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Re: block sizes > 4K ?? possible w/large page support?

To: xfs@xxxxxxxxxxx
Subject: Re: block sizes > 4K ?? possible w/large page support?
From: Carlos Maiolino <cmaiolino@xxxxxxxxxx>
Date: Mon, 11 Jun 2012 13:21:03 -0300
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References: <4FD5643F.5070801@xxxxxxxxx> <20120611132932.GA18432@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> <4FD606C1.1070103@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
> > The maximum block size of a XFS filesystem is 64kiB. But in linux it's 
> > limited
> > to the PAGE_SIZE value. 
> 
> Correct.
> 
> > so, on x86 architectures, the maximum block size is
> > 4kiB.
> 
> Not entirely correct.  Since ~1996, 16 years ago, PPro and higher 32bit
> CPUs with PSE/PSE36 support pages of 4MB, or 2MB with PAE enabled.
> 
I know we can use hugepages with these sizes, but didn't know we can use this as
common usage. I tried to look at MM code and didn't find anything which would
make PAGE_SIZE greater than 4096 (at least in x86), but well, I'm not a MM
developer too.

> x86-64 CPUs in long mode also support a 2MB page size.  But the problem
> of internal fragmentation may outweigh the TLB and other benefits of
> these very large pages.  I'm not an MM dev so I can't elaborate further.
>  There may be other issues.
> 
> > although it could benefit from a 16kiB page size, you'll need to be running 
> > an
> > operating system which supports this page size value.
> 
> And AFAIK the kernel MM team doesn't have x86 2MB pages on their radar.
>  Or do they?
No clue, I'm not a MM developer too =/ maybe I can be someday :D

-- 
--Carlos

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