[j-nsp] MX104 limitation

[adamv0025 at netconsultings.com]

>  Saku Ytti
> Sent: Thursday, March 23, 2017 9:13 AM
> 
> It's still about 75Gbps (i.e. for example 35Gbps+40Gbps) and 55Mpps.
> 
> But memory bandwidth is dependant on how well packet aligns into cells, in
> manufactured example you could have packet which cause singly byte to be
> transferred on second cell, essentially doubling internal memory bandwidth
> requirement.
> Traffic hitting QX will also experience significantly lower memory bandwidth.
> 
> This is not MX104 specific, same applies to MX80, and MPC1, MPC2, MPC3 on
> per Trio basis.
> 
I'd generalize even further, this actually applies to any NPU of any vendor.

Every NPU has it's "pps budget" -this budget gets consumed by packets coming in and features being enabled. 
So the more pps budget you have the better the NPU performs,
So yeah ideally you'd get line-rate performance for 64B frames. Line-rate @10Gbps at L2(64B)@L1(84B) is 29.761Mpps (two directions).  
Of course you don't need 30Mpps budget to send 64B frames back to back in both directions but what it gives you is space for features. 

Personally I keep a table of all NPUs I was considering and their pps budget to help me with design choices and the overall trend I see, especially with advent of 100GE LCs, is that pps budget is far from ideal cause the chips are not there yet and vendors are desperately trying to achieve 100GE for at least some packet sizes, but there are exceptions and yeah there's a difference in price for premium performance.

And as Saku mentioned there is also the memory performance dimension -the "bps budget". 
That is how fast can data be read from all the memories and how fast they can be moved across the chassis (between NPUs).   
That's where mem tx/rx performance comes to play along with memory addressing structure/efficiency,  cell/frame to packet alignment (although this is being addressed in modern platforms -by adding flexibility back), etc...     

I tend to say it a lot lately in my posts, but I say it again, you get what you pay for.      

adam

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