Make sure that you protect your connection table from concurrent access! There should be no race condition bugs. Use synchronization objects to handle this. You should simultaneously remove the unsafe global move_done variable and replace it with a synchronization object.

Phase 4: Congestion

In Phase 4, the server sometimes behaves as if it is congested. A congested server responds to a request not with 0 OK, but with a positive number, such as this:

+1948 STOP

This means that the server is overloaded. pong61 is not allowed to send any more requests for (in this case) 1948 milliseconds—not even from concurrent threads. This should give the server enough time to cool down. The display will show a stop sign during the cool-down period, and if pong61 ignores the stop request and sends a message anyway, the server will explode. But after the cool-down period, the client should go right back to sending requests. A congestion response can sometimes be delayed, as in Phase 2; your client threads should proceed during the delay, entering the cool-down period only once the whole response is available.

Phases 1 through 3 are still active in Phase 4. Phase 4 may catch some race conditions in your code from Phase 3.

The best solutions will avoid all race conditions for non-delayed responses, meaning that the main thread will remain blocked during an immediate congestion signal (i.e., a complete congestion signal included in the server’s initial response). This may require changes to your Phase 2 code.

Phase 5: Evil

Phase 5 is a mystery, but run it and you’ll figure out the problem soon enough.

Hints and advice

For full credit, your code must not suffer from race condition bugs. You’ll need to think this through carefully, as race conditions may not show up during normal testing. Both ./pong61 and ./simpong61 should run cleanly with thread sanitizers. The ./pong61 -f flag, which runs pong61 faster than normal, might be useful as you look for race conditions.

We are only concerned with race conditions inside your client (i.e., between different client threads). We are not concerned with rare issues with scheduling between the server and the client, such as network reordering. It is impossible to avoid all client–server race conditions in this pset. But as usual, your code should never execute undefined behavior.

Extra credit

If you have extra time, implement something fun. For example, two teams could get together and implement Space Invaders (one team programming the monsters, and one team programming the spaceship)! Here are some RPCs the server implements that might be useful.

Or add features to simpong61. Implement obstacles on the board, or paddles (moving obstacles, each controlled by their own thread).

Turnin

You will turn in your code by pushing your git repository and informing the grading server. Inform us ASAP if you have changed partner or repository from pset 5.

Remember to fill out README.md and AUTHORS.md.


This pset was created for CS61.