4 ms·

Silent data corruptions at scale (2021)

84 points by losfair 3y ago

twhitmore 3y ago
Interesting. The corruption was in a math.pow() calculation, representing a compressed filesize prior to a file decompression step. Compressing data, with the increased information density & greater number of CPU instructions involved, seems obviously to increase the exposure to corruption/ bitflips. What I did wonder was why compress the filesize as an exponent? One would imagine that representing as a floating-point exponent would take lots of cycles, pretty much as many bits, and have nasty precision inaccuracies at larger sizes.
SomeoneFromCA 3y ago
Interesting paper, but has some technical errors. First of all, they keep mentioning SRAM+ECC, instead of DRAM+ECC; you cannot use gcj to inspect assembly code generated for Java method, as it will be completely different from the code generated by Hotspot; you do not need all that acrobatics to get disasm of the method, you could just add an infinite loop to the code and attach gdb to the JVM process and inspect the code or dump the core.
- MertsA 3y ago
  Disclaimer: I work at Meta and I know a couple of the authors of the paper but my work is completely unrelated to the subject of the paper. That's not a technical error, they mean SRAM in the CPU itself. You're right about gcj but that's kind of a moot point when investigating some reproducible CPU bug like this. The paper mentions all the acrobatics they went through when trying to find the root cause but if gcj would have been practical then it also would have been immediately clear if the gcj output reproduced the error or not. If it didn't reproduce, no big deal, try another approach. You might be right about it being easier to root cause with gdb directly but I'm not so sure. Starting out, you have no idea which instructions under what state are triggering the issue so you'd be looking for a needle in a haystack. A crashdump or gdb doesn't let you bisect that haystack so good luck finding your needle.
  - SomeoneFromCA 3y ago
    GCJs implementation could be so vastly different from Hotspot, you could as well rewrite it in C and check if it is failing or not. ChatGPT would generate testcase within a minute. It all depends how good you are with x64 assembly. If you are good enough, you can easily deduce what the instructions at the location do, and can potentially simply copy-paste into an asm file, compile it and check result. Would be much faster to me. Bluntly speaking, people who are not familiar with low-level debugging make an honest and succesful attempt to investigate a low-level issue. A seasoned kernel developer or reverse engineer would have just used gdb straight away.
    - MertsA 3y ago
      >A seasoned kernel developer I think you should take another look at the author list. Chris Mason counts as a seasoned kernel developer in my book. Either way I think you're missing the point. Yes gcj would be different, but there's a decent chance it could hand you a binary that reproduces the issue that you can bisect to the root cause from there. It's one thing to run it through gcj and see if it reproduces, rewriting it in C is a ton of work compared to gcj for something that might not pan out.
      - SomeoneFromCA 3y ago
        I am not missing the point, as I do not believe in authorities and someone else's evaluations of skill level of yet another person. To rewrite a simple exponentiation in C would not cause "lots of work", and pinpointing the culprit, the exponentiation does not require any gdb debugging and disassembling. In fact, just knowing that exponentiation has caused that suggests faulty hardware and not further investigation required. You should probably invite these people themselves to the discussion instead of speaking on their behalf. Not productive.
ekelsen 3y ago
I wrote an article about these affecting LLM training at https://www.adept.ai/blog/sherlock-sdc https://www.adept.ai/blog/sherlock-sdc
- metricspaces 3y ago
  [dead]
- walterbell 3y ago
  Thanks, does your blog have a working RSS feed?
userbinator 3y ago
Very interesting topic, but rather low on detail --- really wanted to see what those 60 lines of Asm that allegedly show a faulty CPU instruction were, and also surprised that it wasn't intermittent; in my experience, CPU problems usually are intermittent and heavily dependent upon prior state, and manually stepping through with a debugger has never shown the "1+1=3" type of situation they claim. That said, I wonder if LINPACK'ing would've found it, as that is known to be a very powerful stress-test with divisive opinions among the overclocking community; some, including me, claim that a system can never be considered stable it if fails LINPACK since that is essentially showing intermittent "1+1=3" behaviour, while others are fine with "occasional" discrepancies in its output since the system otherwise appears to be stable.
- thfuran 3y ago
  >while others are fine with "occasional" discrepancies I guess I'd probably be okay with that if the only thing I ever used the computer for was gaming.
- jorticka 3y ago
  Like all stress tests, linpack will find some errors, but not all. I had memory stability issues which would immediatly show under Prime95 (less than 1 minute) but pass hours of Linpack.
  - sirlancer 3y ago
    Prime95 is my gold standard for CPU and memory testing. Everything from desktops to HPC and clustered filesystems get a 24 hour “blend” of tests. If that passes without any instability or bit flips then we’re ready for production.
    - c0l0 3y ago
      In my experience, LINPACK (at least the Intel MKL on GenuineIntel combination) is both quicker and more thorough in finding setups that are not actually stable/reliable.
opisthenar84 3y ago
Might be a noob question but for truly important data, couldn't SDCs be detected by using ECC everywhere?
- jandrewrogers 3y ago
  ECC isn’t free and ECC has a limited ability to detect all statistically plausible errors. Additionally, error correction in hardware is frequently defined by standards, some of which have backward compatibility requirements that go back decades. This is why, for example, reliable software often uses (quasi-)cryptographic checksums at all I/O boundaries. There is error correction in the hardware but in some parts of the silicon that error correction is weak enough that it is likely to eventually deliver a false negative in large scale systems. None of this is free, and there are both hardware and software solutions for mitigating various categories of risk. It is explicitly modeled as an economics problem i.e. how does the cost of not mitigating a risk, if it materializes, compare to the cost of minimizing or eliminating it. In many cases, the optimal solution is unintuitive, such as computing everything twice or thrice and comparing the results rather than using error correction.
- lobochrome 3y ago
  In those cases, the CPU makes a false calculation independent of what's done in RAM. It can be solved by having flop redundancy as in system z - but nobody at Google or Meta would be considering big metal. From my point of view, this technology problem may be interesting academically (and good for pretending to be important in the hierarchy at those companies) but a non-issue at scale business-wise in modern data centers. Have a blade that once in a while acts funny? Trash and replace. Who cares what particular hiccup the CPU had.
  - delroth 3y ago
    > a non-issue at scale business-wise in modern data centers. I've worked on similar stuff in the past at Google and you couldn't be more wrong. For example, if your CPU screwed up an AES calculation involved in wrapping an encryption key, you might end up with fairly large amounts of data that can't be decrypted anymore. Sometimes the failures are symmetric enough that the same machine might be able to decrypt the data it corrupted, which means a single machine might not be able to easily detect such problems. We used to run extensive crypto self testing as part of the initialization of our KMS service for that reason.
dataflow 3y ago
Google also had a "Cores That Don't Count" paper on so-called "mercurial cores" https://news.ycombinator.com/item?id=27378624 https://news.ycombinator.com/item?id=27378624 as well as a presentation https://www.youtube.com/watch?v=QMF3rqhjYuM https://www.youtube.com/watch?v=QMF3rqhjYuM
- deleted 3y ago
  [deleted]
dang 3y ago
Related: Meta quickly detects silent data corruptions at scale - https://news.ycombinator.com/item?id=30905636 https://news.ycombinator.com/item?id=30905636 - April 2022 (95 comments) Silent Data Corruptions at Scale - https://news.ycombinator.com/item?id=27484866 https://news.ycombinator.com/item?id=27484866 - June 2021 (12 comments)

