Re: wait queue process state

[Terje Eggestad]

Well, the only reason I'm aware of is that your have system calls that
can't be interrupted, and sys calls that can (and when they do they
return -1 and errno=EINTR). 

The number of syscall that actually can return EINTR is actually fairly
small. And some like read()/write() may or may not depending on the type
of file. If the file is a socket it usually can, but if it's a regular
file it can't. 

A regular file access is as you uninteruptable as you point out, and to
top it of it can be made nonblocking. The reason is that a regular file
IO is deterministic, it must succeed or fail ASAP, and if it fail it's
either a full FS, or a corrupt FS or a HW failure. (The latter two req
reboot/HW repair, the first is a "normal" case). Socket IO is
nondeterministic and we need a way to interrupt it. 

I guess deterministic/nondeterministic activities is one way to describe
either a kernel task is interruptable or nor. Another rule of thumb is
that you're noninteruptable if you're waiting for HW to complete a task.
(If you don't want to wait, you should have done it async anyway). 
You're interruptable if you wait for another program to do a task.

Make sense?

TJ
 

On Mon, 2002-05-27 at 23:11, Joseph Cordina wrote:
> Hi,
>    I am quite new to this list and thus does not know if this question 
> has been answered many a times. I have looked in the archive but could 
> not find it. Here goes anyway:
>      I realised that when processes are placed in the wait queue, they 
> are set at either INTERRUPTIBLE or NONINTERRUPTIBLE. I also noticed that 
> something like file access is set as NONINTERRUPTIBLE. Could someone 
> please tell me the reason for having these two states. I can understand 
> that INTERRUPTIBLE can be made to be interrupted by a timer or a signal 
> and vice versa for UNTERRUPTIBLE. Yet what makes blocking system calls 
> as INTERRUPTIBLE or NONINTERRUPTIBLE. Also why is file access considered 
> as NONINTERRUPTIBLE.
> 
> In addition, inside the kernel running, are these two different states 
> treated differently (apart from the allowance to be interrupted or 
> otherwise).
> 
> The reason I am asking is that I am working on scheduler activations 
> which allow new kernel threads to be created when a kernel thread blocks 
> inside the kernel. Yet this only works for INTERRUPTIBLE processes, I 
> was thinking of making it work also for NONINTERRUPTIBLE processes. Just 
> wondering if this would have any repurcusions. Also when a process 
> generates a page fault which causes a page to be retreived from the 
> filesystem, it such a process placed in the wait queue as 
> NONINTERRUPTIBLE also ?
> 
> Cheers
> 
> Joseph Cordina
> University of Malta
> 
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Terje Eggestad                  mailto:[e-mail]
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