[Simh] some performance issues

[Henry Bent]

On Tue, 5 Mar 2019 at 09:14, gérard Calliet <gerard.calliet at pia-sofer.fr>
wrote:

> Hello,
>
> Again I have some performance issues with simh.
>
> I built it  on OpenVMS Itanium. It runs on a process with enought
> memory, it takes 1 of the 8 CPU Itanium with 100%, no page faults, and
> 100 BIO by second.
>
> On the emulated VAX we are running processes CPU intensive (old Ada
> compiler), they take 100% of the CPU, also very few page fault, very few
> IO.
>
> And the CPU time is about 2 or 3 times the CPU time on the hardware, and
> so elapsed time 3 or 4 times. (For my understanding, it is difficult to
> think the same stream of instructions can use different CPU times
> between hardware and SIMH).
>
> Where can I do something to improve performance? Are there parameters
> (or constant in compilation) in SIMH which could help? And the specific
> context is processes doing almost only CPU computing.
>
>
I'm pretty sure I said this the last time you asked, but compile SIMH with
profile-directed feedback.  I'm not a VMS person so I know very little
about the compiler there, but I took a quick look at the manual and it
appears to be a supported option.  In my testing, profile-directed feedback
from a real-life workload increased SIMH's speed significantly, up to 66%
if memory serves.

-Henry
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