linux-kernel - Re: [RFC 2/2] x86_64, mm: Reinsert the absent memory

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Message-ID: <51C9D1D6.20405@sgi.com>
Date:	Tue, 25 Jun 2013 10:22:30 -0700
From:	Mike Travis <travis@....com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Nathan Zimmer <nzimmer@....com>, holt@....com, rob@...dley.net,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	yinghai@...nel.org, akpm@...ux-foundation.org,
	gregkh@...uxfoundation.org, x86@...nel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC 2/2] x86_64, mm: Reinsert the absent memory



On 6/25/2013 12:38 AM, Ingo Molnar wrote:
> 
> * Nathan Zimmer <nzimmer@....com> wrote:
> 
>> On Sun, Jun 23, 2013 at 11:28:40AM +0200, Ingo Molnar wrote:
>>>
>>> That's 4.5 GB/sec initialization speed - that feels a bit slow and the 
>>> boot time effect should be felt on smaller 'a couple of gigabytes' 
>>> desktop boxes as well. Do we know exactly where the 2 hours of boot 
>>> time on a 32 TB system is spent?
>>
>> There are other several spots that could be improved on a large system 
>> but memory initialization is by far the biggest.
> 
> My feeling is that deferred/on-demand initialization triggered from the 
> buddy allocator is the better long term solution.

I haven't caught up with all of Nathan's changes yet (just
got back from vacation), but there was an option to either
start the memory insertion on boot, or trigger it later
using the /sys/.../memory interface.  There is also a monitor
program that calculates the memory insertion rate.  This was
extremely useful to determine how changes in the kernel
affected the rate.

> 
> That will also make it much easier to profile/test memory init 
> performance: boot up a large system and run a simple testprogram that 
> allocates a lot of RAM.
> 
> ( It will also make people want to optimize the initialization sequence 
>   better, as it will be part of any freshly booted system's memory 
>   allocation overhead. )
> 
> Thanks,
> 
> 	Ingo
> 
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