Re: [gforth] Performance anomality with dynamic superinstructions on MIPSel

[Bernd Paysan]

Am Samstag, 22. März 2014, 07:24:55 schrieb David Kuehling:
> Hi,
> 
> I'm using a recent gforth revision from git (6ec9915f6277de) and noticed
> that running gforth --dynamic produces pretty extreme performance
> degradation (about a factor of 5) for the benchmark I was running [1].
> This happens on Loongson-2f MIPS (debian squeeze mipsel, 32bit).  Note
> that on MIPS dynamic superinstructions aren't enabled by default as they
> may violate load delay slot requirements on some very old MIPS CPUs.
> 
> The minimum code I could come up with that clearly shows the anomaly is:
> 
>   time gforth-fast -r 600M \
>       -e '30000000 :noname 1- DUP 0> IF RECURSE THEN ; EXECUTE BYE'
>   user        0m1.680s
> 
> vs.
> 
>   time gforth-fast --dynamic -r 600M \
>       -e '30000000 :noname 1- DUP 0> IF RECURSE THEN ; EXECUTE BYE'
>   user        0m12.529s
> 
> I.e. a degradation by a factor of 7.
> 
> Any ideas how to proceed further?

How does this affect other microbenchmarks, e.g. onebench.fs? And: SEE-CODE 
<word> shows the dynamically generated code; could you provide that for the 
microbenchmark above?

>  This could be a side effect of the
> BTB errata of Loongson2f [2] maybe doing speculative loads to invalid
> addresses causing instruction stalls or cache flushes.  But then how
> could the micro-benchmark shown above ever cause a BTB prediction miss?
> (the Loongson2 BTB has 16 entries).  Any ideas how to explain the result
> without invoking CPU bugs?

Not with more diagnostics.

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
http://bernd-paysan.de/

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