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Re: Newbie R5K questions -- -mips2 vs -mips4; is n32 ABI supported by Li

To: Jeffrey Baitis <baitisj@evolution.com>
Subject: Re: Newbie R5K questions -- -mips2 vs -mips4; is n32 ABI supported by Linux?
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 17 Nov 2003 23:52:48 +0100
Cc: linux-mips@linux-mips.org, Adam_Kiepul@pmc-sierra.com, "Mr. Brian R. Gunnison" <brian@evolution.com>, Francis Yu <francisyu@synergyrep.com>, Johnny Lam <Johnny_Lam@pmc-sierra.com>
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On Mon, Nov 17, 2003 at 02:04:26PM -0800, Jeffrey Baitis wrote:

> I'm currently trying to increase performance on our PMC-Sierra RM5231
> system by taking advantage of the MIPS IV ISA. This processor has a
> 32-bit address bus interface with 64-bit GPRs, so I guess that the
> choice of -mabi=n32 is ideal for this processor.

In addition to what Daniel just said ...

N32 requires a 64-bit kernel to run on which is significantly larger
thereby causing more cache misses so a 64-bit kernel is often slower.
On the kernel side a 64-bit kernel is drastically better at handling
large amounts of memory, so once a 32-bit kernel needs highmem the
64-bit kernel will win the race.  Often these effects influence
performance more than what you might gain from exploiting a new ISA.

  Ralf

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