From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Message-ID: <18031.15747.57370.99388@cargo.ozlabs.ibm.com> Date: Wed, 13 Jun 2007 10:42:43 +1000 From: Paul Mackerras To: Segher Boessenkool Subject: Re: [patch 02/10] powerpc: Move CONFIG_PPC64 into CPU selection In-Reply-To: References: <20070611232824.489103547@arndb.de> <20070611233353.734704855@arndb.de> <18030.15937.699275.958545@cargo.ozlabs.ibm.com> <200706121008.01661.arnd@arndb.de> <18030.32600.921348.218117@cargo.ozlabs.ibm.com> Cc: linuxppc-dev@ozlabs.org, Arnd Bergmann List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Segher Boessenkool writes: > It would be good to converge the 32- and 64-bit implementations, > and/or allow a single kernel image to work on both 32-bit and > 64-bit systems, The current situation is far removed from that, > so this patch is a bit premature in that aspect. There will always be differences in emphasis and focus between 32-bit and 64-bit. For example, the vast majority of 32-bit systems are UP, with a few 2-way SMP systems, an insignificant number of 4-way systems, and nothing bigger than 4-way. In contrast, most 64-bit machines are SMP, with enough large SMP machines (64-way and bigger) to be worth worrying about. Hence the 64-bit code focuses on SMP scalability whereas the 32-bit code focuses on performance for UP and 2-way systems - which is why the hash table invalidations get batched up on 64-bit machines but not on 32-bit machines, for instance. Similarly, 64-bit supports large memory (>4GB) and NUMA machines, whereas 32-bit doesn't. Paul.