threading

[Roy Smith]

In article <mailman.9078.1397053837.18130.python-list at python.org>,
 "Frank Millman" <frank at chagford.com> wrote:

> "Chris Angelico" <rosuav at gmail.com> wrote in message 
> news:CAPTjJmqwhb8O8vq84mMTv+-Rkc3Ff1AQDXe5cs8Y5gY02kHyNg at mail.gmail.com...
> > On Wed, Apr 9, 2014 at 11:23 PM, Frank Millman <frank at chagford.com> wrote:
> >
> >> How does one distinguish betwen 'blocking' and 'non-blocking'? Is it
> >> either/or, or is it some arbitrary timeout - if a handler returns within
> >> that time it is non-blocking, but if it exceeds it it is blocking?
> >
> > No; a blocking request is one that waits until it has a response, and
> > a non-blocking request is one that goes off and does something, and
> > then comes back to you when it's done.
> 
> Does reading from disk count as blocking? Strictly speaking I would have 
> thought 'yes'.

Of course it does.  But, the bigger question is, "What counts as reading 
from disk?"

In the old days, all Unix system calls were divided up into two groups, 
based on whether they were "fast" or "slow".  Processes executing a 
"fast" system call would block, and could not be interrupted; i.e. any 
signals delivered to them would be queued up and delivered after the 
system call had finished.  Typically, that meant, if you typed 
Control-C, your process wouldn't get killed until the system call it was 
executing completed.

Disk reads were considered fast.  You type Control-C, the read takes 
another few ms to finish, then your process gets whacked.  You never 
even notice the delay.  But, a read on a tty was slow.  It would sit 
there forever, until you hit return.  Slow system calls got interrupted.

Then, along came the network, and everything got confusing.  If I open a 
file that lives on an NFS server, and read from it, am I doing a disk 
read?  Should I be able to interrupt an NFS operation?