[Jprogramming] stalled J processes

[Roger Hui]

I guess the benchmarks show that you are right.  But the thing to worry
about are the O(n^2) monsters, not mere factors of 2 or 3 or 10.

But regarding the benchmarks: the interpreter may have infelicities in
treating the types of empty things.

​   array =. (100000 2 ?@$ 200 10)  (<;.0~ ,.)~"1 _  a.
   ~. $&.> array
┌─┬─┬─┬─┬─┬─┬─┬─┬─┬─┐
│3│8│2│7│9│1│4│6│0│5│
└─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘
   array =. (100000 2 ?@$ 200 10)  (<;.0~ ,.)~"1 _  a.
   ~. $&.> array
┌─┬─┬─┬─┬─┬─┬─┬─┬─┬─┐
│8│1│6│5│3│4│2│0│9│7│
└─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘
​


On Tue, Aug 19, 2014 at 4:55 PM, Henry Rich <HenryHRich at nc.rr.com> wrote:

> My experiments show ~. faster than ~.!.0 for boxed character arrays of
> rank 2 (disagreeing with Roger's point 1)
>
>    array =. (100000 2 ?@$ 200 10)  (<;.0~ ,.)~"1 _  a.
>    50 {."1 ": array
> +--++-------+-------+--++---+--------+--------+---
> |YZ||�������|ABCDEFG|  ||   |��������|��������|���
> +--++-------+-------+--++---+--------+--------+---
>    array2 =. ,.~ array
>    $array2
> 100000 2
>    10 ts '~. array'
> 0.0212086 1.57357e6
>    10 ts '~.!.0 array'  NB. No change for rank 1
> 0.0211375 1.57389e6
>    10 ts '~. array2'
> 0.0508845 2.09786e6
>    10 ts '~.!.0 array2'  NB. Slowdown for rank 2
> 0.151277 2.22925e6
>
> Henry Rich
>
>
> On 8/19/2014 7:43 PM, Roger Hui wrote:
>
>> 0. For a floating point vector, ~.!.0 is faster than ~., but not to the
>> extent that Bill Lam implied.
>>
>>     timer=: 6!:2
>>     x=: 1e6 ?@$ 0
>>
>>     10 timer '~.x'
>> 0.136236
>>     10 timer '~.!.0 x'
>> 0.0814054
>>
>> Same comments apply to other functions in the index-of family.
>>
>> 1. For non-numeric arguments (and boxed arrays not involving numbers),
>> ~.!.0 should run at the same speed as ~.
>>
>>     x=: a.{~ 1e6 12 ?@$ 256
>>     10 timer '~.x'
>> 0.13204
>>     10 timer '~.!.0 x'
>> 0.130772
>>
>> 2. Floating point matrices can have O(n^2) performance.  It's best to use
>> ~.!.0 if you can get away with it.
>>
>>     3 : '10 timer ''~.t'' [ t=. 1.5,.y ?@$ 0'"0 ]10^0 1 2 3 4
>> 3.03225e_6 5.45805e_6 0.000412806 0.012665 1.01016
>>
>>     3 : '10 timer ''~.!.0 t'' [ t=. 1.5,.y ?@$ 0'"0 ]10^0 1 2 3 4
>> 4.05855e_6 4.3851e_6 3.11155e_5 0.000177643 0.0015026
>>
>> 3. Boxed arrays involving numerics (even all integers) can have O(n^2)
>> performance.  It's best to use ~.!.0 if you can get away with it.
>>
>> I don't remember the exact details of how this comes about.  Best to check
>> the source, or at least run a few (small) benchmarks.
>>
>> 3. The interpreter checks for interrupts predominantly during memory
>> allocation.  If there is a computation that runs for a long time and
>> appears to be uninteruptible, this is the most likely culprit.  Note: most
>> anything in J can not proceed without allocating memory.
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