Re: CONFIG_PHYSICAL_ALIGN limited to 4M?

[Vivek Goyal]

On Tue, Jan 02, 2007 at 12:05:18PM +0100, Rene Herman wrote:
> Good day.
> 
> A while ago it was remarked on list here that keeping the kernel 4M
> aligned physically might be a performance win if the added 1M (it
> normally loads at 1M) meant it would fit on one 4M aligned hugepage
> instead of 2 and since that time I've been doing such.
> 
> In fact, while I was at it, I ran the kernel at 16M; while admittedly a
> bit of a non-issue, having never experienced ZONE_DMA shortage, I am an
> ISA user on a >16M machine so this seemed to make sense -- no kernel
> eating up "precious" ISA-DMAable memory.
> 
> Recently CONFIG_PHYSICAL_START was replaced by CONFIG_PHYSICAL_ALIGN
> (commit e69f202d0a1419219198566e1c22218a5c71a9a6) and while 4M alignment
> is still possible, that's also the strictest alignment allowed meaning I
> can't load my (non-relocatable) kernel at 16M anymore.
> 
> If I just apply the following and set it to 16M, things seem to be
> working for me. Was there an important reason to limit the alignment to
> 4M, and if so, even on non relocatable kernels?

Hi Rene,

Can't think of any reason why we can't keep alignment uppper limit to
16M. That time I had kept 4M as upper limit as that seemed to be only
practical usage.

Rencetly I have restored back CONFIG_PHYSICAL_START option. That patch
is still in -mm. IMHO, your case will fit more if we set
CONFIG_PHYSICAL_START to 16M rather than increasing alignment upper limit
for CONFIG_PHYSICAL_ALIGN. 

http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.20-rc2/2.6.20-rc2-mm1/broken-out/i386-restore-config_physical_start-option.patch

Andrew, Can you please push this patch to 2.6.20-rc3?

Thanks
Vivek
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