Date: Mon, 23 Nov 2009 18:48:39 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>, Greg KH <gregkh@suse.de>
Cc: Mike Galbraith <efault@gmx.de>, Robert Swan <swan.r.l@gmail.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org
Subject: Re: [bisected] pty performance problem
Message-ID: <20091123174839.GC6717@elte.hu>
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 <20091123114845.GC25575@elte.hu>
 <20091123115949.29bcf89d@lxorguk.ukuu.org.uk>
 <20091123120409.GA32009@elte.hu>
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* Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> > > So you'd prefer to detect devices that are byte based or message based 
> > > by what method ?
> > 
> > I'd not delay the worklet by default - i.e. i'd do Mike's patch.
> 
> Certainly stuff like pty should not delay
> > 
> > Havent tested all effects of it though - do you have any estimation 
> > about negative effects from such a change? We do have hard numbers 
> > (latencies in the millisecs range) from the opposite direction and those 
> > numbers arent pretty.
> 
> On a PC I'm not too worried - we might burn a bit more CPU and Arjan
> might even manage to measure it somewhere. There is the theoretical bad
> case where we end up at 100% CPU because the irq, wake, process one char,
> irq wake, process one char sequence fits the CPU so we don't sleep.
> 
> Embedded might be more of a concern, the old behaviour comes from 386/486
> days with low CPU power.
> 
> USB doesn't worry me - USB devices generally have their own buffering
> algorithm and use a timer so that they batch data efficiently into USB
> buffers.
> 
> The drivers/serial layer is often run with low latency set anyway so that
> seems to be ok for the most part.
> 
> Give it a go, send the patch to the maintainer, try it in -next and see
> if anyone screams.

(Doh, i should have Cc:-ed Greg first time around - fixed that.)

	Ingo
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