[Info-vax] OS implementation languages

[Bob Gezelter]

On Thursday, August 31, 2023 at 12:35:05 AM UTC-4, gah4 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. 
> 
> Otherwise, there are many stories of running old OS in emulation on modern 
> hardware, running into problems that never would have occurred years ago. 
> 
> One is that emulators often do synchronous I/O. The I/O interrupt occurs almost 
> immediately, as seen by the OS. Some OS assume that there is time in between. 
> 
> It is, then, possible that surprises will be found when running I/O at higher speed. 
> 
> It might be useful to say which host architecture you were asking about. 
> I am sure no-one thought about 400Gb/s Ethernet for VAX.
gah4,

Ethernet (and other CSMA/CD networking approaches) in a configuration with more than a single, full duplex connection connecting two adapters are essentially limited to a maximum effective utilization of 30% before contention backoff becomes unacceptable.

Active switches, as opposed to hubs, can increase this threshold as each cable is physically connected to an interface and a switch port.

Instant I/O completion has uncovered all manner of bugs and deficiencies over the years. Most such problems are at the driver level, where a driver fails to ensure that data structures are correctly prepared for an immediate interrupt when the "Go" bit is triggered. On occasion, an application has been written to run very I/O bound with the presumption that I/O delay will slow it down, e.g., OpenVMS COPY, RSX-11 PIP, linux dd. Combine a greedy application with a high dispatch priority and voila, monopolized machine for the duration. On OpenVMS, put a process at a base priority above most users, say 6-7, and run a large COPY from one instantaneous mass storage device to another. Other users see an effectively dead machine.

Virtual machine-level synchronous I/O is a gargantuan performance impediment on many levels. My aforementioned Ph.D. dissertation on I/O performance, one of the main conclusions was that unnecessary serialization at any point in the I/O stack was a performance cliff. Serialization obstructs lower-level optimization and workload management. The mathematics could not be more definitive.

- Bob Gezelter, http://www.rlgsc.com