linux-kernel - Re: [patch 05/11] syslets: core code

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Date:	Wed, 14 Feb 2007 18:52:46 -0800
From:	Zach Brown <zach.brown@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 05/11] syslets: core code

>> But the whole point is that the notion of a "register" is wrong in  
>> the
>> first place. [...]
>
> forget about it then. The thing we "register" is dead-simple:
>
>  struct async_head_user {
>          struct syslet_uatom __user              **completion_ring;
>          unsigned long                           ring_size_bytes;
>          unsigned long                           max_nr_threads;
>  };
>
> this can be passed in to sys_async_exec() as a second pointer, and the
> kernel can put the expected-completion pointer (and the user ring idx
> pointer) into its struct atom. It's just a few instructions, and  
> only in
> the cachemiss case.
>
> that would make completions arbitrarily split-up-able. No registration
> whatsoever. A waiter could specify which ring's events it is  
> interested
> in. A 'ring' could be a single-entry thing as well, for a single
> instance of pending IO.

I like this, too.  (Not surprisingly, having outlined something like  
it in a mail in one of the previous threads :)).

I'll bring up the POSIX AIO "list" IO case.  It wants to issue a  
group of IOs and sleep until they all return.  Being able to cheaply  
instantiate a ring implicitly with the submission of the IO calls in  
the list will make implementing this almost too easy.  It'd obviously  
just wait for that list's ring to drain.

I hope.  There might be complications around the edges (waiting for  
multiple list IOs to drain?), but it seems like this would be on the  
right track.

I might be alone in caring about having a less ridiculous POSIX AIO  
interface in glibc, though, I'll admit.  It seems like it'd be a  
pretty sad missed opportunity if we rolled a fantastic general AIO  
interface and left glibc to still screw around with it's own manual  
threading :/.

- z
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