[Info-vax] OS implementation languages

[Dan Cross]

In article <215e5a5a-d9b6-40fb-ad94-3ee8e8ad92e8n at googlegroups.com>,
gah4  <gah4 at u.washington.edu> wrote:
>On Tuesday, August 29, 2023 at 10:25:31 AM UTC-7, Simon Clubley wrote:
>  
> (snip)
>
>> 400GB/s ??? Is that all ??? Amateurs!!! :-) 
> 
>> On a more serious note, I wonder what the maximum rate VMS is capable 
>> of emitting data at if it was using the fastest network hardware 
>> available.
> 
>I am not sure what hardware can do now.
>
>Traditionally, Ethernet was much faster than processors, such that the
>shared media could handle the load. 
>
>That is less obvious now, but a 400Gb/s network doesn't mean that one host
>can go that fast.

400Gbps is at the high-end of what one can deliver to a single
system at this point; one or two infiniband cards into a PCIe
gen4 backplane will get you there.

This will overwhelm just about any general purpose CPU currently
on the market, so a lot of overhead is offloaded to accelerator
hardware on the NIC, but making effective use of _that_ requires
specialized drivers and cooperation with the host.  As a simple
example, the NIC may support offloading layer 3 checksum
calculations, but in order to use that effectively the host
software has to know about it, configure the hardware to do it,
and configure itself to avoid repeating the calculations higher
up in the stack (otherwise, what's the point of offloading?).

This also implies that, "throw more hardware at it!" is only
part of a possible solution to a performance problem: if the
software isn't similarly modified to take advantage of the
capabilities of that hardware, you may not see much in terms of
actual gains.

All of this is to say that the OS can have large effects on
realized, real-world performance on high-end hardware, and it
can be useful to quantify that so as to better understand
performance in context.

	- Dan C.