09-09-2008
Process based code vs. Thread based code
I am just wondering on which is the best way to write programs on UNIX. Which one is better from below:
a) Spawning threads per client connection/request?
b) fork-exec new processes per client connection/request?
Assume that I am doing some database system on linux which is supposed to run on unix systems as well.
The basic differences as I know are:
Using processes - have it's own address space, communicate using SysV/POSIX shared memory + semaphores etc & context switching is costly. Also implementing processes instead of threads is that process are more stable and reliable than threads. Just because if one thread malfunctions, the entire process would break.
On the other side, using thread the context switching is less cheaper compared to that of process context switches. No need for shared memory as the memory is shared and accessible by all threads with in the process & hence can avoid costly IPC mechanism's like SysV/POSIX shared memory and semaphores. Instead use simple mutexes and condition variables.
Being said all the above, I am still not in a position to decide up on which way to go i.e. use threads or use processes?
Please help.
Regards
Kalyan
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FORK(2) BSD System Calls Manual FORK(2)
NAME
fork -- create a new process
SYNOPSIS
#include <unistd.h>
pid_t
fork(void);
DESCRIPTION
fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for
the following:
o The child process has a unique process ID.
o The child process has a different parent process ID (i.e., the process ID of the parent process).
o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that,
for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the
child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish
standard input and output for newly created processes as well as to set up pipes.
o The child processes resource utilizations are set to 0; see setrlimit(2).
RETURN VALUES
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
indicate the error.
ERRORS
fork() will fail and no child process will be created if:
[EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-
dependent.
[EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would
be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
The include file <sys/types.h> is necessary.
SEE ALSO
execve(2), sigaction(2), wait(2), compat(5)
HISTORY
A fork() function call appeared in Version 6 AT&T UNIX.
CAVEATS
There are limits to what you can do in the child process. To be totally safe you should restrict yourself to only executing async-signal
safe operations until such time as one of the exec functions is called. All APIs, including global data symbols, in any framework or library
should be assumed to be unsafe after a fork() unless explicitly documented to be safe or async-signal safe. If you need to use these frame-
works in the child process, you must exec. In this situation it is reasonable to exec yourself.
4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution