[Simh] VAX Emulation Speed Ramblings...

[Robert Lawrence]

This "Accelerated CPU Emulation" sounds very similar in concept to JIT
used in many emulators such a ARanyM (Atari ST), Baskilisk (Mac 68K),
UAE (Amiga), and PearPC (PowerPC Mac).

I am not an emulator-level programmer, but I wonder if JIT can be
applied to simh?

Robert

Norman Lastovica wrote:

>assuming that this REVIVER is (or is based on) charon-vax, SRI claims:
>
>	"Using a technology known as Accelerated CPU Emulation (ACE), 
>	the emulator dynamically optimizes the emulation of the VAX 
>	instructions, providing an improvement of 3 to 5 times over 
>	emulators without ACE.
>
>	"The ACE technology in CHARON-VAX/Industrial Plus does not 
>	require intermediate storage of translated code, but provides 
>	its full capability instantly. As the optimization takes place 
>	below the emulated hardware level, the emulator remains fully 
>	VAX hardware compatible and is completely transparent to VAX 
>	operating systems and applications, like the earlier CHARON-VAX 
>	emulators. "
>
>  
>
>>-----Original Message-----
>>From: simh-bounces at trailing-edge.com
>>[mailto:simh-bounces at trailing-edge.com]On Behalf Of Kevin Brunt
>>Sent: Thursday, August 26, 2004 2:48 PM
>>To: Kevin Handy
>>Cc: simh at trailing-edge.com
>>Subject: Re: [Simh] VAX Emulation Speed Ramblings...
>>
>>
>>
>>On Thu, 26 Aug 2004 13:01:56 -0600 Kevin Handy <kth at srv.net> wrote:
>>
>>    
>>
>>>Robert Lawrence wrote:
>>>
>>>      
>>>
>>>>According to 
>>>>http://www.comwaretech.com/products/legacy_solutions/microvax.shtml,
>>>>"The REVIVER-VAX performance scales linearly with the PC's CPU speed. 
>>>>For integer operations, REVIVER-VAX provides a performance of about 
>>>>one VAX Unit of Processing (VUP) per 100 MHz of CPU clock. Disk I/O is 
>>>>5 to 20 times faster than on the original VAX."
>>>>
>>>>How are they doing this?  Are they using some sort of JIT?  I get 
>>>>about 5 VUPs from a 1.2ghz using simh.  According to their 
>>>>calculations, 12 VUPs should be possible.
>>>>
>>>>What methods can be used to increase performance of simh/vax on any 
>>>>given pc?
>>>>
>>>>        
>>>>
>>>You are only looking at a ~2x difference in speed. Some possibilities:
>>>
>>>1. Hand tuned machine language code.
>>>
>>>Compilers can do a good job, but handcrafted machine code can
>>>make a large improvement, and a 2x difference should be possible.
>>>
>>>However, you would then tie the emulator to run on only one
>>>specific CPU.
>>>
>>>2. Improve the optimizers.
>>>
>>>Make the compilers produce better/faster code.
>>>
>>>3. Hand optimization.
>>>
>>>Look for hot-spots in the simh code, and speed them up.
>>>
>>>4. Less accurate emulation.
>>>
>>>If it doesn't have to produce the correct answer, you can make it
>>>as fast as you want.
>>>
>>>      
>>>
>>>>Thanks,
>>>>        
>>>>
>>You've possibly missed the underlying point which is that it's not 
>>really fair comparing simh with a commercial product. Somebody is being 
>>*paid* to make 'REVIVER-VAX' fast, no doubt getting the performance at 
>>the expense of maintainability and portability - tuning a program up is 
>>always something that is done *after* you've finished writing it.
>>
>>The first place to look at in trying to speed up any emulator has to be 
>>the memory access code - it's accessed repeatedly during (almost) every 
>>instruction cycle. Every native instruction squeezed out here would 
>>have the greatest overall effect.
>>
>>Kevin
>>
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>>    
>>
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