Falcon FPU overclocking results

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Falcon FPU overclocking results

Post by exxos »

While this has been talked about before in the past, It is something I started to test today while I had my falcon "out".. Nobody posted any results from what I could see , so here they are...68882 40mhz FPU was used.

I'm using Frank's fractal program to test the FPU speed out with.
fract.zip
So far the results are as follows..

16mhz MB clock - 65.105 seconds
16mhz oscillator - 63.139 seconds
25mhz MB clock - 51.37 seconds
32mhz MB clock - 46.115 seconds
50mhz oscillator - 44.98 seconds
64mhz oscillator - 44.69 seconds
75mhz oscillator - crashed

Interesting that a external 16mhz osc actually runs faster than the stock 16mhz clock! Also after 50mhz not much speed gain at all. Overall it gives 32% speed boost at 50mhz-64mhz. I suspect even if overclocking was possible over 64mhz, it wouldn't give any real speed boost.
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Re: Falcon FPU overclocking results

Post by Frank B »

Just be careful with anything changing timer c. It is relying on it being a 200 hertz clock.
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Re: Falcon FPU overclocking results

Post by exxos »

Frank B wrote:Just be careful with anything changing timer c. It is relying on it being a 200 hertz clock.
Its a stock machine, The only overclock is the FPU itself, so all should be good :)
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Re: Falcon FPU overclocking results

Post by mfro »

if you have, say, 40% FPU instructions in a given piece of code (I don't think you'll find much more than that in any real-world program) and you force these executing twice as fast, you'll have an overall performance gain of 20%. If you'd (theoretically) manage to execute the FPU code in zero time (infinity times faster), the performance gain would be - obviously - 40%.

You pretty much already passed the cost/value optimum ;)
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Re: Falcon FPU overclocking results

Post by dml »

What's the FPU program written in?

I have some hand-assembled FPU code lying around, albeit not in a demo form. 4x4 matrix multiplies are expensive enough :D

The main problem with C-compiling float stuff is you don't know what its actually doing without disassembling it to make sure. It could be a bunch of library function calls with very little FPU bottlenecking once the CPU/FPU clock ratio gets really biased. Even if it's not making calls, it could be 30% memory accesses or a bunch of CPU stuff in between.

Also - the 16mhz clock performance difference is probably to do with drifting phase. The FPU device supports async bus activity, but it's timing behaviour when that occurs may not be ideal.
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Re: Falcon FPU overclocking results

Post by Cyprian »

Exxos is it 16MHz or 32MHz FPU?
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Re: Falcon FPU overclocking results

Post by exxos »

Cyprian wrote:Exxos is it 16MHz or 32MHz FPU?
40mhz ;)
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Re: Falcon FPU overclocking results

Post by stimpy »

dml wrote: Also - the 16mhz clock performance difference is probably to do with drifting phase. The FPU device supports async bus activity, but it's timing behaviour when that occurs may not be ideal.
I was going to say, try inverting the clock from the MB too.
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Re: Falcon FPU overclocking results

Post by Cyprian »

Below figures for TT:
32MHz - 20.199
16MHz - 30.108

Video Mode: ST LOW

FPU: MC68882FN33A
OC35H
QERQ9108

It seems that the CPU is the bottleneck.
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Re: Falcon FPU overclocking results

Post by alexh »

Wow that's a big difference on the TT. Looks like the test must be CPU/RAM limited rather than FPU limited?
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Re: Falcon FPU overclocking results

Post by Frank B »

alexh wrote:Wow that's a big difference on the TT. Looks like the test must be CPU/RAM limited rather than FPU limited?
It is drawing lines into a buffer then blitting it line by line to the screen. It should be doing more FP work than RAM accesses.
It was written in C. Note that it adapts to the current resolution. From memory it'll run in any planar mode >= 16 colours.

Be careful when comparing different machines. The size of the fractal might be wildly different.
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Re: Falcon FPU overclocking results

Post by exxos »

Frank B wrote:
It is drawing lines into a buffer then blitting it line by line to the screen. It should be doing more FP work than RAM accesses.
It was written in C. Note that it adapts to the current resolution. From memory it'll run in any planar mode >= 16 colours.

Be careful when comparing different machines. The size of the fractal might be wildly different.

