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Old 2008-10-02, 14:15   #1
spartanroc
 
Oct 2008

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Default L2 cache size NOT RECOGNIZED for Core2 E8400

I have a linux system running mprime on Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz. During the mprime setup, here is what it outputs:

Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz
CPU speed: 2992.63 MHz
CPU features: RDTSC, CMOV, Prefetch, MMX, SSE, SSE2
L1 cache size: 32 KB
L2 cache size: unknown
L1 cache line size: 64 bytes
L2 cache line size: unknown
Prime95 32-bit version 24.14, RdtscTiming=1

However, I can easily find that the L2 cache size is 6144 KB from ,e.g., /proc/cpuinfo. It seems that its inability to find the correct L2 cache size does affect the computing speed. The benchmark for the same type machine is 0.0499 for FFT size of 2560K, while my number is much higher at about 0.056. Is there a way to let mprime know the cache size? Thanks.
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Old 2008-10-02, 15:04   #2
Mini-Geek
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"Tim Sorbera"
Aug 2006
San Antonio, TX USA

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Read:
Quote:
Originally Posted by UNDOC.TXT
The program also supports different code paths for LL testing on a Pentium 4
based on the size of the L2 cache. You can explicitly specify the L2 cache
size although this shouldn't be necessary since the program uses the CPUID
instruction to determine the L2 cache size. In local.ini enter:
CpuL2CacheSize=128 or 256 or 512
CpuL2CacheLineSize=32 or 64 or 128
CpuL2SetAssociative=4 or 8
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Old 2008-10-02, 20:28   #3
cheesehead
 
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"Richard B. Woods"
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It should be noted that

Code:
 CpuL2CacheSize=128 or 256 or 512
 CpuL2CacheLineSize=32 or 64 or 128
 CpuL2SetAssociative=4 or 8
is a bit misleading because it could imply that only 128, 256, and 512 are valid values for CpuL2CacheSize.

Actually, as far as I can determine from a look at the V25.2 source code, any values you specify for CpuL2CacheSize, CpuL2CacheLineSize, or CpuL2SetAssociative will be accepted and used -- there's no range checking where those values are read from the local.ini file.

However, there's still a possibility that wherever the "different code paths for LL testing" are taken, CpuL2CacheSize=6144 may take the same code path as CpuL2CacheSize=512. I didn't try to track that down now. (Maybe later.)

Last fiddled with by cheesehead on 2008-10-02 at 20:35
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Old 2008-10-03, 05:04   #4
S485122
 
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"Jacob"
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Do not forget that Core 2 cache is shared cache, you cannot give all the cache to each of of the cores...

Jacob
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Old 2008-10-03, 14:57   #5
spartanroc
 
Oct 2008

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Thanks for the reply. I dug around and found the following values for my E8400:
CpuL2CacheSize=6144
CpuL2CacheLineSize=64

However, putting these values in local.ini does not improve any performance at all.
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Old 2008-10-03, 14:58   #6
spartanroc
 
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Quote:
Originally Posted by S485122 View Post
Do not forget that Core 2 cache is shared cache, you cannot give all the cache to each of of the cores...

Jacob
are you saying I should try the following?
CpuL2CacheSize=3072
CpuL2CacheLineSize=32
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Old 2008-10-03, 15:23   #7
S485122
 
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"Jacob"
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Quote:
Originally Posted by spartanroc View Post
are you saying I should try the following?
CpuL2CacheSize=3072
CpuL2CacheLineSize=32
CpuL2CacheLineSize=64

or

CpuL2CacheLineSize=32

You started me thinking... I am not sure now.

Jacob
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Old 2008-10-03, 15:34   #8
fivemack
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Feb 2006
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Quote:
Originally Posted by spartanroc View Post
are you saying I should try the following?
CpuL2CacheSize=3072
CpuL2CacheLineSize=32
no, CpuL2CacheLineSize=64; the cache is split among the CPUs on whole lines.
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