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get cache specs via sysfs

15 May 2010

By reading some (virtual) files in sysfs, you can get to know cache architecture on your PC in detail.

My PC has Core i5 M540, which has 2 core, with HyperThreading enabled. Therefore Linux kernel shows us 4 CPUs, and there are four directories, cpu[0-3], in /sys/devices/system/cpu. The four logical cores are symmetric, however, I’ll list files in /sys/devices/system/cpu/cpu0/cache only.

The directory /sys/devices/system/cpu/cpu0/cache has four entries: index[0-3], each of which seems to represent L1 D-cache, L1 I-cache, L2 cache, L3 cache.

$ head index[0-3]/{level,type}
==> index0/level <==
1

==> index1/level <==
1

==> index2/level <==
2

==> index3/level <==
3

==> index0/type <==
Data

==> index1/type <==
Instruction

==> index2/type <==
Unified

==> index3/type <==
Unified

L1 cache is devided into D-cache and I-cache, while L2 and L3 are unified one. Two files, size and ways_of_associativity, tell you the size and associativity as follows.

% head index[0-3]/{size,ways_of_associativity}       
==> index0/size <==
32K

==> index1/size <==
32K

==> index2/size <==
256K

==> index3/size <==
3072K

==> index0/ways_of_associativity <==
8

==> index1/ways_of_associativity <==
4

==> index2/ways_of_associativity <==
8

==> index3/ways_of_associativity <==
12

L1 D-cache and I-cache are same in size, but D-cache has higher associativity, that is, D-cache contains up to 8 data which have the same index (lower bit of address) while I-cache can contain at most 4 data with common index. shared_cpu_list indicates which logical core shares the cache.

% head index[0-3]/shared_cpu_list   
==> index0/shared_cpu_list <==
0-1

==> index1/shared_cpu_list <==
0-1

==> index2/shared_cpu_list <==
0-1

==> index3/shared_cpu_list <==
0-3

In this case, L1 and L2 cache are used by cpu0 and cpu1 which are two threads in a single core. L3 cache are shared by all four threads, or 2 cores.

As you can see, Linux kernel shows you detailed data about CPU cache. It would be better and easier for you to search sysfs first before searching Intel website.

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