Re: How much do the hint bits help?

On 22.12.2010 17:31, Simon Riggs wrote:
> On Wed, 2010-12-22 at 17:01 +0200, Heikki Linnakangas wrote:
>> There's plenty of stuff in memory that's not covered by an
>> application-level CRC. That's what ECC RAM is for.
>
> http://www.google.com/research/pubs/archive/35162.pdf
>
> Google research shows that each DIMM has an 8% chance per annum of
> uncorrectable memory errors, even on ECC.

You misread that paper. From summary:

>   About a third of machines and over 8% of DIMMs in
> our fleet saw at least one *correctable* error per year.

Emphasis mine.

> Our
> per-DIMM rates of correctable errors translate to an aver-
> age of 25,000–75,000 FIT (failures in time per billion hours
> of operation) per Mbit and a median FIT range of 778 –
> 25,000 per Mbit (median for DIMMs with errors), while pre-
> vious studies report 200-5,000 FIT per Mbit. The number of
> correctable errors per DIMM is highly variable, with some
> DIMMs experiencing a huge number of errors, compared to
> others. The annual incidence of uncorrectable errors was
> 1.3% per machine and 0.22% per DIMM.

So the real figure of uncorrectable errors is 0.22% per DIMM.

Anyway, unreliable RAM calls for more ECC bits in DIMMs, not invasive 
architectural changes to every single application in the system.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com