On Fri, Mar 2, 2012 at 4:02 PM, Ted Ts'o <tytso@xxxxxxx> wrote:
> Stupid question. Your patch requires unaligned accesses to not have a
> heavy penalty, right? Wasn't it the case that some generations of x86
> had pretty large penalties for aligned accesses? Is that something we
> need to worry about?
There are basically no x86's with heavy penalties.
Sure, unaligned accesses are often *slightly* more expensive,
especially if they cross the cache access boundary (which tends to be
8 bytes on older 32-bit cpu's, and generally 16 bytes on more modern
CPUs - so it's not that they are unaligned per se, but that they cross
the bank size). But even then, it's usually not a huge deal (ie it
takes up two read slots instead of just one).
There are x86 chips that are extremely bad at unaligned SSE/MMX
accesses, but not regular words.
Linus
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