Food for thought on ram...

[Mattas27]

I don't know a TON on the mechanics of a PC, I know enough to build one and how to troubleshoot etc... So more than the average PC user in the world maybe, but my question is...

 

Is RAM still really needed in the small size and form? I mean... Why cant manufacturers throw a PCIe slot there for an M.2 and use that as ram? 

 

Please explain.

Edited November 11, 2019 by Mattas27
Type

[martward]

As far as I know RAM is still considerably faster than SSD's, even over PCIe.

[AbydosOne]

3 minutes ago, Mattas27 said:

I don't know a TON on the mechanics of a PC, I know enough to build one and how to troubleshoot etc... So more than the average PC user in the world maybe, but my question is...

 

Is RAM still really needed in the small size and form? I mean... Why cant manufacturers throw a PCIe slot there for an M.2 and use that as ram? 

 

Please explain.

Not enough bandwidth/speed. DDR4 RAM is something like 10-20GB/sec, while NVMe is maybe 2.5GB/sec in real use, best case. The CPU controls the RAM directly*, instead of translating to a serial bus and having that information decoded on the other end by the SSD controller, leading to much better response times (nanoseconds vs milliseconds).

 

* = Modern CPUs control directly. Previous systems would have a separate chip communicating with the CPU passing RAM information. As demand for RAM speed increased, the controller moved onto the CPU directly.

[porina]

Dual channel 3200 ram is about 50 GB/s bandwidth. A top end SSD might go around 5GB/s so it is quite a gap.

 

However the main reason for ever considering this is cost. SSDs are cheaper, per capacity, compared to ram. Intel Xpoint is kinda in between ram and flash SSDs and they are offering it in server configurations to be used as cheaper ram.

 

The other concern is life. Flash SSDs may only have write cycles in the ball part if hundreds. In comparison, ram is practically unlimited. Flash SSDs would wear out very quickly in this use case.

[Vishera]

RAM must be close to the CPU for direct communication and lower latency,

You don't want the CPU to look for the RAM among all the devices on the PCI-E bus,it will take longer and might cause "confusion" to the CPU.

[AbydosOne]

10 minutes ago, porina said:

Dual channel 3200 ram is about 50 GB/s bandwidth.

I actually have been running MemTest a bunch on my 3200 CAS14 system (trying and failing to track down a Heisenbug), and it tells me that it's something like 18GB/sec, which is where I got my number from. Maybe that's per stick?

[porina]

2 hours ago, AbydosOne said:

I actually have been running MemTest a bunch on my 3200 CAS14 system (trying and failing to track down a Heisenbug), and it tells me that it's something like 18GB/sec, which is where I got my number from. Maybe that's per stick?

I'm not familiar with how memtest measures it, but certainly the max bandwidth scales with the number of channels. Also, practical measurements using software tends to be reduced somewhat from the theoretical maximum. I have dual channel 3200 in my gaming system (I'm using it now) and a quick run with Aida64 read bandwidth gives 45GB/s.