3 ms·

These things exist, using trade names like chipkill or lockstep memory. Though they don't need to sacrifice half of the memory chips to get good error recovery

These things exist, using trade names like chipkill or lockstep memory. Though they don't need to sacrifice half of the memory chips to get good error recovery properties. Note that this is still not end-to-end protection of data integrity. Bit flips happen in networking, storage, buses between everything, caches, CPUs, etc. See eg [1] [1] https://arxiv.org/abs/2102.11245 https://arxiv.org/abs/2102.11245 Silent Data Corruptions at Scale (based on empirical data at Facebook)

formerly_proven 5y ago
According to the Intel developer's manual L1 has parity and all caches up from that have ECC. This would seem to imply that the ring / mesh also has at least parity (to retry on error). Parity instead of ECC on L1(D) makes sense since the L1(D) has to handle small writes well, while the other caches deal in lines.
- eru 5y ago
  Of course, parity can only detect an odd number of bit flips.
  - RL_Quine 5y ago
    Some systems do have two parity bits but obviously there’s efficiency loss there.
- namibj 5y ago
  Zen3 has parity on L1I and ECC on L1D and beyond. Even ECC on consumer hardware (it's just a few extra traces on the motherboard).
willis936 5y ago
Checksumming filesystems and file transfer protocols cover many cases. SCP, rsync, btrfs, and zfs all fix this problem. As for guaranteeing the computed data is correct: I know space systems often have two redundant computers that calculate everything and compare results. It's crazy expensive and power demanding, but it all but solves the problem.
- belter 5y ago
  If they have two computers and results differ, how do they decide which one is correct ? ;-)
  - willis936 5y ago
    They don't. They do it again. If they never agree then Houston has a real problem.
  - bonzini 5y ago
    Usually they have an odd number. The Space Shuttle had five, of which the fifth was running completely different software. In case of a 2/2 split the crew could shut down a pair (in case the failure was clear) or switch to the backup computer.
    - X6S1x6Okd1st 5y ago
      Interestingly etherum also takes the completely different software approach as well: there are 5 different main clients
  - luckman212 5y ago
    a third system?

formerly_proven 5y ago

According to the Intel developer's manual L1 has parity and all caches up from that have ECC. This would seem to imply that the ring / mesh also has at least parity (to retry on error). Parity instead of ECC on L1(D) makes sense since the L1(D) has to handle small writes well, while the other caches deal in lines.

eru 5y ago
Of course, parity can only detect an odd number of bit flips.
- RL_Quine 5y ago
  Some systems do have two parity bits but obviously there’s efficiency loss there.
namibj 5y ago
Zen3 has parity on L1I and ECC on L1D and beyond. Even ECC on consumer hardware (it's just a few extra traces on the motherboard).

willis936 5y ago

Checksumming filesystems and file transfer protocols cover many cases. SCP, rsync, btrfs, and zfs all fix this problem. As for guaranteeing the computed data is correct: I know space systems often have two redundant computers that calculate everything and compare results. It's crazy expensive and power demanding, but it all but solves the problem.

belter 5y ago
If they have two computers and results differ, how do they decide which one is correct ? ;-)
- willis936 5y ago
  They don't. They do it again. If they never agree then Houston has a real problem.
- bonzini 5y ago
  Usually they have an odd number. The Space Shuttle had five, of which the fifth was running completely different software. In case of a 2/2 split the crew could shut down a pair (in case the failure was clear) or switch to the backup computer.
  - X6S1x6Okd1st 5y ago
    Interestingly etherum also takes the completely different software approach as well: there are 5 different main clients
- luckman212 5y ago
  a third system?