Mark Roulo (nothanks.delete@this.xxx.com) on January 7, 2021 7:00 pm wrote:
> Jörn Engel (joern.delete@this.purestorage.com) on January 7, 2021 4:29 pm wrote:
> > rwessel (rwessel.delete@this.yahoo.com) on January 7, 2021 9:25 am wrote:
> > >
> > > So SECDEC on a 32-bit word requires seven bits, eight bits on a 64-bit word, nine bits on a 128-bit word.
> > > Doing that on 64-bit words has the advantage of allowing fairly simple 8x9 or 9x8 RAM configurations.
> >
> > If the memory interface is 64bit, but the cacheline size is 64 _Bytes_, you need to do 8 reads anyway.
> > I wonder how hard it would be to do 9 reads to get the extra ECC information. That would simplify
> > things quite a bit, you can use the same DIMMs and motherboards. You can also get away with relatively
> > fewer bits for ECC, so it might be possible to reduce overhead from 12.5% to something closer to
> > 2.15%. Memory bandwidth is reduced by 11%, which would be fine in my book.
> >
> > Any CPU manufacturer should be able to do something like that. And I believe I can find old
> > implementations of that going back close to 20 years. So why isn't it done all the time?
>
> Random Googling suggests that many motherboards accept ECC DRAM even if the CPU in the motherboard
> does not support ECC. Everything works, you just don't get the ECC protection.

Desktop DIMMs are 64 bits wide, 72 bits if the system can do ECC. All desktop systems will work just fine with 72 bit wide DIMMs but they are seen as 64 bit DIMMs and in most cases neither CPU nor motherboard support the top 8 ECC bits and many will not have traces to power or connect to said bits. But the ECC capable DIMMs will work as if the were "not capable" :)

> If the CPU supports ECC at all (current schemes or your 64 byte scheme), I don't think the
> extra cost of "real" ECC DRAM would be off-putting to the customers who wanted ECC.

For decades now they have been off-putting as if you could find any. Double the price at half the speed (yes, slight, but only a slight exaggeration).

> So the answer to your question, "why isn't it done all the time," is that the CPU vendor has to build ECC
> into the CPU and the vendors that don't do your scheme don't want to support ANY ECC on those chips. Almost
> certainly for market segmentation. This is not a technical limitation. It is a business decision.

The "business" decision made by Intel deletes the technically sane choice of ECC memory in the market.