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chuckula
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PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 2:36 pm

A feature-complete draft of the spec for PCIe 4.0 has just been published today! [Edit: If you want to actually read it, you have to be a member of the PCI SIG. I'm not!]

Here's a story with more info

At this stage the draft looks to be pretty much what you'll get with PCIe 4.0 from a technological perspective, but there is still more legal wrangling and other bureaucracy before it's at the 1.0 release stage. It should be enough for hardware vendors to make sure prototypes (that they have already been working on) actually meet the technical standard though.
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chuckula
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 2:44 pm

As a followup: There was also a press release about developments for PCIe 5.0 that is obviously still a ways out: http://news.sys-con.com/node/4099882
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whm1974
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 2:47 pm

At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...
 
TwistedKestrel
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:15 pm

It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.
 
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:45 pm

whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...

For consumers? Probably not, but it'll drop the cost of boards if you only need 8 lanes or something for the same bandwidth that 32 gave you before.

For everyone else? Absolutely. MOAR BANDWIDTH.
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chuckula
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:48 pm

Waco wrote:
whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...

For consumers? Probably not, but it'll drop the cost of boards if you only need 8 lanes or something for the same bandwidth that 32 gave you before.

For everyone else? Absolutely. MOAR BANDWIDTH.


Imagine a board that delivers the same total bandwidth of 64 PCIe 3.0 lanes like the Threadripper but only needs wiring for 32 lanes due to PCIe 4.0.

You might not think it's too impressive, until you see the reduced price of the board.
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whm1974
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:49 pm

TwistedKestrel wrote:
It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.

Even so, we will still be better off increasing the number of lanes instead.
 
whm1974
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:49 pm

chuckula wrote:
Waco wrote:
whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...

For consumers? Probably not, but it'll drop the cost of boards if you only need 8 lanes or something for the same bandwidth that 32 gave you before.

For everyone else? Absolutely. MOAR BANDWIDTH.


Imagine a board that delivers the same total bandwidth of 64 PCIe 3.0 lanes like the Threadripper but only needs wiring for 32 lanes due to PCIe 4.0.

You might not think it's too impressive, until you see the reduced price of the board.

OK you convinced me, it can be useful. Now what about DDR5?
 
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:52 pm

chuckula wrote:
Waco wrote:
whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...

For consumers? Probably not, but it'll drop the cost of boards if you only need 8 lanes or something for the same bandwidth that 32 gave you before.

For everyone else? Absolutely. MOAR BANDWIDTH.


Imagine a board that delivers the same total bandwidth of 64 PCIe 3.0 lanes like the Threadripper but only needs wiring for 32 lanes due to PCIe 4.0.

You might not think it's too impressive, until you see the reduced price of the board.

I'd rather see 64 lanes of PCIe 4.0. :P
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:53 pm

whm1974 wrote:
OK you convinced me, it can be useful. Now what about DDR5?

I'm a bad person to ask. The workloads that tend to run on my stuff are almost entirely memory-bandwidth bound for much of the compute cycle. :)
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chuckula
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:53 pm

whm1974 wrote:
TwistedKestrel wrote:
It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.

Even so, we will still be better off increasing the number of lanes instead.


Will we? Lanes cost money and power to implement. Waco is super excited about the Threadripper because he will actually use all 60 lanes* on the board that hook directly to the CPU. However, he's in the 1% of users of that platform that will actually be using all that bandwidth.

However, if you go to PCIe 4.0, then a 16x connector will be delivering twice the bandwidth to a single GPU, even if you never use any of the other lanes (and there's never going to be an x32 PCIe 3.0 connector that could do the same job). As of right now that bandwidth probably isn't going to make for huge gains, but these I/O standards need to be put in place prior to the launch of products that actually use them.

* There are 64 lanes but 4 go to the chipset.
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:57 pm

whm1974 wrote:
OK you convinced me, it can be useful. Now what about DDR5?


What are you asking whm? If DDR5 would be useful?

Of course it will be. Higher speeds with less power. Gimme dat bandwidth.
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 3:59 pm

"Not useful"?? If you use an M.2 port then you will find PCIe 4.0 useful. M.2 ports are going to stay x4 width for a long time, and there are a couple NVMe M.2 SSDs that are flirting with the 4GB/s limitation. Doubling that to 8GB/s will give the spec plenty of headroom... because if somebody needs >8GB/s from storage then they should probably be using PCIe cards anyway. :P
 
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 4:01 pm

chuckula wrote:
whm1974 wrote:
TwistedKestrel wrote:
It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.

