linux-kernel - Re: [PATCH] drivers/char/hvc_console.c: reduce max idle timeout

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Date:	Mon, 1 Nov 2010 22:21:15 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	<michael@...erman.id.au>
CC:	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] drivers/char/hvc_console.c: reduce max idle timeout

On 11/1/2010 8:35 PM, Michael Ellerman wrote:
> On Wed, 2010-10-27 at 12:54 -0400, Chris Metcalf wrote:
>> The tile architecture uses this framework for our serial console,
>> and our users complain that the delay of up to two seconds feels like
>> the machine has gone non-responsive and is disturbing.  By contrast,
>> a delay of up to half a second feels like just the normal sort of
>> delay caused by swapping, network lag, etc. and is not noticeable.
>> The overhead is obviously not much greater.
> It's 4 times greater.
>
> We picked 2 seconds because it gave a reasonable trade off between
> responsiveness and load. I'm not convinced that half a second is a
> better number.

Perhaps the tradeoff should be tunable, then?  I think on our architecture
we're willing to pay a higher cost on the core running this task, since we
have many cores; we often have a core that mostly just runs miscellaneous
Linux administrative tasks anyway, so adding a bit more overhead there is
not significant for us.

This issue has caused multiple reports of unresponsiveness from our users,
so I'd be interested in finding a way to strike a balance.  We could use a
config option defaulting to 2 seconds (for example), or something more
dynamic (probably unnecessary).

Let me know your preference, if this sounds plausible, and I'll write up a
proposed patch.  Thanks.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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