4 ms·
> Unfortunately, the laws of physics driving DRAM cells have not improved much over the last couple of years (or decades, for that matter), so memory chips stil
by theandrewbailey 3y ago
> Unfortunately, the laws of physics driving DRAM cells have not improved much over the last couple of years (or decades, for that matter), so memory chips still must operate with similar absolute latencies, driving up the relative CAS latency. In this case 14ns remains the gold standard, with CAS latencies at the new speeds being set to hold absolute latencies around that mark.
Some gaming memory kits can do 10ns or less latency. Though I guess if memory latency is your bottleneck, you should look at HBM.
- spintin 3y agoHBM is slower than DDR per pin, the speed gain is from a hugely parallel bus. Parallel means latency if you have non "embarrassingly parallelizable" tasks?
- Tuna-Fish 3y agoThe smallest transfer done from memory is a single cache line, which on most desktop machines is 64 bytes, or 512 bits. You could imagine a memory bus that was 512 bits wide and transferred a cache line per clock, and this would improve latency when compared to a serial bus with higher clock speed. HBM doesn't do that, though, instead every HBM3 module has 16 individual 64-bit channels, with 8n prefetch (that is, when you send a single request to a single channel, it will respond with 512 bits over 8 cycles).
- dist-epoch 3y agoDDR5 has 2 independent 32-bit lanes. Multiple transfers are required for 64 bytes.
- Tuna-Fish 3y agoDDR5 has a 16n prefetch, so a single transfer from a 32-wide channel moves 64 bytes.
- moffkalast 3y agoI don't think they make HBM RAM kits. /s
- nsteel 3y agoAs others have said, there is nothing low latency about HBM. Renesas did have a special Low Latency HBM thing at one point, but I don't think it ever saw the light of day.
- jeffbee 3y ago> Some gaming memory kits can do 10ns or less latency Without a thorough analysis by real engineers my interpretation of this statement is "DRAM marketers can print anything they want on the sticker".
- NavinF 3y ago> Some gaming memory kits can do 10ns or less latency Source? My overclocked desktop RAM shows 45ns in benchmarks. I call bullshit on 4.5x faster RAM. Most people fight for an extra 5% latency reduction
- wmf 3y agoThat's probably 10 ns for the DRAM and 35 ns for the caches and memory controller.
- gautamcgoel 3y agoJust to make sure I understand: you're saying that checking L1/L2/L3 takes around 35ns, and then the CPU accesses DRAM which takes 10ns? If that's so, how is L3 cache any faster than DRAM? Also, can you explain why the memory controller adds some latency?
- wmf 3y agoAn L3 hit only takes ~15 ns so that means another 15-20 ns is spent traversing the fabric and memory controller. I'm not sure what all is involved there but for Intel it has to go around the ring and for AMD it has to cross chiplets.
- gautamcgoel 3y agoInteresting. If an L3 hit takes 15 ns, then based on your argument a hypothetical CPU with only one core (and hence no fabric) would be better off without L3, since a DRAM read can be performed in just 10 ns.
- nsteel 3y agoYou still need a memory controller, you still need to get to that controller on the edge of the die. And going to RAM more often will surely consume more power.
- 3y ago