Even so, we will still be better off increasing the number of lanes instead.


Will we? Lanes cost money and power to implement. Waco is super excited about the Threadripper because he will actually use all 60 lanes* on the board that hook directly to the CPU. However, he's in the 1% of users of that platform that will actually be using all that bandwidth.

However, if you go to PCIe 4.0, then a 16x connector will be delivering twice the bandwidth to a single GPU, even if you never use any of the other lanes (and there's never going to be an x32 PCIe 3.0 connector that could do the same job). As of right now that bandwidth probably isn't going to make for huge gains, but these I/O standards need to be put in place prior to the launch of products that actually use them.

* There are 64 lanes but 4 go to the chipset.

It is my understanding that still is more than plenty of bandwidth left on PCIe 16x v3 slots even for future high end GPUs.
 
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 4:03 pm

It's not just about bandwidth for single GPUs whm. Although it's not wrong that PCIe 3.0 x16 is plenty of bandwidth for cardss today, GPUs are FAR from the only thing that uses PCIe on a motherboard. Think bigger, man.
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 4:05 pm

whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...


It'd be incredibly useful for Thunderbolt 3. TB3 currently mandates up to 4 x PCI-E 3.0 lanes per port. Keeping the 4 lanes but increasing their bandwidth would help A LOT for things like external GPUs. Imagine those 4 lanes giving the equivalent bandwidth of a full 16 lanes of PCI-E 2.0, for example.
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 4:06 pm

Kurotetsu wrote:
whm1974 wrote:
At this point I'm not sure if PCIe v4 could be useful during the foreseeable future. Although I supposed enough time has passed to update the standard...


It'd be incredibly useful for Thunderbolt 3. TB3 currently mandates up to 4 x PCI-E 3.0 lanes per port. Keeping the 4 lanes but increasing their bandwidth would help A LOT for things like external GPUs. Imagine those 4 lanes giving the equivalent bandwidth of a full 16 lanes of PCI-E 2.0, for example.


Or if things don't need more bandwidth, using the excess lanes for other purposes. Like another TB3 port.
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whm1974
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 4:33 pm

OK thanks guys. I was wrong about this.
 
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Re: PCIe 4.0 ALMOST Ready

Wed Jun 07, 2017 6:25 pm

whm1974 wrote:
TwistedKestrel wrote:
It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.

Even so, we will still be better off increasing the number of lanes instead.

Pin count matters. Number of PCB traces matters. You can increase bandwidth without adding pins/traces if you increase the speed of each lane.
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Re: PCIe 4.0 ALMOST Ready

Thu Jun 08, 2017 12:16 am

just brew it! wrote:
whm1974 wrote:
TwistedKestrel wrote:
It could definitely be useful. Not for single devices like GPUs, but getting more bandwidth out of the same number lanes on a system-wide basis could make current lane allocation shenanigans less of an issue.

Even so, we will still be better off increasing the number of lanes instead.

Pin count matters. Number of PCB traces matters. You can increase bandwidth without adding pins/traces if you increase the speed of each lane.


a) AMD/NVidia will create 4 lane cards to save on costs
b) MB vendors will not create electrically 16 lane and tell GPU vendors to go away
c) Will now have around 31.5GB/sec transfer capability for all those 8GB VRAM GPUs
 
whm1974
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Re: PCIe 4.0 ALMOST Ready

Thu Jun 08, 2017 1:29 am

blahsaysblah wrote:
a) AMD/NVidia will create 4 lane cards to save on costs
b) MB vendors will not create electrically 16 lane and tell GPU vendors to go away
c) Will now have around 31.5GB/sec transfer capability for all those 8GB VRAM GPUs

I don't believe b will happen.
 
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Re: PCIe 4.0 ALMOST Ready

Thu Jun 08, 2017 3:13 am

What does this mean for NVLink? Irrelevant POWER thingy, or does it still hold an advantage?
 
chuckula
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Re: PCIe 4.0 ALMOST Ready

Thu Jun 08, 2017 6:46 am

Zizy wrote:
What does this mean for NVLink? Irrelevant POWER thingy, or does it still hold an advantage?


NVLink may still have some advantages for dedicated high-end Nvidia GPU systems, but it's never going to be a standard that will replace PCIe in the vast majority of systems, even for GPUs.
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