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Re:Intel’s first Optane SSD: 375GB that you can also use as RAM by Const-me on Sunday March 19, 2017 @03:36PM (#13909158)
L2 cache is several nanoseconds. DDR is much slower.

This page says 100ns: https://gist.github.com/jboner/2841832

This one says 60ns: http://stackoverflow.com/q/4087280/126995

Re:Intel’s first Optane SSD: 375GB that you can also use as RAM by comboy on Sunday March 19, 2017 @04:19PM (#13909382)
Got these 9ns from the wikipedia [1]. But 60-100ns would still be 3 orders of magnitude faster than this SSD.

I think the difference comes from the latency of operation from the CPU point of view vs actual memory/drive latency.In the article they seem to be talking about the drive latency.

1. https://en.wikipedia.org/wiki/DDR3_SDRAM#Latencies

Re:Intel’s first Optane SSD: 375GB that you can also use as RAM by Tuna-Fish on Sunday March 19, 2017 @06:32PM (#13910081)
That text is talking about the CAS latency, not full read/write latency. CAS latency is the latency for reading from an open row. Rows are small enough that the only way you can persistently hit an open row is if you are either reading sequentially, (in which case the CPU prefetch logic makes the memory latency go away completely) or if you work on such a small working set that you actually hit the cache all the time anyway.

On DDR3, the full latency of a miss to ram is, in a low-bandwidth scenario:

      latency of the full cache system, typically expressed as the L3 latency
    + latency of opening a row (Trcd)
    + latency for reading from an open row (Tcas)
    + latency of passing data from memory controller to L1 cache
In a situation with some but not anywhere near full load on the memory subsystem:

      caches
    + latency of closing a row (Trp)
    + latency of opening a row (Trcd)
    + latency of reading from an open row (Tcas)
    + latency of passing data to L1
and when doing full tilt random access:

      caches
    + time remaining waiting out the minimum allowed row active time of the previous memory access (Tras)
    + latency of closing a row (Trp)
    + latency of opening a row (Trcd)
    + latency of reading from an open row (Tcas)
    + latency of passing data to L1
The total latency is typically between 50 to 150 ns depending on the CPU and RAM in use.