On 06/12/2018 11:27, Dale wrote:
I've bought but not yet installed a FX-8350 CPU. I have this in my
make.conf file:
CFLAGS="-march=native -O2 -pipe"
USE_CPU="
USE_CPU does not do anything, AFAICT. CPU features are specified in
CPU_FLAGS_X86. You can get appropriate flags using the
app-portage/cpuid2cpuflags tool. For example, here:
$ cpuid2cpuflags
CPU_FLAGS_X86: aes avx mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1
sse4_2 ssse3
So in my make.conf, I use:
CPU_FLAGS_X86="aes avx mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1
sse4_2 ssse3"
Those were put there ages ago, likely when I built and installed Gentoo
on this rig. Do I need to change those to something that is compatible
with both CPUs and then change to the new CPU after it is installed? Or
will the new CPU be close enough that it won't matter? Right now, I
don't know for sure what the new CPU supports or doesn't.
Just install the new CPU and run cpuid2cpuflags to see what to put in
CPU_FLAGS_X86. You can delete USE_CPU as that doesn't seem to be used
for anything.
While at it, going from a 4 core CPU at 3.2GHz to a 8 core CPU at
4.0/4.2GHz, just how much increase can I expect? Will it double and
that's about it or will it be more than that?
You won't get anything close to double the speed. The extra cores will
mostly go unused, unless you use applications that make use of them.
You will still get a speed up due to the newer CPU architecture and the
higher frequency.
Also, since it has two
speeds, will it run at the slower or faster one? Will it depend on
load? I've never had a CPU with two clock speeds like this before.
The two speeds specify the lower and upper speeds, depending on how many
CPU cores are currently being under load, and also how much load there
is. You don't have to worry about it though. It's all automatic. When
you're not running anything that stressed the CPU, clock speeds are
actually lower than 4GHz (some CPUs can clock down to 1GHz or so when
they're idle and not doing anything.) Once something CPU-heavy runs, it
will clock up to 4.2GHz. If you run something that stresses all CPU
cores, then it will go to 4.0GHz to avoid overheating.
But again, all this is automatic.
