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It’s unclear to me what eight channel memory actually means, I’m not used to seeing that mentioned in casual architecture overviews. What’s the benefit of that?

It’s unclear to me what eight channel memory actually means, I’m not used to seeing that mentioned in casual architecture overviews. What’s the benefit of that? Is that something that a programmer must explicitly take advantage of, or is that just an automatic part of the CPU?

recrof 6y ago
from the software standpoint, it's invisible to the programmer. it's about bandwidth of the memory bus. when you have single shared memory lane, bandwidth gets split between memory modules. typically(on most of the consumer hardware) there are 2 memory channels.
kllrnohj 6y ago
Think RAID 0 but for RAM done by the CPU. Nearly every consumer CPU for the last 20 years is dual channel, meaning "2 drive RAID 0". If you've seen things like recommendations to get paired DDR memory sticks, this is why. You only get this RAID-like benefit with multiple RAM sticks (just like RAID 0 of a single drive doesn't do anything). It's also why some consumer products have unexpectedly bad performance for the CPU specs - cheaper laptops may skimp here and just run a single stick of memory instead of 2. Which means half the memory bandwidth. 8 channel then means the CPU can do this up to 8 sticks for 8x the bandwidth instead of the more common 2x.
- Hello71 6y ago
  also notably this doesn't help with latency, which is more often the sticking point.
  - gameswithgo 6y ago
    with 64 cores and 4 channels, throughput may often be a sticking point. that is like have 32 cores on a normal 2 channel desktop, throughput would often be a limiter there.
Laforet 6y ago
More channels allow more memory bandwidth, though actual performance gain is marginal unless the data being processed has very strict spatial locality (e.g. fluid dynamics simulations), most other workloads are more constrained by cache. The real reason for adding more memory channels to a general purpose CPU is actually to increase the maximum amount of memory per node. A few years ago, workstation and server platforms often allowed 3 or more DIMMs per channel with performance declining sharply with memory density. Since then both AMD and Intel have limited each channel to 2 DIMMs and increase the number of memory channels from 4 to 6/8 to make up for the loss. While more memory channels do enable better performance scaling, it has also made the CPU socket more complex and fragile than ever: The TRX40 socket has 4094 pins and literally requires the CPU to be bolted down using a torque wrench to ensure good contact.

recrof 6y ago

from the software standpoint, it's invisible to the programmer. it's about bandwidth of the memory bus. when you have single shared memory lane, bandwidth gets split between memory modules. typically(on most of the consumer hardware) there are 2 memory channels.

kllrnohj 6y ago

Think RAID 0 but for RAM done by the CPU. Nearly every consumer CPU for the last 20 years is dual channel, meaning "2 drive RAID 0". If you've seen things like recommendations to get paired DDR memory sticks, this is why. You only get this RAID-like benefit with multiple RAM sticks (just like RAID 0 of a single drive doesn't do anything). It's also why some consumer products have unexpectedly bad performance for the CPU specs - cheaper laptops may skimp here and just run a single stick of memory instead of 2. Which means half the memory bandwidth. 8 channel then means the CPU can do this up to 8 sticks for 8x the bandwidth instead of the more common 2x.

Hello71 6y ago
also notably this doesn't help with latency, which is more often the sticking point.
- gameswithgo 6y ago
  with 64 cores and 4 channels, throughput may often be a sticking point. that is like have 32 cores on a normal 2 channel desktop, throughput would often be a limiter there.

Laforet 6y ago

More channels allow more memory bandwidth, though actual performance gain is marginal unless the data being processed has very strict spatial locality (e.g. fluid dynamics simulations), most other workloads are more constrained by cache. The real reason for adding more memory channels to a general purpose CPU is actually to increase the maximum amount of memory per node. A few years ago, workstation and server platforms often allowed 3 or more DIMMs per channel with performance declining sharply with memory density. Since then both AMD and Intel have limited each channel to 2 DIMMs and increase the number of memory channels from 4 to 6/8 to make up for the loss. While more memory channels do enable better performance scaling, it has also made the CPU socket more complex and fragile than ever: The TRX40 socket has 4094 pins and literally requires the CPU to be bolted down using a torque wrench to ensure good contact.