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

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Date:	Thu, 15 Feb 2007 09:39:33 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	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>,
	Zach Brown <zach.brown@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 05/11] syslets: core code

On Thu, 15 Feb 2007, Evgeniy Polyakov wrote:

> On Thu, Feb 15, 2007 at 09:05:13AM -0800, Davide Libenzi (davidel@...ilserver.org) wrote:
> > 
> > I actually think that building chains of syscalls bring you back to a 
> > multithreaded solution. Why? Because suddendly the service thread become 
> > from servicing a syscall (with possible cachehit optimization), to 
> > servicing a whole session. So the number of service threads needed (locked 
> > down by a chain) becomes big because requests goes from being short-lived 
> > syscalls to long-lived chains of them. Think about the trivial web server, 
> > and think about a chain that does open->fstat->sendhdrs->sendfile after an 
> > accept. What's the difference with a multithreaded solution that does 
> > accept->clone and execute the above code in the new thread? Nada, NIL.
> 
> That is more ideological question about micro-thread design at all.
> If syslet will be able to perform only one syscall, one will have 4
> threads for above case, not one, so it is even more broken.

Nope, just one thread. Well, two, if you consider the "main" dispatch 
thread, and the syscall service thread.



> So, if Linux moves that way of doing AIO (IMO incorrect, I think that
> the correct state machine made not of syscalls, but specially crafted
> entries - like populate pages into VFS, send chunk, recv chunk without
> blocking and continue on completion and the like), syslets with attached 
> state machines are the (smallest evil) best choice.

But at that point you don't need to have complex atom interfaces, with 
chains, whips and leather pants :) Just code it in C and submit that to 
the async engine. The longer is the chain though, the closer you get to a 
fully multithreaded solution, in terms of service thread consuption. And 
what do you save WRT a multithreaded solution? Not thread 
creation/destroy, because that cost is fully amortized inside the chain 
execution cost (plus a pool would even save that).
IMO the plus of a generic async engine is mostly from a kernel code 
maintainance POV. You don't need anymore to have AIO-aware code paths, 
that automatically transalte to smaller and more maintainable code.



- Davide


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