84 points by losfair 3y ago

Interesting. The corruption was in a math.pow() calculation, representing a compressed filesize prior to a file decompression step. Compressing data, with the increased information density & greater number of CPU instructions involved, seems obviously to increase the exposure to corruption/ bitflips. What I did wonder was why compress the filesize as an exponent? One would imagine that representing as a floating-point exponent would take lots of cycles, pretty much as many bits, and have nasty precision inaccuracies at larger sizes.

SomeoneFromCA 3y ago

Interesting paper, but has some technical errors. First of all, they keep mentioning SRAM+ECC, instead of DRAM+ECC; you cannot use gcj to inspect assembly code generated for Java method, as it will be completely different from the code generated by Hotspot; you do not need all that acrobatics to get disasm of the method, you could just add an infinite loop to the code and attach gdb to the JVM process and inspect the code or dump the core.

MertsA 3y ago
Disclaimer: I work at Meta and I know a couple of the authors of the paper but my work is completely unrelated to the subject of the paper. That's not a technical error, they mean SRAM in the CPU itself. You're right about gcj but that's kind of a moot point when investigating some reproducible CPU bug like this. The paper mentions all the acrobatics they went through when trying to find the root cause but if gcj would have been practical then it also would have been immediately clear if the gcj output reproduced the error or not. If it didn't reproduce, no big deal, try another approach. You might be right about it being easier to root cause with gdb directly but I'm not so sure. Starting out, you have no idea which instructions under what state are triggering the issue so you'd be looking for a needle in a haystack. A crashdump or gdb doesn't let you bisect that haystack so good luck finding your needle.
- SomeoneFromCA 3y ago
  GCJs implementation could be so vastly different from Hotspot, you could as well rewrite it in C and check if it is failing or not. ChatGPT would generate testcase within a minute. It all depends how good you are with x64 assembly. If you are good enough, you can easily deduce what the instructions at the location do, and can potentially simply copy-paste into an asm file, compile it and check result. Would be much faster to me. Bluntly speaking, people who are not familiar with low-level debugging make an honest and succesful attempt to investigate a low-level issue. A seasoned kernel developer or reverse engineer would have just used gdb straight away.
  - MertsA 3y ago
    >A seasoned kernel developer I think you should take another look at the author list. Chris Mason counts as a seasoned kernel developer in my book. Either way I think you're missing the point. Yes gcj would be different, but there's a decent chance it could hand you a binary that reproduces the issue that you can bisect to the root cause from there. It's one thing to run it through gcj and see if it reproduces, rewriting it in C is a ton of work compared to gcj for something that might not pan out.
    - SomeoneFromCA 3y ago
      I am not missing the point, as I do not believe in authorities and someone else's evaluations of skill level of yet another person. To rewrite a simple exponentiation in C would not cause "lots of work", and pinpointing the culprit, the exponentiation does not require any gdb debugging and disassembling. In fact, just knowing that exponentiation has caused that suggests faulty hardware and not further investigation required. You should probably invite these people themselves to the discussion instead of speaking on their behalf. Not productive.

