Project 2 - User-Level Threads

Outline
For schedule, See main project page.

For this project, we assume that you will be working in the same groups as for Project 1.

Tasks:

  1. Implement a user-level thread manager.
  2. Add a set of common synchronization primitives to your thread package.
  3. Implement a simple synchronization problem.
  4. Add preemption to your thread package.
  5. Implement a multithreaded web server to test your thread package.
  6. Analyze your design and report test results.
Assignment Goals
Background
In the beginning (well, the relative beginning), there was UNIX. UNIX supported multiprogramming; that is, there could be multiple independent processes each running with a single thread of control.

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 Setup
Where to Work
You will be using the simplethreads package for this assignment. Since it does not involve modifying the kernel, this project does not require VMWare. The simplethreads package has been tested on various platforms. We will be testing your implementation on forkbomb, so you should make sure to test there before submitting. (We support simplethreads only on forkbomb.) You can do the bulk of your development on forkbomb, or on a native Linux lab workstation, or on some non-departmental Unix (i386) machine.

Please do not use attu.

Create Your Copy of the Project Files
The simplethreads distribution is available at forkbomb:/cse451/projects/simplethreads-1.X.tar.gz or by download. (Where X is the release number (31 as of 5/21/09). Use the latest version you find there, though.) To begin, untar the distribution. On forkbomb, this can be done by
[forkbomb] /cse451/LOGIN> tar -xvzf /cse451/projects/simplethreads-1.X.tar.gz
(Remember that /cse451/LOGIN is not backed up.) 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.{c,h} Support for creating new stacks and switching between them.
lib/sthread_switch_{powerpc,i386}.h Assembly functions for saving registers and switching stacks.
lib/sthread_queue.h A simple queue that you may find useful.
lib/sthread_preempt.h Support for generating timer interrupts and controlling them (see part 5)
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 webserver for part 4.
Configure the Build
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 build steps as simplethreads). In the simplethreads-1.X directory, run ./configure to generate an appropriately configured Makefile.
Build
The distributed code should compile and link. In the top directory, type 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 to the source, it is only necessary to repeat the last step (make).

Test
Run make check. The test programs in test/ will be run, along with an indicate for each whether it got the correct result. Success at this step is simply having the test programs execute at all -- they will all fail, because you haven't yet completed the implementation of simplethreads.
To Remember For Later

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. Because both provide the same interface (once you've completed parts 1 and 2, anyway), applications don't even know which version they're using. The configure script will accept a --with-pthreads argument to select the version in use (the default is user threads). To switch back and forth, re-run configure and then run make clean.

Summary
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 [--with-pthreads]
  5. make
  6. make check (Run the programs in test/. At this point, they should run, but not get correct results.)

To Add a Source File

If you add a new source file, do the following:

  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. The edited Makefile.am has to be processed by the automake tool before it can be used by make. From within the top-level directory:
    • On forkbomb: run autoreconf
  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 a new variable prog_SOURCES 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
  2. Follow steps 2-4 above.

To Add a New Arbitrary File

  1. Edit the Makefile.am in the directory containing your file. Add your file to the list EXTRA_DIST (see top-level Makefile.am for an example).
  2. Follow steps 2-4 above.
The Assignment
Part 1: Implement Thread Scheduling
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 routine to initialize your data structures (sthread_user_init()).
  3. A thread creation routine (sthread_user_create()).
  4. A thread destruction routine (sthread_user_exit()).
  5. A mechanism for a thread to voluntarily yield, allowing another thread to run (sthread_user_yield()).
  6. A mechanism for a (single) thread to wait for another to finish (sthread_user_join()).
  7. A simple non-preemptive thread scheduler.

The routines in sthread_ctx.h do all of the stack manipulation, register storing, and nasty stuff like that. You should be able to use them based on the semantics explained in that file. Rather than focusing on the details of that low level manipulation, this assignment focuses on managing the thread contexts. Viewed as a layered system, you need to implement the grey box below:

Simplethread Layers

At the top layer, applications use the sthread package (through the API defined in sthread.h). Immediately below that, sthread.c performs its function by using either the routines in sthread_pthread.c (a thin veneer over pthreads) or your implementation in sthread_user.c. (The choice depends on a switch you gave (or didn't give) when you ran configure before building the sthread library.) 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 yields/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 immediately 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.
  • 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 (eventually) 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) and there can be no more threads attempting to join with it. (At most one thread can join, and to do so it must have indicated that intention on the create call.) 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 mostly garbage (though it is probably important that you have an sthread_t, 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.
  • When debugging with gdb(1), you may see messages like [New Thread 1024 (LWP 18771)]. These messages refer to kernel threads.
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 mutexs (struct _sthread_mutex) and condition variables (struct _sthread_cond).
  • Implement the mutex operations (sthread_user_mutex_*()).
  • Implement the condition variable operations (sthread_user_cond_*()).

