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The major upcoming configuration change in Linux 2.6.39 is to get rid of the '686' flavour. This may be surprising, because it's the most widely used flavour of the 4 we have a present:
|Name||Minimum CPU features||Maximum total CPU threads||Physical address space|
|686||686-class; CMOV instruction||32||4 GiB|
|686-bigmem||686-class; PAE||32||64 GiB|
However, the physical address limitation means that an increasing proportion of new PCs and the majority of PC servers need the '686-bigmem' flavour. Even those that have less than 4 GiB RAM do support PAE and can run the '686-bigmem' flavour. There is a small cost (up to about 0.1% of RAM) in the use of larger hardware page tables. There is also an important benefit on recent processors: the larger page table entries include an NX bit (also known as XD) which provides protection against some buffer overflow attacks, both in the kernel and in user-space..
There are only a few 686-class processors that support CMOV but not PAE: most Intel Pentium M models, the VIA C3 'Nehemiah' and the AMD Geode LX. These also all lack SMP support, which means the '486' flavour is suitable for them.
Some benchmarking on two of those - a Pentium M model 745 and a C3 'Nehemiah' - indicated that they run the '486' flavour slightly faster than the '686' flavour. It appears that the performance gain from using plain uniprocessor code (rather than SMP-alternatives, which are patched with NOPs on uniprocessor systems) outweighs the performance loss from avoiding use of some newer instructions.
We intend to get rid of 'amd64' too, but we need to ensure that upgrades from linux-image-2.6-amd64:i386 to linux-image-2.6-amd64:amd64 work properly.
Yes, it is. Therefore '686-bigmem' will be renamed to '686-pae'.