mersenneforum.org L2 cache size NOT RECOGNIZED for Core2 E8400
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 2008-10-02, 14:15 #1 spartanroc   Oct 2008 38 Posts 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.
2008-10-02, 15:04   #2
Mini-Geek
Account Deleted

"Tim Sorbera"
Aug 2006
San Antonio, TX USA

10B516 Posts

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

 2008-10-02, 20:28 #3 cheesehead     "Richard B. Woods" Aug 2002 Wisconsin USA 769210 Posts 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
 2008-10-03, 05:04 #4 S485122     "Jacob" Sep 2006 Brussels, Belgium 34308 Posts Do not forget that Core 2 cache is shared cache, you cannot give all the cache to each of of the cores... Jacob
 2008-10-03, 14:57 #5 spartanroc   Oct 2008 3 Posts 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.
2008-10-03, 14:58   #6
spartanroc

Oct 2008

316 Posts

Quote:
 Originally Posted by S485122 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

2008-10-03, 15:23   #7
S485122

"Jacob"
Sep 2006
Brussels, Belgium

71816 Posts

Quote:
 Originally Posted by spartanroc 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

2008-10-03, 15:34   #8
fivemack
(loop (#_fork))

Feb 2006
Cambridge, England

33·239 Posts

Quote:
 Originally Posted by spartanroc 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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