So far, your threads are non-preemptive, which gives you atomic critical sections.  For this part, get your synchronization primitives working with this non-preemptive assumption and start thinking about where the critical sections are (add comments if you find it useful).  When you add preemption, you will have to go back and add appropriate protection to critical sections.  The details about this are in part 4.

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? (Some of this is similar to what you did for join in Part 1.)
  • Locks do not immediately switch threads when unlocked.
  • When you finish this part, all tests in the test directory should work except for test-preempt.c
Part 3: Implement a Simple Synchronization Problem
There are several famous synchronization problems in computer science. For part 3, your job is to implement a "food services" problem, an instance of the better-known multiple-producer, multiple-consumer problem. There are N cooks (each a separate thread) that constantly produce burgers. We'll assume for debugging purposes that each burger has a unique id assigned to it at the time it is created. The cooks place their burgers on a stack. Each time a cook produces a burger, she prints a message containing the id of the burger justed cooked. There are M hungry students (each a separate thread) that constantly grab burgers from the stack and eat them. Each time a student eats a burger, she prints a message containing the id of the burger she just ate. Ensure, for health and sanity reasons, that a given burger is consumed at most once by at most one student.

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 your solution to this problem.

Place your solution in a new file called test-burgers.c in the test directory.  Make the program take 3 command-line parameters: N, M, and the total number of burgers to produce. For example, test-burgers 3 2 100 should simulate 3 cooks, 2 students, and 100 total burgers.

Hints
Part 4: Implement a Multithreaded Web Server
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 implements the server side of the above algorithm.

For this part of the project 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 eliminate thread creation costs from your experimental data). Make sure your threads are properly and efficiently synchronized. Use the routines you implemented in part 2.

Don't forget that the only interface to the thread system that you should use is that described by sthread.h. Do not use functions internal to sthreads directly from sioux.

You should 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.

Because we have not used asynchronous IO in sioux, it will be very difficult to obtain good performance using the user-level threads. In some cases, it may be difficult to even get correct behavior at all times (e.g., if no new requests are sent, existing requests may not be serviced at all). We recommend using kernel-level threads for testing this part (meaning you should configure your library with pthreads).

At this point you should perform a sanity-check by doing the "Run the Web Benchmark and Report the Results" portion of Part 6, below.

Part 5: Add Preemption
In this part, you will add preemption to your threads system. This part of the project is not a lot of work (it represents perhaps only 10% of the code you will write), but it's a little tricky. We've made it the last part of the project (aside from the "Report" part) so you won't get stuck on it and fail to get the multi-threaded web server running. (The multi-threaded web server does not require preemption.)

We provide you with:

  • A facility to generate timer interrupts
  • Primitives to turn interrupts on and off
  • Synchronization primitives atomic_test_and_set and atomic_clear

See sthread_preempt.h for more details on the functions you are given.

It is your job to:

  1. Add code that will run every time you get a timer interrupt
  2. Add synchronization to your implementation of threads, mutexes, and condition variables.

To initialize the preemption system, you must make a call to sthread_preemption_init, which takes two arguments: a function to run on every interrupt, and a period in microseconds, specifying how often to generate the timer interrupts. For example, sthread_preemption_init(func,50) will call func every 50 microseconds. You should add a call to sthread_preemption_init as the last line in your sthread_user_init() function. Make it so that the thread scheduler switches to a different thread on each interrupt.

The hard part will be figuring out where to add synchronization to your thread management routines. Think about what would happen if you were interrupted at various points in your code. For example, we don't want to be preempted when we're inside yield() and in the middle of switching to a different thread. The safest way to ensure this never happens is by disabling interrupts. You are provided with a function splx(int splval), where splx(HIGH) disables interrupts and splx(LOW) enables them. Here is an example: 

int oldvalue = splx(HIGH); // disable interrupts; put old interrupt 
                           // state(disable/enabled) into oldvalue
                           // {critical_section};
splx(oldvalue); // restore interrupts to original state
You are also provided with two other synchronization primitives: atomic_test_and_set and atomic_clear. See sthread_preempt.h for a usage example. These will be useful for less important critical sections, such as ready queue manipulation. Note that it is also necessary to synchronize your mutex and condition variable code. For example, in sthread_user_mutex_lock, you will want to use atomic_test_and_set for grabbing a lock.

