linux-kernel - Re: [PATCH 1/3] i/o bandwidth controller documentation

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Date:	Fri, 04 Jul 2008 17:35:58 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	ptb@....it.uc3m.es
CC:	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] i/o bandwidth controller documentation

Peter T. Breuer wrote:
>> +               Block device I/O bandwidth controller
> 
> How can this work?  You will limit the number of available buffer heads 
> per second?
> 
> Unfortunaely, the problem is the fs above the block device.  If the
> block device is artificially slowed then the fs will still happily allow
> a process to fill buffers forever until memory is full, while the block
> device continues to trickle the buffers away.
> 
> What one wants is for the fs buffering to be linked to the underlying
> block device i/o speed. One wants the rate at which fs buffers are
> filled to be no more than (modulu brief spurts) the rate  at which the
> device operates.
> 
> That way networked block devices have a chance of having some memory
> left to send the dirty buffers out to the net with. B/w limiting the
> device itself doesn't seem to me to do any good.
> 
> Peter

Peter,

I see your message only now, it seems you didn't add me in to or cc.

Anyway, I totally agree with you, but it seems there's a
misunderstanding here. The block device i/o bw controller *does*
throttling slowing down applications' requests and not the dispatching
of the already submitted i/o requests.

IMHO, for the same reason you pointed, delaying the dispatching of i/o
requests simply leads to an excessive page cache and buffers
consumption, because userspace apps dirty ratio is actually never
limited.

As reported in the io-throttle documentation:

"This controller allows to limit the I/O bandwidth of specific block
devices for specific process containers (cgroups) imposing additional
delays on I/O requests for those processes that exceed the limits
defined in the control group filesystem."

Do you think we can use a better wording to describe this concept?

-Andrea
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