Is my L3 cache really slower than my DRAM?

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I've recently been running a memtest86 on my PC (Threadripper 1920x, Kingston ECC DDR4-2666 CAS19) because I suspected that the stock timings were conservative. And I was right: the memory will run at stock voltages at 3200 speed with JEDEC standard CAS20 timings, which gives a small but noticeable performance increase on benchmarks.

However, when running the memtest86, I noted that it reports my L3 cache as running at 17.36GB/s but my RAM as running at 17.73GB/s, meaning my RAM is actually faster than the processor's L3 cache.

  1. Is this likely?
  2. Should I disable on processor L3 cache?
  3. How can I disable on processor L3 cache it that's appropriate?

Christi

Posted 2018-12-15T10:31:53.870

Reputation: 915

There are multiple aspects to CPU and cache performance and I understand this can be very complex. They cannot be properly evaluated on the basis of a single benchmark number. CPU manufacturers have done extensive testing and designed the caches to be of benefit under real world conditions, not to do well with a specific benchmark. And I suspect this benchmark number is of little relevance to real world performance.The determination that L3 cache is not beneficial could only be done after extensive and difficult testing, which memtest86 was not designed to do. – LMiller7 – 2018-12-15T14:33:05.673

Answers

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The slight difference is perhaps because the RAM is buffered, but the L3 cache is not (L3 is a kind of buffer by itself).

Doing a write to the RAM will result in the Memory Controller storing it in a buffer and sending a response back stating that the transaction is complete (even though it's not). The exception to this is when doing large blocks of writes and go beyond the capabilities of the Memory Controller buffer.

My guess is that the L3 cache and the RAM are very similar in hardware, but the Memory Controller gives the RAM a slight advantage, a small one since memtest86 is writing large block of memory.

So:

Is this likely?

Yes, as described above, although it might only be an illusion.

Should I disable on processor L3 cache?

Absolutely not, even if this is at all possible.

How can I disable on processor L3 cache it that's appropriate?

Not appropriate, as the CPU is hard-wired to use it.

harrymc

Posted 2018-12-15T10:31:53.870

Reputation: 306 093

Asked: 2018-12-15T10:31:53.870

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Active: 2018-12-15T10:51:16.910