linux-kernel - Re: [RFC 0/2] Delay initializing of large sections of memory

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Date:	Fri, 21 Jun 2013 12:18:21 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Greg KH <gregkh@...uxfoundation.org>, <holt@....com>,
	<travis@....com>, <rob@...dley.net>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <yinghai@...nel.org>,
	<akpm@...ux-foundation.org>, <x86@...nel.org>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 0/2] Delay initializing of large sections of memory

On 06/21/2013 12:03 PM, H. Peter Anvin wrote:
> On 06/21/2013 09:51 AM, Greg KH wrote:
>> On Fri, Jun 21, 2013 at 11:25:32AM -0500, Nathan Zimmer wrote:
>>> This rfc patch set delays initializing large sections of memory until we have
>>> started cpus.  This has the effect of reducing startup times on large memory
>>> systems.  On 16TB it can take over an hour to boot and most of that time
>>> is spent initializing memory.
>>>
>>> We avoid that bottleneck by delaying initialization until after we have
>>> started multiple cpus and can initialize in a multithreaded manner.
>>> This allows us to actually reduce boot time rather then just moving around
>>> the point of initialization.
>>>
>>> Mike and I have worked on this set for a while, with him doing the most of the
>>> heavy lifting, and are eager for some feedback.
>> Why make this a config option at all, why not just always do this if the
>> memory size is larger than some specific number (like 8TB?)
>>
>> Otherwise the distros will always enable this option, and having it be a
>> configuration choice doesn't make any sense.
>>
> Since you made it a compile time option, it would be good to know how
> much code it adds, but otherwise I agree with Greg here... this really
> shouldn't need to be an option.  It *especially* shouldn't need to be a
> hand-set runtime option (which looks quite complex, to boot.)
The patchset as a whole is just over 400 lines so it doesn't add alot.
If I were to pull the .config option it would probably remove 30 lines.

The command line option is too complex but some of the data I haven't 
found a way
to get at runtime yet.


>
> I suspect the cutoff for this should be a lot lower than 8 TB even, more
> like 128 GB or so.  The only concern is to not set the cutoff so low
> that we can end up running out of memory or with suboptimal NUMA
> placement just because of this.
Even at lower amounts of ram there is an positive impact.I it knocks 
time off
boot even at as small as a 1TB of ram.

> Also, in case it is not bloody obvious: whatever memory the kernel image
> was loaded into MUST be considered "online", even if it is loaded way high.
>
> 	-hpa
>
>
>
>


Ok
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