Programmatically get accurate CPU cache hierarchy information on Linux

Question

I'm trying to get an accurate description of the data cache hierarchy of the current CPU on Linux: not just the size of individual L1/L2/L3 (and possibly L4) data caches, but also the way they are split or shared across cores.

For instance, on my CPU (AMD Ryzen Threadripper 3970X), each core has its own 32 KB of L1 data cache and 512 KB of L2 cache, however the L3 cache is shared across cores within a core complex (CCX). In other words, there are 8 distinct L3 caches, each of 16 MB.

The "Cache" section of this screenshot of CPU-Z on Windows is basically what I'm trying to find out:

I have no problem getting these information on Windows with GetLogicalProcessorInformation().

However, on Linux, it appears that sysconf() only gives me either the per-core cache size for L1 and L2 data caches (_SC_LEVEL1_DCACHE_SIZE and _SC_LEVEL2_DCACHE_SIZE), or the total L3 cache size (_SC_LEVEL3_CACHE_SIZE).

EDIT: lstopo's output under VMWare. The virtual machine has 8 cores. L1 and L2 cache information are fine but L3 cache size does not appear to be correct:

Answer 1

A full picture of the cache hierarchy can be found programmatically by opening files in /sys (sysfs).

Each "thread" or "logical processor" is represented by a sub-directory in /sys/devices/system/cpu/. Within that directory you'll find a cache directory. For example, cache information for the first logical processor can be found here:

$ ls /sys/devices/system/cpu/cpu0/cache/
index0
index1
index2
index3
power
uevent

Each cache entity associated with that logical processor is represented by an index[0-9]* directory. The number after index does not represent the level. The same cache entity may be listed multiple times under different logical processors. Within these directories you can find all the properties of the cache entity (level, sets, line size, etc).

$ ls /sys/devices/system/cpu/cpu0/cache/index0
coherency_line_size
level
number_of_sets
physical_line_partition
power
shared_cpu_list
shared_cpu_map
size
type
uevent
ways_of_associativity

Full documentation can be found here.

Most importantly, to get the output you want, you'll need to inspect shared_cpu_list:

$ cat /sys/devices/system/cpu/cpu0/cache/index0/shared_cpu_list
0,28

This will show you what logical processors share this cache entity. By inspecting all entities (/sys/devices/system/cpu/cpu*/cache/index*/), and eliminating duplicates using shared_cpu_list, you can programmatically access all the data you require.

Note that your hypervisor isn't required to pass along accurate information. This will only show you the cache hierarchy as the guest kernel sees it.