[Jprogramming] stalled J processes

[Henry Rich]

With zero tolerance, the O(n^2) problem does not arise.

Henry Rich

On 8/19/2014 10:14 PM, bill lam wrote:
> 2.
> for numeric matrix and zero tolerance, will ~. perform better than O(n^2)
> if it computes hash for each row? (or it has already done so)
> On Aug 20, 2014 7:43 AM, "Roger Hui" <rogerhui.canada at gmail.com> 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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