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

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Date:	Wed, 28 Feb 2007 08:17:01 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <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>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

On Wed, 28 Feb 2007, Ingo Molnar wrote:

> 
> * Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > Why can't aio_* be implemented with *simple* (or parallel/unrelated) 
> > syscall submit w/out the burden of a complex, limiting and heavy API
> 
> there are so many variants of what people think 'asynchronous IO' should 
> look like - i'd not like to limit them. I agree that once a particular 
> syslet script becomes really popular, it might (and should) in fact be 
> pushed into a separate system call.
> 
> But i also agree that a one-shot-syscall sys_async() syscall could be 
> done too - for those uses where only a single system call is needed and 
> where the fetching of a single uatom would be small but nevertheless 
> unnecessary overhead. A one-shot async syscall needs to get /8/ 
> parameters (the syscall nr is the seventh parameter and the return code 
> of the nested syscall is the eighth). So at least two parameters will 
> have to be passed in indirectly and validated, and 32/64-bit compat 
> conversions added, etc. anyway!

At this point, given how threadlets can be easily/effectively dispatched 
from userspace, I'd argue the presence of either single/parallel or syslet 
submission altogether. Threadlets allows you to code chains *way* more 
naturally than syslets, and since they basically are like functions calls 
in the fast path, they can be used even for single/parallel submissions. 
No compat code required (ok, besides the trivial async_wait).
My point is, the syslet infrastructure is expensive for the kernel in 
terms of compat, and extra code added to handle the cond/jumps/etc. Is 
also non-trivial to use from userspace. Are those big performance 
advantages there to justify its existence? I doubt that the price of a 
sysenter is a lot bigger than a atom decoding, but I'm looking forward in 
being proven wrong by real life performance numbers ;)

- Davide

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