Date:	Mon, 05 Feb 2007 16:27:44 -0800
From:	Scot McKinley <scot.mckinley@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	bert hubert <bert.hubert@...herlabs.nl>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Zach Brown <zach.brown@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-aio@...ck.org, Suparna Bhattacharya <suparna@...ibm.com>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [PATCH 2 of 4] Introduce i386 fibril scheduling


As Joel mentioned earlier, from an Oracle perspective, one of the key 
things we are looking for is a nice clean *common* wait point. We don't 
really care whether this common wait point is the old libaio:async-poll, 
epoll, or "wait_for_async". And if "wait_for_async" has the added 
benefit of scaling, all the better.

However, it is desirable for that common wait-routine to have the 
ability to return explicit completions, instead of requiring a follow-on 
call to some other query/wait for events/completions for each of the 
different type of async submissions done (poll, pid, i/o, ...). 
Obviously not a "must-have", but desirable.

It is also desirable (if possible) to have immediate completions (either 
immediate errs or async submissions that complete synchronously) 
communicated at submission time, instead of via the common wait-routine.

Finally, it is agreed that neg-errno is a much better approach for the 
return code. The threading/concurrency issues associated w/ the current 
unix errno has always been buggy area for Oracle Networking code.

Regards, -Scot 

Linus Torvalds wrote:

>On Mon, 5 Feb 2007, bert hubert wrote:
>  
>
>>>From my end as an application developer, yes please. Either make it
>>perfectly ok to have thousands of outstanding asynchronous system calls (I
>>work with thousands of separate sockets), or allow me to select/poll/epoll
>>on the "async fd".
>>    
>>
>
>No can do.
>
>Allocating an fd is actually too expensive, exactly because a lot of these 
>operations are supposed to be a few hundred ns, and taking locks is simply 
>a bad idea.
>
>But if you want to, we could have a *separate* "convert async cookie to 
>fd" so that you can poll for it, or something.
>
>I doubt very many people want to do that. It would tend to simply be nicer 
>to do
>
>	async(poll);
>	async(waitpid);
>	async(.. wait foranything else ..)
>
>followed by a
>
>	wait_for_async();
>
>That's just a much NICER approach, I would argue. And it automatically 
>and very naturally solves the "wait for different kinds of events" 
>question, in a way that "poll()" never did (except by turning all events 
>into file descriptors or signals).
>
>  
>
>>Alternatively, something like SIGIO ('SIGASYS'?) might be considered, but,
>>well, the fd might be easier.
>>    
>>
>
>Again. NO WAY. Signals are just damn expensive. At most, it would be an 
>option again, but if you want high performance, signals simply aren't very 
>good. They are also a nice way to make your user-space code very racy.
>
>  
>
>>In fact, perhaps the communication channel might simply *be* an fd. Queueing
>>up syscalls sounds remarkably like sending datagrams. 
>>    
>>
>
>I'm the first to say that file descriptors is the UNIX way, but so are 
>processes, and I think this is MUCH better done as a "process" interface. 
>In other words, instead of doing it as a filedescriptor, do it as a 
>"micro-fork/exec", and have the "wait()" equivalent. It's just that we 
>don't fork a "real process", and we don't exec a "real program", we just 
>exec a single system call.
>
>If you think of it in those terms, it all makes sense *without* any file 
>descriptors what-so-ever, and the "wait_for_async()" interface also makes 
>a ton of sense (it really *is* "waitpid()" for the system call).
>
>		Linus
>
>--
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>  
>

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