Aleksandar Milivojevic wrote:
> My experience with Alpha processors and competing 32/64-bit counterparts
> of that time (UltraSparc for example) was that small increase of size
> code is mostly overrated. In real-world, it was not much of an concern.
> If 256 megs was enough to make 32-bit application happy on UltraSparc,
> 256 megs was enough to make it happy on completely 64-bit Alpha. On
> UltraSparc itself, running 32-bit vs 64-bit kernel has almost no
> performance penalty in real world applications.
My understanding is that it's not the ~ 256 MB main memory that's the
problem (not much of it is code anyway). It's the ~ 512 KB Level 2
cache, and the ~64 KB Level 1 instruction cache. Stuff gets evicted
from that very regularly, and cache misses are *expensive* (in terms of
wasted clock cycles).
As processors have got faster, memory hasn't kept up. So a cache miss is
proportionately much more expensive, and using cache effectively more
important. You can even find people reckoning that x86 these days is
competitive with RISC because it's effectively a compression mechanism
for RISC code!
Fortunately, as you note, the slowdown due to increased code size is
very small on AMD64, and dwarfed by the other improvements it brings.
> So I wouldn't waste much time worrying about that. Yeah, in lab tests
> you might see some slowdown. But when you load your applications and
> start using the system in real world, you are not going to see any
> noticable performance degradation (even if number of registers and
> cache is the same). Add addtional cache and registers that you get
> when swtiching to AMD64's 64-bit mode, and as both you and James
> wrote, things get actually much faster. Add to that ability that all
> applications are able to handle large files without being specifically
> modified for that is big enough bonus on its own.
James.
--
E-mail address: james | Strangely enough, this last one is interrupted by
@westexe.demon.co.uk | twelve dancing rabbits singing about toilet paper.
| -- The megahal program, trained on my quote file.
