Rowhammer mitigation

[naimc]

cowbutt thank you for info.

David does passmark display refresh interval (ms) of the ram. Is anyone aware of any windows utility that would display the tREFI and tREFIX9 values. I have some Haswell Lenovo PC that don't expose memory timing setting and I would like to figure out what the refresh rate is.

[cowbutt]

Yes, it is surprising that you had an error with DDR4. This is the first report or errors with have seen with DDR4.

Yup. The reading I've done since suggests that DDR4's rowhammer mitigations aren't automatic, but that they require some support, presumably from the BIOS/UEFI and how it sets up the memory controller to enable TRR.Or maybe that option is obscured by the BIOS terminology (there's an option for 'enhanced stability' that I haven't changed from 'normal'). Or maybe because this is "crazy overclocker RAM" it doesn't do TRR (or do it properly).

Reducing the refresh rate was always known to be a solution, but it is nice to see the details. As you say it does reduce performance and battery life (in laptops)

My intent was that it'd save a few hours of research for anyone else trying to mitigate, whether on DDR4 or not.

[David (PassMark)]

Yes, it is surprising that you had an error with DDR4. This is the first report or errors with have seen with DDR4.

Reducing the refresh rate was always known to be a solution, but it is nice to see the details. As you say it does reduce performance and battery life (in laptops)

[cowbutt]

My i7-5820K/GA-X99-UD4/2400MHz Crucial Ballistix DDR4 system was failing rowhammer (a few hundred errors per pass) until I reduced the refresh interval timing from the default of 7.8ms, in spite of the fact that DDR4 is supposed to include rowhammer mitigation (source: https://en.wikipedia.org/wiki/Row_hammer#Mitigation)

In my board's BIOS, the two settings were tREFI (default of 9360) and tREFIX9 (default of 82).


refresh interval (ms) = tREFI / (RAM clock (MHz) / 2)

tREFIX9 = 8.9 * tREFI / 1024


so...


9360/(2400/2)=7.8ms

(Source: page 123 of http://www.intel.com/content/dam/www...-datasheet.pdf)


The standard recommendation is to reduce the refresh interval to 3.9ms and thereby double the refresh rate (source: http://support.lenovo.com/us/en/prod...ity/row_hammer). Doing that gave me one error per pass at the same address both times, so I reduced the interval to 75% of 3.9ms (i.e. tREFI=3510, tREFIX9=31) and it's now error free over 8 passes overnight.


Because the memory controller is refreshing slightly more frequently, it reduces the frequency which the RAM can be accessed, so it slightly hurts performance, but I don't care too much. 45GB/s is enough for me.