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Full Discussion: Controlling child processes
Top Forums Programming Controlling child processes Post 50239 by DreamWarrior on Monday 19th of April 2004 06:26:03 PM
Old 04-19-2004
Quote:
Originally posted by forumGuy
It is actually a Operating system course and all the major topics mentioned we have studied; on an academic level I understand it, however implementation is something different. In regards to deadlocks I do not know if this helps, however the Bankers algorithm may be a start (lots of overhead and predefined info is needed). Do not beat yourself up about the deadlock issue even the big boys (Oracle) occasionally end up in this state and most Operating systems including UNIX ignore it. Do you know the type of deadlock that is being encountered i.e. mutual exclusion, hold and wait, circular waiting or non preemptive (Studied for a final, it is fresh in the noggin)?
FI
Actually I myself am not encountering any problems with deadlocks. Although I understand them quite well, and am familar with the banker's algorithm. However, I'm lucky in that I don't need it because I am in complete control of my resources and can simply generate (and strickly follow) locking rules that guarentee I won't deadlock.

In my case, this is simple. In the case of an OS or DB, it's not so simple because you're not in complete control over what the end interface (process/client) will request or do with the resources. Funny enough that in these circumstances the Banker's Algorithm is generally of no help because most of the time the OS/DB has no knowledge of what resources are going to (or ever will be) requested by the processes/clients that it is scheduling. Therefore, this renders the Banker's Algorithm useless, as it requires all processes to notify the scheduler of all its resource requirements before entering a "critical section" and unless the OS/DB places strict requirements on its interface (which they'd prefer not to do) they can deadlock and there's nothing that can be done about it besides detect it and tell the application that it must correct it.

In *nix, the EDEADLK errno is set in response to the failed system call that generated the potential deadlock. The application then can either die, release resources and try again, or just spin trying infiniately and hoping that the condition is corrected (which is unlikely).

In Oracle, the deadlock is detected and broken by automatically "rolling back" the client having less work done at the time of detection. Afterwhich, the client can attempt to re-do all its work from the beginning again.

Neither of these are "faults" of the OS/DB, but just inherient issues due to the fact that they can not predict the resource usage of their clientelle (ironic in that Oracle's name suggests that it should be capable of doing so however Smilie.)

Anyway, be greatful that your OS course is actually making you implement or play with these things in code. Theory means little without any hands on knowledge. In my opinion, too many schools teach theory without ever giving students practical work that makes use of it.
 

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KILL(1) 							   User Commands							   KILL(1)

NAME
kill - terminate a process SYNOPSIS
kill [-s signal|-p] [-q sigval] [-a] [--] pid... kill -l [signal] DESCRIPTION
The command kill sends the specified signal to the specified process or process group. If no signal is specified, the TERM signal is sent. The TERM signal will kill processes which do not catch this signal. For other processes, it may be necessary to use the KILL (9) signal, since this signal cannot be caught. Most modern shells have a builtin kill function, with a usage rather similar to that of the command described here. The '-a' and '-p' options, and the possibility to specify processes by command name are a local extension. If sig is 0, then no signal is sent, but error checking is still performed. OPTIONS
pid... Specify the list of processes that kill should signal. Each pid can be one of five things: n where n is larger than 0. The process with pid n will be signaled. 0 All processes in the current process group are signaled. -1 All processes with pid larger than 1 will be signaled. -n where n is larger than 1. All processes in process group n are signaled. When an argument of the form '-n' is given, and it is meant to denote a process group, either the signal must be specified first, or the argument must be preceded by a '--' option, otherwise it will be taken as the signal to send. commandname All processes invoked using that name will be signaled. -s, --signal signal Specify the signal to send. The signal may be given as a signal name or number. -l, --list [signal] Print a list of signal names, or convert signal given as argument to a name. The signals are found in /usr/include/linux/signal.h -L, --table Similar to -l, but will print signal names and their corresponding numbers. -a, --all Do not restrict the commandname-to-pid conversion to processes with the same uid as the present process. -p, --pid Specify that kill should only print the process id (pid) of the named processes, and not send any signals. -q, --queue sigval Use sigqueue(2) rather than kill(2) and the sigval argument is used to specify an integer to be sent with the signal. If the receiving process has installed a handler for this signal using the SA_SIGINFO flag to sigaction(2), then it can obtain this data via the si_value field of the siginfo_t structure. NOTES
It is not possible to send a signal to explicitly selected thread in a multithreaded process by kill(2) syscall. If kill(2) is used to send a signal to a thread group, then kernel selects arbitrary member of the thread group that has not blocked the signal. For more details see clone(2) CLONE_THREAD description. The command kill(1) as well as syscall kill(2) accepts TID (thread ID, see gettid(2)) as argument. In this case the kill behavior is not changed and the signal is also delivered to the thread group rather than to the specified thread. SEE ALSO
bash(1), tcsh(1), kill(2), sigvec(2), signal(7) AUTHOR
Taken from BSD 4.4. The ability to translate process names to process ids was added by Salvatore Valente <svalente@mit.edu>. AVAILABILITY
The kill command is part of the util-linux package and is available from Linux Kernel Archive <ftp://ftp.kernel.org/pub/linux/utils/util- linux/>. util-linux March 2013 KILL(1)
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