Project 2 - User-Level Threads

Outline

Tasks:

1. Implement a user-level thread manager.
2. Add a set of common synchronization primitives to your thread package.
3. Implement a multithreaded web-server to test your thread package.
4. Analyze your design and test results in a report.

Updates:

• 10-13-03: Current code is available on spinlock/coredump at /cse451/projects/simplethreads-1.02.tar.gz.

Assignment Goals

• To gain understanding of how thread packages are implemented, including data structures used, the interface provided, and common problems they face.
• To make use of common synchronization primitives such as locks and condition variables.
• To understand some of the problems with user-level threads.
• To interact with the instructor and TAs.
• To understand the concept of overlapping I/O and computation.
• To practice discussion and analysis of your designs.

Background

As new applications were developed, programmers increasingly desired multiple threads of control within a single process so they could all access common data. For example, databases might be able to process several queries at the same time, all using the same data. Sometimes, they used multiple processes to get this effect, but it was difficult to share data between processes (which is typically viewed as advantage, since it provides isolation, but here it was a problem).

What they wanted was multithreading support: the ability to run several threads of control within a single address space, all able to access the same memory (because they all have the same mapping function). The programmers realized that they could implement this entirely in the user-level, without modifying the kernel at all, if they were clever.

As you'll discover in the last part of this assignment, there were some problems with this approach, which motivated kernel developers to include thread support in the kernel itself (and motivated researchers to do it better; see Scheduler Activations).

Simplethreads

This project, since it does not involve modifying the kernel, does not require VMWare. The simplethreads package should work on any Linux 2.4 machine (it will not work on Linux 2.2 due to missing support for a necessary syscall), and other varieties of UNIX that support pthreads as well. (Please do not use attu.) It will not work with Cygwin. It has been reported to work with Mac OS X v10.1, but this hasn't been reproducable. Please send mail to the list if you get it working on Cygwin or Mac OS X.

To begin, copy that file to your work directory (/cse451/LOGIN) and run
tar -xvzf simplethreads-1.X.tar.gz
to expand it (if you are working on a different machine, see scp(1)).

Simplethreads contains a lot of files, but most are safe to ignore. Pay attention to:

Dir/File	Contents
lib/	The simplethreads thread library itself.
lib/sthread_user.c	Your part 1 and part 2 implementations go here.
lib/sthread_ctx.h	Support for creating new stacks and switching between them.
lib/sthread_queue.h	A simple queue that you may find useful.
include/	Contains sthread.h, the public API to the library (the functions available for apps using the library).
test/	Test programs for the library.
web/	The multithreaded webserver for part 3.

Like many UNIX programs, simplethreads uses a configure script to determine parameters of the machine needed for compilation (in fact, you'll find many UNIX packages follow exactly the same steps as simplethreads for compilation). In the simplethreads-1.X directory, run ./configure to test all these parameters.

Finally, build the package by typing make. You can also build an individual part, such as the library, by running make in a subdirectory. Or, if you just want to compile one file, run make myfile.o (from within the directory containing myfile.c).

Note that, as you make changes, it is only necessary to repeat the last step (make).

One interesting feature of simplethreads is that applications built using it can use either the native, kernel-provided threads or the user-level threads that you'll implement. When running the test programs, use --sthread-pthread to select kernel threads or --sthread-user to select user threads (they default to kernel threads).

In summary, the steps are:

1. Copy the tar.gz source to your directory.
2. tar -xvzf simplethreads-1.X.tar.gz
3. cd simplethreads-1.X
4. ./configure
5. make
6. Run the programs in test/

To Add a Source File

1. Edit the Makefile.am in the directory containing the new file, adding it to the _SOURCES for the library/executable the file is a part of. E.g., to add a new file in the lib/ directory that will become part of the sthread library, add it to the libsthread_la_SOURCES line.
2. From the top-level directory (simplethreads-1.X), run automake.
3. Also from the top-level directory, run ./configure.
4. Your file is now added; run make as usual to build it.

