threading

[Roy Smith]

In article <mailman.9077.1397051723.18130.python-list at python.org>,
 Chris Angelico <rosuav at gmail.com> wrote:

> For instance, the above code assumes that print() won't block. You 
> might think "Duh, how can printing to the screen block?!?", but if 
> your program's output is being piped into something else, it most 
> certainly can :)

<nostalgia-mode>

Heh.  One day, a long time ago, I had to investigate why our Vax-11/750 
had crashed.  I vaguely recollect being called at home on a weekend and 
having to schlepp into work, but my memory may just be running in 
auto-story-embelishment mode.

Turns out, the machine hadn't really crashed, but it was hung.  The 
console was a LA-120 (http://tinyurl.com/mljyegv), on which was printed 
various log messages from time to time.  It had run out of paper, which 
was detected by the little out-of-paper microswitch, so it stopped 
printing.  When its input buffer got full, it sent a control-s, which 
tells the thing on the other end of the serial line to stop sending.  
Which of course caused the kernel tty driver output buffer to fill, 
which eventually caused all the print statements by the various system 
loggers to block, and eventually the whole mess ground to a halt.

I put in new paper.  The printer proceeded to spit out several hours 
worth of buffered log messages and the system picked up where it left 
off.

</nostalgia-mode>

At Songza, we've been using gevent to do asynchronous I/O.  It's an 
interesting concept.  Basically, you write your application code as you 
normally would, using blocking I/O calls.  Gevent then monkey-patches 
the heck out of the Python library to intercept every call that could 
possibly block and splice it into a asynchronous task scheduler 
framework.  The amazing thing is that it works.  It let us reduce the 
number of gunicorn worker processes we use by a factor of 6, and handle 
the same amount of traffic.

Of course, monkey-patching is black magic, and sometimes we get hit by 
really bizarre and difficult to track down bugs.  But, to go back to my 
"technology evolution" scale, gevent is somewhere between avant-garde 
and what the kewl kids are using (version 1.0 was released in December), 
so that shouldn't be too surprising.