linux-kernel - Re: [PATCH 0/2] x86: Speed up ioremap operations

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Date:	Wed, 27 Aug 2014 16:15:28 -0700
From:	Mike Travis <travis@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
	msalter@...hat.com, dyoung@...hat.com, riel@...hat.com,
	peterz@...radead.org, mgorman@...e.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/2] x86: Speed up ioremap operations



On 8/27/2014 4:06 PM, Andrew Morton wrote:
> On Wed, 27 Aug 2014 17:59:27 -0500 Mike Travis <travis@....com> wrote:
> 
>>
>> We have a large university system in the UK that is experiencing
>> very long delays modprobing the driver for a specific I/O device.
>> The delay is from 8-10 minutes per device and there are 31 devices
>> in the system.  This 4 to 5 hour delay in starting up those I/O
>> devices is very much a burden on the customer.
> 
> That's nuts.

Exactly!  The customer was (as expected) not terribly pleased... :)
> 
>> There are two causes for requiring a restart/reload of the drivers.
>> First is periodic preventive maintenance (PM) and the second is if
>> any of the devices experience a fatal error.  Both of these trigger
>> this excessively long delay in bringing the system back up to full
>> capability.
>>
>> The problem was tracked down to a very slow IOREMAP operation and
>> the excessively long ioresource lookup to insure that the user is
>> not attempting to ioremap RAM.  These patches provide a speed up
>> to that function.
> 
> With what result?
> 

Early measurements on our in house lab system (with far fewer cpus
and memory) shows about a 60-75% increase.  They have a 31 devices,
3000+ cpus, 10+Tb of memory.  We have 20 devices, 480 cpus, ~2Tb of
memory.  I expect their ioresource list to be about 5-10 times longer.
[But their system is in production so we have to wait for the next
scheduled PM interval before a live test can be done.]

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