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Date: Thu, 21 Feb 2008 17:47:02 +0300
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Subject: Re: [PHP-DEV] Multi-threading
From: indeyets@gmail.com ("Alexey Zakhlestin")

On 2/21/08, Nathan Rixham <nrixham@gmail.com> wrote:

> Thankyou for taking the time to shed some light on the mater for me; I'd
>  missed the all vital "sharing resources" part of the concept, probably
>  because (if I'm correct in saying) php shares memory of
>  variables/classes/resources between parent and child processes until a
>  write occurs, at which point a new version of the aforementioned is
>  created. [please correct if wrong].

pcntl_fork() uses fork() call which is implemented in OS. Forked
process is a copy of parent process, which means, that it occupies
similiar amount of memory. Each time you "fork" your memory usage gets
bigger

>  Would I therefore be correct in assuming the following:
>
>  In PHP we can create mutli-process(tasking) applications that have most
>  of the benefits of multi-threaded applications and less bugs.
>
>  I'm working on the premise that a processor can only run so fast, so
>  multithreading say 100k calculations would not speed up and application,
>  if anything it would add overhead, and slow it down. The only way to
>  speed up such an application would be to throw more hardware at the
>  problem (be it servers or processors).

If your task is CPU-bound, you can't do much, but give it more hadrware.
(we are not talking about optimization of algorithms here)

>  Meaning that the only time multi-process/multi-threading would have any
>  positive bearing on an application would be when dealing with things
>  such as external resources, due to the slower access times of these
>  external resources we find threads/processes "waiting" for the resource
>  to be usable (fully downloaded/opened) - in such a situation
>  multi-threading has it's distinct advantages.
>
>  However forking an app (?multi-process) can already achieve the same
>  results, the only difference being that we may have a few duplicate
>  classes/variables around the application doing it this way (subject to
>  us not taking the time to define all "shared" resources in the parent
>  process.

Multithreading can be dramatially good for memory-bound tasks.
It is possible to say (although, this is oversimplification), that
threads are "light processes".

They allow to execute a lot of parallel tasks with less overhead

>  If I'm correct in all of the above, then I'd personally think that we
>  don't need multithreading in PHP, however a neat(er) predefined method
>  of interprocess communication would be beneficial.


-- 
Alexey Zakhlestin
http://blog.milkfarmsoft.com/