A note: timer interrupts are set up so that they only fire when you are executing your code (either the user application or the thread library).  Interrupts occuring inside printf or other system functions will be dropped.  Also, interrupts in the critical assembly code inside sthread_switch are also dropped.  This simplifies your task greatly.

"Stress testing" is important. It's common to get this part of the project "90% correct." You forget to disable interrupts in just one or two situations. This may not show up with cursory testing. That's what "race conditions" are all about -- they're devilishly difficult to track down because they're timing-dependent. But eventually they will show up!

IMPORTANT! The code that we have provided to support preemption works correctly only on the x86 architecture! Do not attempt this portion of the assignment on a Mac or other non-x86 architecture!

Hints

  • Start by initializing preemption to run a function which only prints something out to the screen, and make it run every second (pass 1000000 to sthread_preemption_init). This will check that you indeed can receive interrupts.
  • If you disable interrupts in one place, make sure you reenable them on all code paths. This is a common cause if your application suddenly freezes and stops receiving interrupts. Be particularly careful with your scheduler (your yield function). You will probably want to disable interrupts for the whole time you are inside yield, and reenable them after you've completed sthread_switch. However, sthread_switch could actually return to two different places: to the next line after a call to sthread_switch and to your user thread starter function when you switch to a new user thread for the first time! Be sure to reenable interrupts in both places.
  • You should never execute any application code with interrupts disabled.
  • To aid in debugging, you can press ctrl-\ (ctrl-backslash) at any time while your application is running to see the total number of interrupts generated so far.
  • There is a test provided for you, test-preempt.c, for testing your library with preemption. It has no yield calls and relies solely on interrupts to make progress. Also, go back to the other tests and check that they still work as expected. In particular, you should see a bit of randomless introduced to your part 3 solution because of preemption.
  • It is up to you to set the interrupt period. Good values are around 10-50 microseconds.
  • To compare programs with and without preemption, it is useful to turn preemption off, which you can do by either commenting out your call to sthread_preemption_init, or doing a ./configure --without-preemption and then make clean and make from the shell.
Part 6: Report
Include the following in a report to be turned in electronically on the due date.   This should be at most 3 pages long.

Design Discussion

Briefly describe the design of your user-level threads, synchronization primitives, and webserver.  Mention any design decisions you had to make, and why you made them. 

Functionality

Does your implementation work?  Which parts work and which don't?  For the ones that don't work, how do you think you would fix them?

Run the Web Benchmark and Report the Results

Consider these two web server configurations:

  1. pthreads, 1 thread in thread pool
  2. pthreads, 5 threads in thread pool

Using the web benchmark described below, measure the throughput and response time for these two web servers using 1, 5, and 25 clients.  Each client should fetch the file emacs.html (found under /cse451/projects), so you should copy this file into your web/docs directory (which is the web server root directory) and include it in urls to pass to the webclient benchmark.  For best results, make sure you run sioux and webclient on different hosts  (for example, run sioux on forkbomb and webclient on attu - this is the only place where using attu is ok!).

Report these results in a few easy to understand graphs.   Explain your results.  How does the multithreaded aspect of the web server affect throughput?  What about response time?  How would web server performance be affected if you were to use your user threads? (You're welcome to try it and measure it, although this is not required.)

Conclusions

Discuss conclusions you have reached from this project. Was this a good experience?

Using the Web Benchmark

The WebStone web benchmark tool is installed on forkbomb as /cse451/projects/bin/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).

All of the above parameters are required. The URLLIST file should contain one relative URL (just the file name) per line followed by a space and then a number (the number is the weight representing how often to request the file relative to other files in the list - most likely, 1 is the value you want).

For example, to test a webserver on almond.cs.washington.edu, with two simulated clients each fetching the index.html file twice, one would run:

/cse451/projects/bin/webclient -w almond.cs.washington.edu -p 12703 -l 2 -n 2 -u ./urls

Where the file urls would contain the following single line:

/index.html 1
Turnin

Please turnin one copy of the project per group. It would help us if you select one group member to execute all turnins (only the last submission will be graded).

Parts 1, 2, 3

In your top-level simplethreads directory, run make dist. This will produce a file named simplethreads-1.X.tar.gz. Make a new directory called username where username is your CSE login and move simplethreads-1.X.tar.gz into that directory. Submit this directory using the turnin(1L) program under project name project2a. Turnin will not work on forkbomb, so you'll need to use attu.

Run tar tvzf simplethreads-1.X.tar.gz and check to make sure all simplethreads files, and any new files you have added, are listed.

Parts 4, 5, and 6

Follow the same instructions as above. Turnin a final version of your code, including any scripts or other files you used in part 6, as well as your report. It should be submitted under the project name project2b.