On Wed, Oct 13, 2004 at 09:30:07 -0000, matrix reloaded wrote:
> Actually intervention of schedular comes for kernel threads and not for the use
> level threads. Ok try to do this thing if you have some machine with 2.4.18 :-
> Start the program using those 2 pthreads and then do ps -aef for that process, you
> will see that there are 4 processes with the same name, but with different PIDs. On
> 2.5 this thing was removed as we moved from Linux Legacy Thread to NPTL
> thing... Explaination of this is first pid process is the main program, 2nd is the
> thread manager and 3rd & 4th are the created threads. Here, you can see that these
> threads are visible to the OS, so you can send signals to them as you send to a
> process, but 2.4.20 onwards this thing has been changed and now you can't see 4
> PIDs, because now actual use-level thread implementation has been done. OS can't
> see individual threads inside a program, so is schedular. The time slot is given to
> the whole process [whithin which these threads are residing]. It's the responsibility
> of the thread library manager to divide this time slot among individual threads.
> Schedular doesn;t have to worry about them...
1) Kernel can't replace kernel-threads with user-threads. Libc (since
libpthreads are part of it) can do it.
2) User-threads are mostly useless. When one thread blocks, the whole
process does. And it can't be run in parallel on SMP.
3) There is another thing kernel can do -- it can change semantics of
/proc to be by-pid and not by-tid. While each thread has different
tid, it has the same pid (pthreads want it). If ps does not see tids,
it can't show them...
Note: If you want to try out, declare the gettid syscall (see man
gettid) and call it in a threaded application. Since the __syscall0
macro expands to an inline function with just asm("int 0x80", ...), the
library won't be able to fake it and you will know whether you run
kernel-threads or user-threads.
-------------------------------------------------------------------------------
Jan 'Bulb' Hudec <bulb@xxxxxx>
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