To Add a New Test Program

1. Edit test/Makefile.am. Add your program to the list bin_PROGRAMS. Create new variables prog_SOURCES and prog_LDADD following the examples of the programs already there. For example, if the new program is named test-silly, add:

   test_silly_SOURCES = test-silly.c other sources here
   test_silly_LDADD = $(ldadd)
   

2. Follow steps 2-4 above.

The Assignment

For part 1, we give you:

1. An implementation of a simple queue (sthread_queue.h).
2. A context-switch function that, given a new stack pointer, will switch contexts (sthread_ctx.h).
3. A new-stack function that will create and initialize a new stack such that it is ready to run.
4. Skeleton routines for a user-level thread system (sthread_user.c).

It is your job to complete the thread system, implementing:

1. Data structures to represent threads (struct _sthread in sthread_user.c).
2. A thread creation routine (sthread_user_create()).
3. A thread destruction routine (sthread_user_exit()).
4. A simple non-preemptive thread scheduler.
5. A mechanism for a thread to voluntarily yield, allowing another thread to run (sthread_user_yield()).
6. A routine to initialize your data structures (sthread_user_init()).

The routines in sthread_ctx.h do all of the stack manipulation, PC changing, register storing, and nasty stuff like that. Rather than focusing on that, this assignment focuses on managing the thread contexts. Viewed as a layered system, you need to implement the grey box below:

At the top layer, applications will use the sthread package (through the API defined in sthread.h). sthread.c will then call either the routines in sthread_pthread.c or your routines in sthread_user.c (it chooses based on the value passed to sthread_init()). Your sthread_user.c, in turn, builds on the sthread_ctx functions (as described in sthread_ctx.h).

Applications (the top-layer) may not use any routines except those listed in sthread.h. They must not know how the threads are implemented; they simply request threads be created (after initializing the library), and maybe request yeilds/exits. For example, they have no access to sthread_queue. Nor do they keep lists of running threads around; that is the job of the grey box.

Similarly, your grey box - sthread_user.c - should not need to know how sthread_ctx is implemented. Do not attempt to modify the sthread_ctx_t directly; use the routines declared in sthread_ctx.h.

Recommended Procedure

1. Figure out what each function you need to implement does. Look at some of the test programs to see usage examples.
2. Examine the supporting routines we've provided (primarily in sthread_queue.h and sthread_ctx.h).
3. Design your threading algorithm: When are threads put on the ready queue? When are threads removed? Where is sthread_switch called?
4. Figure out how to start a new thread and what to do about the initial thread (the one that calls sthread_user_init).
5. Talk to your fellow students and the TAs about your design.
6. Implement it.
7. Test it. The test programs provided are not adequate; for full confidence in your implementation, you'll need to create some of your own.

Hints

• sthread_create should not immediatly run the new thread.
• Use the provided routines in sthread_ctx.h. You don't need to write any assembly or try to directly manipulate registers for this assignment, nor is an understanding of the exact layout of the stack required (in fact, the routines provided go to great lengths to avoid requiring such knowledge).
• If the routine passed to sthread_create() finishes (returns), you need to make sure the thread's resources are cleaned up (i.e. sthread_exit() is called).
• The start routine passed to sthread_new_ctx does not take any arguments (unlike the routine that your sthread_user_create is passed). So you can't create a new stack directly with the user-supplied routine; you need to supply a routine that takes no arguments but somehow winds up invoking the user's routine with the user's argument.
• You should free any resources allocated when a thread exits (whether it exits explicitly, by calling sthread_exit(), or implicitly, because the start routine returns). However, you should not attempt to free the stack of a running thread (note: to free a stack, use shread_free_ctx()). This requires a few tricks.
• Dealing with the initial thread is tricky. You need to make sure that an sthread_t struct is created for it at some point, so it can be scheduled like the other threads. However, it wasn't created by your sthread_user_create() function. Remember that, while a thread is running, the state stored in the sthread_t is garbage (though it is probably important that you have a such a struct, so you've got some place to put the state when you want to stop the thread).
• Be very careful using any local variables after calling sthread_switch() as their values are quite likely different from what they were before (you're on a different stack).
• While globals are bad in general, there will be places in this assignment where they are necessary.
• To debug with gdb(1) or any other debugger, run the configure script with --disable-shared (and then re-run make from the top level). This disables shared-libraries, which cause lots of trouble for the debugger. When you run the project, gdb will stop each time you create a new thread with a message like "Program received signal SIGUSR1, User defined signal 1." This is OK - just type continue.

