4 ms·

> Why is it impossible to parallelize this? One of the mentioned articles touches on this: http://utaharch.blogspot.com/2013/11/a-dram-refresh-tutorial.html ht

by majke 8y ago

> Why is it impossible to parallelize this? One of the mentioned articles touches on this: http://utaharch.blogspot.com/2013/11/a-dram-refresh-tutorial.html http://utaharch.blogspot.com/2013/11/a-dram-refresh-tutorial... > Upon receiving a refresh command, the DRAM chips enter a refresh mode that has been carefully designed to perform the maximum amount of cell refresh in as little time as possible. During this time, the current carrying capabilities of the power delivery network and the charge pumps are stretched to the limit. I guess: it's actually hard to deliver power to refresh all the bits at once? Also note that ancient cpus like Z80 had the memory refresh machinery built into CPU as opposed to memory https://en.wikipedia.org/wiki/Memory_refresh https://en.wikipedia.org/wiki/Memory_refresh > What's the cause of the 3200ns+ readings? No idea. Random noise? Timing interrupt? Some peripheral doing DMA transfer? Kernel context switch? System Management Mode? https://en.wikipedia.org/wiki/System_Management_Mode https://en.wikipedia.org/wiki/System_Management_Mode Feel encouraged to run the code and try to debug it!

dragontamer 8y ago
> I guess: it's actually hard to deliver power to refresh all the bits at once? You can't refresh all the bits at once. You only have something like 256kB worth of sense amplifiers across 2GB of RAM (Guesstimates from my memory: but the point is that you have much much FEWER sense-amplifiers than actual RAM). You need a sense amplifier to read RAM safely. Each time you read from DRAM, it destroys the data. Only a sense amplifier can read data safely, store it for a bit, and then write it back. Since you only have 256kB of sense amplifiers, you have to refresh the data in chunks of 256kB at a time. Not all sense amplifiers are "gang'ed up" together: they're actually broken up into 16-banks of sense amplifiers. But for whatever reason, modern DDR4 spec seems to ask all sense amplifiers to refresh together in a single command. In any case, you can at best, get 16x the parallelism (theoretically: since the spec doesn't allow for this) by imagining a "bank-specific refresh command". That's a lot of complexity though, and I'm not even sure if you really gain anything from it. Its probably best to just refresh all 16-banks at the same time.
- rasz 8y ago
  > modern DDR4 spec seems to ask all sense amplifiers to refresh together in a single command Per bank auto-refresh could be exploited with elaborate memory controller algorithm trying to always prioritize refreshing unused/least used banks, except it was broken by design and you couldnt control which bank to refresh. Nobody even bothered implementing per row refresh counters to skip freshly read rows. Rowhammer is a real shitshow exposing sloppy memory engineering in the industry.

by majke 8y ago

dragontamer 8y ago

> I guess: it's actually hard to deliver power to refresh all the bits at once? You can't refresh all the bits at once. You only have something like 256kB worth of sense amplifiers across 2GB of RAM (Guesstimates from my memory: but the point is that you have much much FEWER sense-amplifiers than actual RAM). You need a sense amplifier to read RAM safely. Each time you read from DRAM, it destroys the data. Only a sense amplifier can read data safely, store it for a bit, and then write it back. Since you only have 256kB of sense amplifiers, you have to refresh the data in chunks of 256kB at a time. Not all sense amplifiers are "gang'ed up" together: they're actually broken up into 16-banks of sense amplifiers. But for whatever reason, modern DDR4 spec seems to ask all sense amplifiers to refresh together in a single command. In any case, you can at best, get 16x the parallelism (theoretically: since the spec doesn't allow for this) by imagining a "bank-specific refresh command". That's a lot of complexity though, and I'm not even sure if you really gain anything from it. Its probably best to just refresh all 16-banks at the same time.

rasz 8y ago
> modern DDR4 spec seems to ask all sense amplifiers to refresh together in a single command Per bank auto-refresh could be exploited with elaborate memory controller algorithm trying to always prioritize refreshing unused/least used banks, except it was broken by design and you couldnt control which bank to refresh. Nobody even bothered implementing per row refresh counters to skip freshly read rows. Rowhammer is a real shitshow exposing sloppy memory engineering in the industry.