缓存行大小

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.

Answer 2

My BusSpeed benchmark was intended to identify cache sizes and performance at different strides, to show burst reading on buses:

http://www.roylongbottom.org.uk/busspd2k%20results.htm

Following are results on a Core i7 with 8 MB L3:

  Memory  Reg2  Reg2  Reg2  Reg2  Reg1  Reg2  Reg1  Reg2  Reg1  Reg8
  KBytes Inc64 Inc32 Inc16  Inc8  Inc4  Inc4  Inc4  Inc4  Inc8  Inc8
   Used   MB/S  MB/S  MB/S  MB/S  MB/S  MB/S  MB/S  MB/S  MB/S  MB/S

      4  10025 10800 11262 11498 11612 11634  5850 11635 23093 23090
      8  10807 11267 11505 11627 11694 11694  5871 11694 23299 23297
     16  11251 11488 11620 11614 11712 11719  5873 11718 23391 23398
     32   9893  9853 10890 11170 11558 11492  5872 11466 21032 21025
     64   3219  4620  7289  9479 10805 10805  5875 10797 14426 14426
    128   3213  4805  7305  9467 10811 10810  5875 10805 14442 14408
    256   3144  4592  7231  9445 10759 10733  5870 10743 14336 14337
    512   2005  3497  5980  9056 10466 10467  5871 10441 13906 13905
   1024   2003  3482  5974  9017 10468 10466  5874 10467 13896 13818
   2048   2004  3497  5958  9088 10447 10448  5870 10447 13857 13857
   4096   1963  3398  5778  8870 10328 10328  5851 10328 13591 13630
   8192   1729  3045  5322  8270  9977  9963  5728  9965 12923 12892
  16384    692  1402  2495  4593  7811  7782  5406  7848  8335  8337
  32768    695  1406  2492  4584  7820  7826  5401  7792  8317  8322
  65536    695  1414  2488  4584  7823  7826  5403  7800  8321  8321
 131072    696  1402  2491  4575  7827  7824  5411  7846  8322  8323
 262144    696  1413  2498  4594  7791  7826  5409  7829  8333  8334
 524288    693  1416  2498  4595  7841  7842  5411  7847  8319  8285
1048576    704  1415  2478  4591  7845  7840  5410  7853  8290  8283

                  End of test Fri Jul 30 16:44:29 2010

  CPUID and RDTSC Assembly Code
  CPU GenuineIntel, Features Code BFEBFBFF, Model Code 000106A5
  Intel(R) Core(TM) i7 CPU         930  @ 2.80GHz Measured 2807 MHz