Hacker News Reader

Comment by spamizbad | Hacker News Reader

I think people should pay more attention to the LPDDR4X RAM that's tightly coupled to the CPU and likely has significantly less latency and more bandwidth than anything on the Ryzen or i9. This is critical to keeping their wide design fed, and if you swapped this out with standard DDR3200 you'd see some lackluster numbers.

Both Intel and AMD try to overcome the memory wall with larger caches and clever prefetching, which is going to chew up more die space and power than x86 decoders.

View on HN

Anandtech measured the ram latency at >90ns, which is not great:

https://www.anandtech.com/show/16252/mac-mini-apple-m1-teste...

The cache on the M1 is also massive. If Intel/AMD have clever prefetchers to make up for this, it looks like the M1 might be doing something more.

I have been trying to explain to people that having the DRAM chips physically next to the SOC on the package does not actually meaningfully improve memory latency.

I get that people on this site are excited, but please be excited about the right things.

People seem to have dramatically missed the point here; it's almost certainly done for power reasons.

And also for silicon budget, probably: short distances require small interface transistors and thin wires compared to massive distances like 5cm.

The chip area can be re-used for compute-sized transistors and wires (much, much smaller than interface transistors and wires, so there can be many more of them).

Yeah, speed of PCB transmission is ~8 inches (~20cm) per nanosecond. Which is about 2/3 of the classic "light-nanosecond", which is roughly a foot.

Why would that RAM have less latency? Do you have any sort of a source for why you think that?

How would one benchmark memory performance (DRAM latency & bandwidth) on the various chips?

Above my pay grade! You probably operating with a fixed speed on M1 so you can't intentionally cripple it, but it would be interesting to see "What would this look like if Apple had paired it with standard DDR4-3200?". AMD's Zen core's are known to respond well to lower latency RAM, but in the PC space you're not getting anything close to what Apple has with the LPDDR4x on package. You could try benchmarking the Intel and Ryzen chips with DDR4 5000 and try carefully tuning the memory frequency and timing to find the right balance of bandwidth and latency.

> in the PC space you're not getting anything close to what Apple has with the LPDDR4x on package.

I mean, that's exactly how an 11th Gen Core laptop is constructed.

> but in the PC space you're not getting anything close to what Apple has with the LPDDR4x on package.

You can run basically any Intel CPU at DDR4-4266 without any issue. Or you can go thread ripper and laugh in quad channel memory. Or Intel x299 and triple-channel memory.

You don't have to tune anything, either, the RAM comes pre-tuned with the right timings in the XMP profile.

DDR5 is coming to consumer platforms probably 2022

this is a reasonable test: https://www.cs.virginia.edu/stream/ref.html

but it's pretty straightforward to just copy giant buffers in order/out of order and squint really hard at the results

Reply on news.ycombinator.com