Cache line size

Question

It might be a very common and simple question but I need some explanation about the curve that I just obtained from a cache benchmarks code. The goal here is to find the cache line size. I used the code from here: (h**ps://github.com/jiewmeng/cs3210-assign1/blob/master/cache-l1-line.cpp)

This is the curve that I have obtained from running the code on my machine (Macbook Pro with core i7 - cache line size is 64byte - L1 data cache is 32KB).

The Time vs different stride size curve

• I think the peak happens on 128 bytes and not on the 64 bytes. if it is true I want to know why?
• Why the time is reduced at 512 bytes?

Update:

I also ran a code to determine the size of the L1 and L2 caches. Here is the figure just to document the data. As you can see there is two peak in 32KB (L1 Cache size) and 256KB (L2 Cache size).

Question:

I am wondering if there is any way to find the size of L3 shared cache.

Cache size figure.

Thanks

Answer 1

I'm guessing that the 128B peak is most likely due to spatial prefetching. You can see in Intels' Optimization guide, under section 2.1.5.4

This prefetcher strives to complete every cache line fetched to the L2 cache with the pair line that completes it to a 128-byte aligned chunk

It wouldn't be a clean jump since this prefetches is not always firing, and even when it does, it only prefetches into the L2, but it's much better than fetching from memory. To make sure this is the case, you can disable prefetches (through BIOS or other means, although some systems may not support that), and check again.

As for the L3 size - you didn't specify your exact model, but i'm guessing you have more than 4M L3 - just keep the curve going and see if it jumps.

EDIT

Just noticed another thing - your k*i expression is probably overflowing int at the max range, which means your access pattern might not be cyclic as you expect.