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

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Message-Id: <1172145159.3531.253.camel@laptopd505.fenrus.org>
Date:	Thu, 22 Feb 2007 12:52:39 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Ingo Molnar <mingo@...e.hu>, Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Zach Brown <zach.brown@...cle.com>,
	"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

> It is not a TUX anymore - you had 1024 threads, and all of them will be
> consumed by tcp_sendmsg() for slow clients - rescheduling will kill a
> machine.

I think it's time to make a split in what "context switch" or
"reschedule" means...

there are two types of context switch:

1) To a different process. This means teardown of the TLB, going to a
new MMU state, saving FPU state etc etc etc. This is obviously quite
expensive

2) To a thread of the same process. No TLB flush no new MMU state,
effectively all it does is getting a new task struct on the kernel side,
and a new ESP/EIP pair on the userspace side. If there is FPU code
involved that gets saved as well.

Number 1 is very expensive and that is what is really worrying normally;
number 2 is a LOT lighter weight, and while Linux is a bit heavy there,
it can be made lighter... there's no fundamental reason for it to be
really expensive.

-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

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