linux-kernel - Re: [bisected] pty performance problem

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Date:	Mon, 23 Nov 2009 18:48:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...e.de>
Cc:	Mike Galbraith <efault@....de>, Robert Swan <swan.r.l@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [bisected] pty performance problem


* Alan Cox <alan@...rguk.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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