Re: MM instruction and the pipeline

On Mon, 21 Oct 2024 06:32:52 GMT
anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:

"Paul A. Clayton" <paaronclayton@gmail.com> writes:

I was thinking primarily about uncorrectable errors in the
source. It would be convenient to software for MM to fail early on
an uncorrectable source error. It would be a little less
convenient (and possibly more complex in hardware) to generate an
ECC exception at the end of the MM instruction (or when it
pauses from a context switch). 

 
Welcome to the DDR5 age (which started in 2021).  DDR5 not just has
on-die ECC, it also has ECS (error check and scrub).  From
<https://in.micron.com/content/dam/micron/global/public/products/white-paper/ddr5-new-features-white-paper.pdf>:
 
|An additional feature of the DDR5 SDRAM ECC is the error check and
|scrub (ECS) function. The ECS function is a read of internal data and
|the writing back of corrected data if an error occurred. ECS can be
|used as a manual function initiated by a Multi-Purpose Command (MPC),
|or the DDR5 SDRAM can run the ECS in automatic mode, where the DRAM
|schedules and performs the ECS commands as needed to complete a full
|scrub of the data bits in the array within the recommended 24-hour
|period. At the completion of a full-array scrub, the DDR5 reports the
|number of errors that were corrected during the scrub (once the error
|count exceeds a minimum fail threshold) and reports the row with the
|highest number of errors, which is also subject to a minimum
|threshold.
 
I am wondering why scrubbing is not performed automatically on
refresh.
>

Typical DDR5 row contains 4096 data bits. That's 32x bigger than
internal ECC block. In order to do scrub at refresh without major
increase in refresh timing one would need a lot more ECC correction
logic than currently present.
Even with a lot of extra logic there will be some slowdown, likely one
clock (== 16T == 2.5 to 3.3 ns).

Even before DDR5, scrubbing is a feature of the memory controller that
it performs in hardware (i.e., without software having to do something
in an interrupt handler or some such).  Of course the My66000 memory
controller may be less capable, and leave scrubbing to software; maybe
also refresh?
 

Hopefully the last part is tongue in cheek.

- anton