I found that out yesterday. With LOW and MED resolutions. While it looked the same, medium res took a lot longer. I'm not sure about the colour modes though ? I think I tried 4 colour and 16 colour, not sure it made a difference there.
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Re: Falcon FPU overclocking results

Post by Cyprian »

I've just redone my test in a different video modes:
ST LOW: 32MHz - 20.199
TT LOW: 32MHz - 50.81
TT MID: 32MHz - 99.15
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Re: Falcon FPU overclocking results

Post by exxos »

Interesting results. I presume the "bottleneck" is ST-RAM. Similar with overclocking the CPU, there comes a point where pushing higher speeds offer little gain as its waiting around for RAM access all the time. I assume if someone has the bus speeded up to 20mhz or 24mhz, that 50mhz FPU speeds should show slightly better times. Though has anyone here got the bus speeded to try it ?
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Re: Falcon FPU overclocking results

Post by Frank B »

I think it showed some gains on a TT in fast Ram. Mainly in the high band width resolutions.
On a falcon some bus accelerators change the 200hz clock. Doug warned me about that. I don't calibrate the timer based on real time. So beware :)
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Re: Falcon FPU overclocking results

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Frank B wrote:I think it showed some gains on a TT in fast Ram. Mainly in the high band width resolutions.
On a falcon some bus accelerators change the 200hz clock. Doug warned me about that. I don't calibrate the timer based on real time. So beware :)
ahh yep, both good points :)
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Re: Falcon FPU overclocking results

Post by Cyprian »

from my figures we can see that total count of computed pixels is also important:

Code: Select all

ST LOW:	20.199 sec	64000 pixels	3168.47 pixels/sec
TT LOW:	50.81 sec	153600 pixels	3023.03 pixels/sec
TT MID:	99.15 sec	307200 pixels	3098.34 pixels/sec
If you did that test in ST Mid (640x200=128000px):

Code: Select all

32mhz MB clock	46.115 seconds	128000 pixels	2775.67 pixels/sec
then final result (pixels/sec) is more or less similar
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Re: Falcon FPU overclocking results

Post by exxos »

46.115 seems familiar :)

I've found a 75mhz osc on evilbay, when it comes I will see what happens.. I doubt the time will change by any great amount.
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Re: Falcon FPU overclocking results

Post by dml »

The FPU is difficult to measure.

It executes something like this (sorry for crappy diagram - CPU pipeline ignore for simplicity):

Code: Select all

 UNIT	DECODE	EXECUTE
 ----------------------
 CPU	2
 CPU	4
 FPU	17------\
 CPU	8	    30+
 CPU	4	    |
 FPU	17-----\|
 <stall>	    *
		         30+
		         |
		         |
 FPU	17-----\*
 CPU	4	    30+
 CPU	4	    |
 ... 		    |

I think the 17-cycle decode periods are tied to the CPU clock, or at least can't run ahead of the CPU. These are also serialized with the CPU.

The execute stage is the most expensive part (can be 100s of cycles) and parallels with all of the decodes (CPU, FPU), unless another execute blocks it, where it waits. It can only execute one item at a time. It also blocks for data dependencies (even more complicated).

So if you have a lot of FP ops packed closely together and mainly using registers, the execute steps will benefit from high overclocking. If the FP ops are spread out though, between CPU or memory access, the gain will drop sharply after 2x or 3x overclock.
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Re: Falcon FPU overclocking results

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dml wrote: So if you have a lot of FP ops packed closely together and mainly using registers, the execute steps will benefit from high overclocking. If the FP ops are spread out though, between CPU or memory access, the gain will drop sharply after 2x or 3x overclock.
That would make sense... we need more FPU benchmarking apps then ;) Possible accessing the screen memory doesn't help then ?
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Re: Falcon FPU overclocking results

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75mhz - crashed :)
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Re: Falcon FPU overclocking results

Post by dml »

Yep for overclocking definitely need one which thrashes the FPU specifically. Apart from timing, it's the only way to be sure it's showing the worst case heat profile when nothing else gets in its way....
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Re: Falcon FPU overclocking results

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dml wrote:Yep for overclocking definitely need one which thrashes the FPU specifically. Apart from timing, it's the only way to be sure it's showing the worst case heat profile when nothing else gets in its way....
They are 40mhz parts to start with. So assuming they are overclocked to 50mhz it shouldn't matter to much I suspect. I don't think much is ever going to flat line the FPU constantly anyway.
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Re: Falcon FPU overclocking results

Post by exxos »

I've designed a small clock board to plug into the FPU socket to simple hold a oscillator and connect to easier. The only issue is the 16mhz clock pin on the socket will have to be cut out. I'm not sure how to go about doing that yet as with the socket being a SMT one , I don't want to risk breaking it. Though hopefully there is a track which can be cut somewhere, maybe someone here has already found a method to isolate the 16mhz clock from the socket ?
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Re: Falcon FPU overclocking results

Post by dml »

exxos wrote:I've designed a small clock board to plug into the FPU socket to simple hold a oscillator and connect to easier. The only issue is the 16mhz clock pin on the socket will have to be cut out. I'm not sure how to go about doing that yet as with the socket being a SMT one , I don't want to risk breaking it. Though hopefully there is a track which can be cut somewhere, maybe someone here has already found a method to isolate the 16mhz clock from the socket ?
Isn't the 16mhz line to FPU independent from those routed elsewhere? i.e. it might be accessed by removing a SMT resistor near the combel and feeding in there without touching the FPU socket. Although the track distance will be increased so the overclocking limit will probably be reduced.

I could be remembering things wrongly - would need to check the schematic.
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