ekelsen 3y ago

I wrote an article about these affecting LLM training at https://www.adept.ai/blog/sherlock-sdc https://www.adept.ai/blog/sherlock-sdc

metricspaces 3y ago
[dead]
walterbell 3y ago
Thanks, does your blog have a working RSS feed?

userbinator 3y ago

Very interesting topic, but rather low on detail --- really wanted to see what those 60 lines of Asm that allegedly show a faulty CPU instruction were, and also surprised that it wasn't intermittent; in my experience, CPU problems usually are intermittent and heavily dependent upon prior state, and manually stepping through with a debugger has never shown the "1+1=3" type of situation they claim. That said, I wonder if LINPACK'ing would've found it, as that is known to be a very powerful stress-test with divisive opinions among the overclocking community; some, including me, claim that a system can never be considered stable it if fails LINPACK since that is essentially showing intermittent "1+1=3" behaviour, while others are fine with "occasional" discrepancies in its output since the system otherwise appears to be stable.

thfuran 3y ago
>while others are fine with "occasional" discrepancies I guess I'd probably be okay with that if the only thing I ever used the computer for was gaming.
jorticka 3y ago
Like all stress tests, linpack will find some errors, but not all. I had memory stability issues which would immediatly show under Prime95 (less than 1 minute) but pass hours of Linpack.
- sirlancer 3y ago
  Prime95 is my gold standard for CPU and memory testing. Everything from desktops to HPC and clustered filesystems get a 24 hour “blend” of tests. If that passes without any instability or bit flips then we’re ready for production.
  - c0l0 3y ago
    In my experience, LINPACK (at least the Intel MKL on GenuineIntel combination) is both quicker and more thorough in finding setups that are not actually stable/reliable.

opisthenar84 3y ago

Might be a noob question but for truly important data, couldn't SDCs be detected by using ECC everywhere?

jandrewrogers 3y ago
ECC isn’t free and ECC has a limited ability to detect all statistically plausible errors. Additionally, error correction in hardware is frequently defined by standards, some of which have backward compatibility requirements that go back decades. This is why, for example, reliable software often uses (quasi-)cryptographic checksums at all I/O boundaries. There is error correction in the hardware but in some parts of the silicon that error correction is weak enough that it is likely to eventually deliver a false negative in large scale systems. None of this is free, and there are both hardware and software solutions for mitigating various categories of risk. It is explicitly modeled as an economics problem i.e. how does the cost of not mitigating a risk, if it materializes, compare to the cost of minimizing or eliminating it. In many cases, the optimal solution is unintuitive, such as computing everything twice or thrice and comparing the results rather than using error correction.
lobochrome 3y ago
In those cases, the CPU makes a false calculation independent of what's done in RAM. It can be solved by having flop redundancy as in system z - but nobody at Google or Meta would be considering big metal. From my point of view, this technology problem may be interesting academically (and good for pretending to be important in the hierarchy at those companies) but a non-issue at scale business-wise in modern data centers. Have a blade that once in a while acts funny? Trash and replace. Who cares what particular hiccup the CPU had.
- delroth 3y ago
  > a non-issue at scale business-wise in modern data centers. I've worked on similar stuff in the past at Google and you couldn't be more wrong. For example, if your CPU screwed up an AES calculation involved in wrapping an encryption key, you might end up with fairly large amounts of data that can't be decrypted anymore. Sometimes the failures are symmetric enough that the same machine might be able to decrypt the data it corrupted, which means a single machine might not be able to easily detect such problems. We used to run extensive crypto self testing as part of the initialization of our KMS service for that reason.

dataflow 3y ago

Google also had a "Cores That Don't Count" paper on so-called "mercurial cores" https://news.ycombinator.com/item?id=27378624 https://news.ycombinator.com/item?id=27378624 as well as a presentation https://www.youtube.com/watch?v=QMF3rqhjYuM https://www.youtube.com/watch?v=QMF3rqhjYuM

deleted 3y ago
[deleted]

dang 3y ago

Related: Meta quickly detects silent data corruptions at scale - https://news.ycombinator.com/item?id=30905636 https://news.ycombinator.com/item?id=30905636 - April 2022 (95 comments) Silent Data Corruptions at Scale - https://news.ycombinator.com/item?id=27484866 https://news.ycombinator.com/item?id=27484866 - June 2021 (12 comments)