linux-kernel - Re: [RFC v3 0/5] Transparent on-demand struct page initialization embedded in the buddy allocator

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Date:	Tue, 13 Aug 2013 18:10:20 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mike Travis <travis@....com>, Nathan Zimmer <nzimmer@....com>,
	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Robin Holt <holt@....com>,
	Rob Landley <rob@...dley.net>,
	Daniel J Blueman <daniel@...ascale-asia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Yinghai Lu <yinghai@...nel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC v3 0/5] Transparent on-demand struct page initialization
	embedded in the buddy allocator

On Tue, Aug 13, 2013 at 10:51:37AM -0700, Linus Torvalds wrote:
> I realize that benchmarking cares, and yes, I also realize that some
> benchmarks actually want to reboot the machine between some runs just
> to get repeatability, but if you're benchmarking a 16TB machine I'm
> guessing any serious benchmark that actually uses that much memory is
> going to take many hours to a few days to run anyway? Having some way
> to wait until the memory is all done (which might even be just a silly
> shell script that does "ps" and waits for the kernel threads to all go
> away) isn't going to kill the benchmark - and the benchmark itself
> will then not have to worry about hittinf the "oops, I need to
> initialize 2GB of RAM now because I hit an uninitialized page".
> 
I am not overly concerned with cost having to setup a page struct on first
touch but what I need to avoid is adding more permanent cost to page faults
on a system that is already "primed".

> Ok, so I don't know all the issues, and in many ways I don't even
> really care. You could do it other ways, I don't think this is a big
> deal. The part I hate is the runtime hook into the core MM page
> allocation code, so I'm just throwing out any random thing that comes
> to my mind that could be used to avoid that part.
> 

The only mm structure we are adding to is a new flag in page->flags.
That didn't seem too much.

I had hoped to restrict the core mm changes to check_new_page and
free_pages_check but I haven't gotten there yet.

Not putting on uninitialized pages on to the lru would work but then I 
would be concerned over any calculations based on totalpages.  I might be
too paranoid there but having that be incorrect until after a system is booted
worries me.


Nate
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