4 ms·

Yes. Even the persistent memories that attach to the memory bus are currently quite a bit slower than DRAM (5-7x from estimates I've seen), while the difference

Yes. Even the persistent memories that attach to the memory bus are currently quite a bit slower than DRAM (5-7x from estimates I've seen), while the difference with PCIe-attached ones is even more. I'm not sure what the future holds in terms of latencies for non-volatile storage but sub-DRAM levels aren't within reach yet.

matt_d 11y ago
On a side note, it's interesting to me that emerging memory technologies currently seem to be mainly focused on addressing the "from-DRAM-to-disk" part of the memory hierarchy. That is, as you mentioned, not directly competing with DRAM, and consistently on the same side of the 1 microsecond dividing line between memory and storage; as in: http://www.rambusblog.com/2015/10/15/mid-when-memory-and-storage-converge/ http://www.rambusblog.com/2015/10/15/mid-when-memory-and-sto... (note SCM placed between DRAM and SSD) http://semiengineering.com/the-memory-and-storage-hierarchy/ http://semiengineering.com/the-memory-and-storage-hierarchy/ As far as the other side of the line is concerned, I think I've only seen proposals for hybrid-cache architectures (HCA) -- other than http://link.springer.com/chapter/10.1007%2F978-1-4419-9551-3_7 http://link.springer.com/chapter/10.1007%2F978-1-4419-9551-3... -- with a hybrid approach (e.g., combining SRAM/eDRAM/STT-RAM/PCRAM) probably making sense due to latency/endurance/bandwidth trade-offs. If anything, there seems to be more development on the DRAM interface itself -- with multiple candidates for the (or a) DDR4's successor, so far involving Wide I/O (Samsung), Hybrid Memory Cube (Intel, Micron), High Bandwidth Memory (SK Hynix, AMD, Nvidia): http://www.extremetech.com/computing/197720-beyond-ddr4-understand-the-differences-between-wide-io-hbm-and-hybrid-memory-cube http://www.extremetech.com/computing/197720-beyond-ddr4-unde... (Latency and bandwidth improvements seem promising, http://semiengineering.com/which-memory-type-should-you-use/ http://semiengineering.com/which-memory-type-should-you-use/) One interesting development I've seen involves reducing SRAM's footprint, by moving from 6T (6-transistors) cell to a 1T (one-transistor) one: http://www.eetimes.com/document.asp?doc_id=1328453 http://www.eetimes.com/document.asp?doc_id=1328453 It's fairly recent development, though, and it remains to be seen how is it going to fare. Other than the above, there doesn't really seem to be much progress around competing with/improving SRAM. However, this may become increasingly important, since some of the technological process scaling issues apply to SRAM, too.

matt_d 11y ago

On a side note, it's interesting to me that emerging memory technologies currently seem to be mainly focused on addressing the "from-DRAM-to-disk" part of the memory hierarchy. That is, as you mentioned, not directly competing with DRAM, and consistently on the same side of the 1 microsecond dividing line between memory and storage; as in: http://www.rambusblog.com/2015/10/15/mid-when-memory-and-storage-converge/ http://www.rambusblog.com/2015/10/15/mid-when-memory-and-sto... (note SCM placed between DRAM and SSD) http://semiengineering.com/the-memory-and-storage-hierarchy/ http://semiengineering.com/the-memory-and-storage-hierarchy/ As far as the other side of the line is concerned, I think I've only seen proposals for hybrid-cache architectures (HCA) -- other than http://link.springer.com/chapter/10.1007%2F978-1-4419-9551-3_7 http://link.springer.com/chapter/10.1007%2F978-1-4419-9551-3... -- with a hybrid approach (e.g., combining SRAM/eDRAM/STT-RAM/PCRAM) probably making sense due to latency/endurance/bandwidth trade-offs. If anything, there seems to be more development on the DRAM interface itself -- with multiple candidates for the (or a) DDR4's successor, so far involving Wide I/O (Samsung), Hybrid Memory Cube (Intel, Micron), High Bandwidth Memory (SK Hynix, AMD, Nvidia): http://www.extremetech.com/computing/197720-beyond-ddr4-understand-the-differences-between-wide-io-hbm-and-hybrid-memory-cube http://www.extremetech.com/computing/197720-beyond-ddr4-unde... (Latency and bandwidth improvements seem promising, http://semiengineering.com/which-memory-type-should-you-use/ http://semiengineering.com/which-memory-type-should-you-use/) One interesting development I've seen involves reducing SRAM's footprint, by moving from 6T (6-transistors) cell to a 1T (one-transistor) one: http://www.eetimes.com/document.asp?doc_id=1328453 http://www.eetimes.com/document.asp?doc_id=1328453 It's fairly recent development, though, and it remains to be seen how is it going to fare. Other than the above, there doesn't really seem to be much progress around competing with/improving SRAM. However, this may become increasingly important, since some of the technological process scaling issues apply to SRAM, too.