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Date: -- (:)
From: John Max Skaller <skaller@o...>
Subject: Re: [Caml-list] ocaml-3.05: a performance experience
Mike Lin wrote:

>>I'm intrigued by the relationship between these two parsers. In
>>
> particular,
> 
>>note that my Felix compiler takes 'pull' code, and automatically
>>translates it
>>to the much more efficient 'push' model: ie. it switches the client/server
>>relationship around, a process I call control inversion.
>>
> 
> I'm just curious if you could elaborate on in what respect a push parser is
> "much more efficient"? 


Blocking IO using OS resources is very slow -- O(log N) is good.
User space dispatching is typically very fast: O(1) [using a hashtable]

So if you're parsing XML being read over the internet,
the push technology is superior -- if you can bypass
the high level socket interface (which is non-trivial :-(

>From my experience pull parsers tend to be easier to
> use because they don't impose an event-driven model on the driver program,


	Absolutely! That is why Felix exists. To allow one to
write 'pull' code, which does blocking reads of data, but have the
code translated to 'push' code, where a thread is resumed by a
dispatcher when the data is available.

	For some applications like telephony, where the number
of connections is rather large, event driven code is the ONLY option.
Unix OS are typically incapable of handling millions of threads, a user
space dispatcher has no trouble at all, even on a small Linux PC.
The reason is: client level code 'knows' the encoding of messages
and can sort out where to send it much faster than any generic
OS facilities can: OS schedulers are designed to run arbitrary
mixtures of programs, not millions of threads of the same application.


> and also it is trivial to build a push parser on top of a pull parser.


	Yep. Just write a dispatch loop :-)


-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850


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