Part 2: Implement Mutexs and Condition Variables

For part 2, you'll use the thread system you wrote in part 1, extending it to provide support for mutexs (a.k.a. locks) and condition variables. Skeleton functions are again provided in lib/sthread_user.c. You need to:

• Design data structures for mutexes (struct _sthread_mutex) and condition variables (struct _sthread_cond).
• Implement the mutex operations (sthread_user_mutex_*()).
• Implement the condition variable operations (sthread_user_cond_*()).

Hints

• Figure out how to block a thread, making it wait on some queue. How do you get the calling thread? How do you switch out of it? How will you wind up back in it?
• Locks do not immediately switch threads when unlocked.

Every web server has the following basic algorithm:

1. Web server starts and listens (see listen(2)) for incoming connections.
2. A client (i.e. web browser) opens a connection.
3. The server accepts (see accept(2)) the connection.
4. The client sends an http request.
5. The server services the request by:
   1. Parsing the request.
   2. Finding the requested file.
   3. Reading the file.
   4. Sending a set of http headers, followed by the contents of the file, to the client.
   5. Closing the connection.
6. The client displays the file to the user.

The sioux webserver in the web directory impelements the server side of the above algorithm.

For part 3 you will make sioux into a multithreaded web server. It must use a thread pool approach; it should not create a new thread for each request. Instead, incoming requests should be distributed to a pool of waiting threads (this is to eleminate thread creation costs from your experimental data). Make sure your threads are properly and efficiently synchronized. Use the routines you implemented in part 2.

Note that you are implementing an application now. That means the only interface to the thread system that you should use is that described by sthread.h (as distributed in the tar.gz). Do not use functions internal to sthreads directly from sioux.

The webserver should accept the --sthread-pthread and --sthread-user arguments (see the provided routine sthread_parse_impl() in sthread.h). This will allow you to test with both user-level and kernel-level threads and compare.

You should also accept a command-line flag indicating how many threads to use.

In testing, you may encounter "Address already in use" errors. TCP connections require a delay before a given port can be reused, so simply waiting a minute or two should be sufficient.

Design Discussion

Experiment

For your analysis, design an experiment to determine the effectiveness of user-level threads as compared to kernel-level threads in your webserver. This experiment should be conducted in the standard scientific method. It should explore the performance of the server under a variety of different conditions chosen to confirm or deny various aspects of your hypothesis.

You should see how your web server responds (i.e. how good is its throughput) with various number of simultaneous clients and with various numbers of threads in the thread pool. Is there a maximum number of threads that is useful?

Include a presentation of your experimental design, data, analysis, and conclusions in your report.

Conclusions

Discuss conclusions you have reached from this project. What recomendations do you have for designers of thread systems? Can you recommend user threads or kernel threads? Why? What other features might be useful?

Using the Web Benchmark

This will be installed by the time you need it.

The WebStone web benchmark tool has been installed on spinlock and coredump as /cse451/projects/webclient. It measures the throughput and latency of a webserver under varying loads. It simulates a number of active clients, all sending requests in parallel (it does this by forking several times). Each client requests a set of files, possibly looping through that set multiple times. When the test is complete, each client outputs the connection delay, response time, bytes transfered, and throughput it observed (depending on the server, the clients may all observe very similar results, or the data may vary widely). The tool takes the following arguments:

• -n CLIENTS specify the number of parallel clients to simulate.
• -l LOOPS specify the number of times each client should fetch the files.
• -w SERVER specify the hostname of the webserver (when sioux is started, it prints a URL including the hostname).
• -p PORT specify the port number (when sioux is started, it prints a port number).
• -u URLLIST specify a file containin a list of URLs to fetch, one per line, each followed by a weight (e.g. 1).

Parts 1 and 2

If you have added any files, run tar -tzf simplethreads-X.X.tar.gz and check to make sure your new files are listed.

Part 3 and 4

Follow the same instructions as for parts 1 and 2. Turnin a final version of your code including any scripts or other files you used in your experiment by 4:00 PM Friday November 7. It should be submitted under the project name project2b.

You should turn in your report on paper in class on Friday, November 7