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>>I am not impressed byt the 2 GFlop results. It might be all we get, but
>>the Altivec should have a peak of 8 flops per clock cycle (4 muladds per
>>cycle), so it should be doing better.
>The main thing I wonder about is why java mode slows down the computation.
>All it should do from my reading is add an extra stage to the pipeline.
>Since we are using enough registers to handle the longer pipeline, it is
>a real mystery to me why adding the extra stage drops us from 2.2
>Gflop to 2. Makes me wonder if that extra stage is not pipelined . . .
The extra stage is definitely pipelined. A common issue in Altivec
code is that one runs out of rename registers; the 7400/7410 has 6
vector rename registers, meaning that only six vector operations can
be scheduled to be in flight at any given time; the seventh and later
will stall until a rename register is freed up. Looking at my own
matmul kernel code, I don't see any place where this could happen
other than at the end of the row loop (where there are vec_madd
operations in flight, and vector permute operations start up to do
the 4x4 transpose.)
>If we weren't memory bound, I thought I'd be able to get some speedup by
>using the normal FPU along with the AltiVec. Essentially, since the AltiVec
>is 4 times faster than the regular FPU, you can imagine doing your loop
>such that 4/5 of the loop is done by the altivec, and 1/5 is done by the
>FPU. I implemented this, and got a very nice slowdown.
This is more than likely related to the issue above; the 7400/7410
has an 8 instruction completion queue, and can retire no more than 2
instructions per cycle. Using the VFPU and FPU simultaneously is an
easy way to quickly fill up the completion queue and cause stalls.
These issues may be quite different on a 7450 machine, which has 16
vector rename registers and can retire three instructions per cycle.
As of Macworld-New York, all of Apple's shipping G4 machines are
Nicholas Coult, Ph.D., web: http://melby.augsburg.edu/~coult
Assistant Professor, Department of Mathematics, Augsburg College
firstname.lastname@example.org, phone: (612) 330-1064 office: Science Hall 137B