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

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Date:	Tue, 27 Feb 2007 11:49:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	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>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	"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

* Evgeniy Polyakov <johnpol@....mipt.ru> wrote:

> > does that work for you?
> 
> Yes, -fomit-frame-point make the deal.
> 
> In average, threadlet runs as fast as epoll.

yeah.

> Just because there are _no_ rescheduling in that case.

in my test it was 'little', not 'no'. But yes, that's exactly my point: 
we can remove the nonblock hackeries from event loops and just 
concentrate on making it schedule in less than 10-20% of the cases. Even 
a relatively high 10-20% rescheduling rate is hardly measurable with 
threadlets, while it gives a 10%-20% regression (and possibly bad 
latencies) for the pure epoll/kevent server.

and such a mixed technique is simply not possible with ordinary 
user-space threads, because there it's an all-or-nothing affair: either 
you go fully to threads (at which point we are again back to a fully 
threaded design, now also saddled with event loop overhead), or you try 
to do user-space threads, which Just Make Little Sense (tm).

so threadlets remove the biggest headache from event loops: they dont 
have to be '100% nonblocking' anymore. No O_NONBLOCK overhead, no 
complex state machines - just handle the most common event type via an 
outer event loop and keep the other 99% of server complexity in plain 
procedural programming. 1% of state-machine code is perfectly 
acceptable.

	Ingo
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