Memory & Caches

[@ramenguy3108]

Hey man, awesome presentation! Great insight and easy to follow! There is not much on youtube of such quality. Hope to see more of your videos in the future, keep it up!

[@TheNewTimeNetwork]

Great video overall. Especially the set/way-mechanism of caches was much better explained than the college materials managed to do. There's a correction/extension about DDR memory I would like to make: The bus clock of DDR3 runs between 400 to 1066 MHz, for a data transfer rate of 800 to 2133 MT/s (because of Double Transfer Rate). Only the actual DRAM cores run at a quarter of the bus frequency (100-266 MHz). In order to satisfy eight transfers (four bus clock cycles with DDR) for each DRAM cycle, eight columns from the sense amp latches are sent in a burst. I also assume that the random access time to DRAM increases well beyond 250 cycles not because of prefetcher noise, but because new DRAM rows have to be openened all the time, which takes a while. Also, PCI-Express is indeed now mostly connected right to the CPUs and can therefore have non-uniform access characteristics like RAM. Most desktop systems have southbridges that can fan out more PCI-E lanes for less important peripherals, high-end servers less so. The southbridge on multi-socket systems is also usually connected to just one socket.

[@ZyklonV]

I recognise a lot of this from the white paper “What every programmer should know about memory” (Drepper, 2007) :) I’m wondering if an up to date / more modern info compendium exists on the subject of cache use (white paper, video presentation, or other).

[@MattGodbolt]

ZyklonV absolutely. I credit that paper at the end. I'm not sure of any comprehensive update to it, or indeed any major changes in the ways caches work. With the exception of the new non-inclusive L3 in intel CPUs (though I think AMD has always been non inclusive). Of course, I don't mention inclusivity here...

[@ZyklonV]

Ah, shame on me for not checking the video description either! Thank you for the video, it does make the whole thing more digestible.

[@MattGodbolt]

ZyklonV oh golly no problem! Thanks for taking the time to comment and for your kind words