TA Notes - Spring 2012

( Last Update - Apr 24, 2012 )

Lab0

• (Jan. 19) lab0 source tar ball updated:
  A system call 'stacktrace' was added to system/stacktrace.c and was also declared in include/prototypes.h
  This system call can be used to diagnose stack corruption.
  
  
• (Jan. 23) lab0 source tar ball updated:
  The code in the release source archive was cleaned up to prevent a TRAP which was being caused whenever xdone() is called (xdone is called when all the user created processes are done executing).
  Also fixed was a compiler warning which was the result of minor mistake in prototypes.h
  The code changes do not affect the content of lab0 as such, but it is STRONGLY RECOMMENDED that you use the new source archive (which is at the same place /u/u3/503/xinu-spring2012-x86.tar.gz)
  
  
Lab0 has been graded please check Instructions for accessing grades.

Lab1

• (Feb. 19) Bug in system/wakeup.c
  There is a minor bug in system/wakeup.c which affects the sleep/wakeup logic in the lab1 source archive (/u/u3/503/xinu-spring2012-x86.tar.gz).
  Current Code :

                                          void    wakeup(void)
                                          {
                                                  /* Awaken all processes that have no more time to sleep */
  
                                                  while (nonempty(sleepq) && (firstkey(sleepq) <= 0)) {
                                                          ready(dequeue(sleepq), RESCHED_NO);
                                                  }
  
                                                  if ( (slnonempty = nonempty(sleepq)) == TRUE ) {
                                                          sltop = &queuetab[firstkey(sleepq)].qkey;
                                                  }
                                                  resched();
                                                  return;
                                          }
                                  

  The bug is on the line which gets the new value of sltop, we should be dereferencing the firstid in the sleep queue and not the firstkey.
  Correct Code :
  

                                          void    wakeup(void)
                                          {
                                                  /* Awaken all processes that have no more time to sleep */
  
                                                  while (nonempty(sleepq) && (firstkey(sleepq) <= 0)) {
                                                          ready(dequeue(sleepq), RESCHED_NO);
                                                  }
  
                                                  if ( (slnonempty = nonempty(sleepq)) == TRUE ) {
                                                          sltop = &queuetab[firstid(sleepq)].qkey;
                                                  }
                                                  resched();
                                                  return;
                                          }
                                  

This bug will be fixed in the archives for future labs, but for the current lab, please fix this bug in your code

Lab2

For lab2 you need to get a fresh copy of the source archive from /u/u3/503/xinu-spring2012-x86.tar.gz if you have not already done so.

Lab3

Though the test cases given in x_tests.c and elsewhere seem to use the get_bs/release_bs set of calls, all the test cases used for grading Part 1 will expect that you have a proper implementation of the allocate_bs/open_bs/close_bs/deallocate_bs functions. For the correct order in which these need to be called, please check the Part 2 handout under the Backing Store Management section. Note that none of this information is new and is already mentioned in the handout. We have just reiterated this for clarification.

Important
read_bs and write_bs call rdsRead() and rdsWrite() correspondingly. The way the remote disk server works, both rdsRead and rdsWrite queue up requests in a list after checking if the required data is present in the cache. Now the operations of checking the cache and adding the requests may not be thread safe. So multiple processes doing read_bs or write_bs simultaenously can lead to race conditions. This can be fixed by having a single global semaphore which is initialized to a count of 1 in rdsInit. The code which checks the cache and appends requests in rdsRead() and rdsWrite() can then be enclosed in a wait()/signal() on this semaphore. Note : Make sure you call signal on this semaphore everywhere there is a "return" and make sure you do not have any blocking calls inside the wait()/signal() block.

Grades for lab3 have been released following is some information pertaining to the test cases and mark division :

test01 20 pts
test02 20 pts
test03 10 pts
test04 30 pts
test05 30 pts
test06 30 pts
test07 30 pts
test08 30 pts
test09 30 pts
test10 15 pts
test11 15 pts
test12 45 pts
test13 45 pts

test 12 does a stress test, and test 13 checks for FIFO replacement, I modified the code on the rdserver to print the block number for a write request, this way I could roughly evaluate that the blocks were written in FIFO order. Please check your grades and contact me via email if there are any discrepancies.
The comments will generally contain the test case that failed, what the test case does / or what your code did erroneously and the points deducted on each line for every test case that failed.

Accessing Grades

The grades for LabY, HWY and comments on the grading are available in text files. You can access these by doing the following from a terminal on any of the CS Linux/Unix machines :

    # cat /u/u3/503/labs/12spring/grades/XXX/labY.rpt

Where "XXX" stands for your CS username (the username you use to login to the CS machines) and "Y" stands for the lab number. For example if your username is "seerpini", your scores and comments for lab1 are at /u/u3/503/labs/12spring/grades/seerpini/lab1.rpt .
NOTE

• .rpt is just and extension, the .rpt files are just plain text files.
• The grade file is only accessible to the concerned student.