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

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Message-ID: <20091123133423.64f34c73@lxorguk.ukuu.org.uk>
Date:	Mon, 23 Nov 2009 13:34:23 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
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

> > 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.

Alan
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