linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Thu, 22 Feb 2007 13:23:57 -0800
From:	Zach Brown <zach.brown@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Ulrich Drepper <drepper@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.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>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

> Plus there's the fundamental killer that KAIO is a /lot/ harder to
> implement (and to maintain) on the kernel side: it has to be  
> implemented
> for every IO discipline, and even for the IO disciplines it  
> supports at
> the moment, it is not truly asynchronous for things like metadata
> blocking or VFS blocking. To handle things like metadata blocking  
> it has
> to resort to non-statemachine techniques like retries - which are bad
> for performance.

Yes, yes, yes.

As one of the poor suckers who has been fixing bugs in fs/aio.c and  
fs/direct-io.c, I really want everyone to read Ingo's paragraph a few  
times.  Have it printed on a t-shirt.

Look at the number of man-years that have gone into fs/aio.c and fs/ 
direct-io.c.  After all that effort it *barely* supports non-blocking  
O_DIRECT IO.

The maintenance overhead of those two files, above all else, is what  
pushed me to finally try that nutty fibril attempt.

> Syslets/threadlets on the other hand, once the core is implemented,  
> have
> near zero ongoing maintainance cost (compared to KAIO pushed into  
> every
> IO subsystem) and cover all IO disciplines and API variants  
> immediately,
> and they are as perfectly asynchronous as it gets.

Amen.

As an experiment, I'm working on backing the sys_io_*() calls with  
syslets.  It's looking very promising so far.

> So all in one, i used to think that AIO state-machines have a long- 
> term
> place within the kernel, but with syslets i think i've proven myself
> embarrasingly wrong =B-)

Welcome to the party :).

- z
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