Subject: Re: [ANN] Userspace M-on-N threading model implementation. Alpha
	release.
From: Arjan van de Ven <arjan@infradead.org>
To: Bill Davidsen <davidsen@tmr.com>
Cc: Jakub Jelinek <jakub@redhat.com>, linux-kernel@vger.kernel.org
In-Reply-To: <45C65353.5000602@tmr.com>
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Content-Type: text/plain
Organization: Intel International BV
Date: Mon, 05 Feb 2007 07:14:18 +0100
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> > On the other side, the overhead you need to add for every single syscall
> > that might block for the M:N threads and the associated complications
> > which make it far harder to conform to POSIX IMHO far outweight the costs
> > of going into the kernel for a context switch.
> 
> That really wasn't my question, Arjan said that switching real threads 
> wasn't a context switch in the hardware sense, and I was asking if I 
> missed something.

a hardware context switch is basically a CR3 change with associated tlb
flush. That is the part that is the most expensive of a context switch.
Just going into the kernel and getting out with a different EIP/ESP is
really cheap, in the order of "a few hundred cycles"; not a heck of a
lot more expensive than a simple getpid or other simple system call.


>  It may be cheap, but it would seem to be a context 
> switch none-the-less.

it includes a privilege level switch, not so much a full context
switch...

-- 
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