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MathGroup Archive 2005

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Re: Re: Re: Slowdown

  • To: mathgroup at smc.vnet.net
  • Subject: [mg53278] Re: [mg53254] Re: [mg53241] Re: Slowdown
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Tue, 4 Jan 2005 03:13:00 -0500 (EST)
  • References: <cr33tt$je6$1@smc.vnet.net> <200501020912.EAA27775@smc.vnet.net> <200501030929.EAA10649@smc.vnet.net> <C570DAF1-5D86-11D9-8B6F-000A95B4967A@mimuw.edu.pl>
  • Sender: owner-wri-mathgroup at wolfram.com

I just noticed another thing, which seems to throw more light at what  
is happenng. Look carefully at the Hash values of the different names  
of the form weirdness$number:


MapIndexed[{First[#2], Hash[#1]} & ,
   ToExpression /@ (StringJoin["weirdness", "$",
       ToString[#1]] & ) /@ Range[20]]


{{1, 1783099877}, {2, 1783099878}, {3, 1783099879},
   {4, 1783099880}, {5, 1783099881}, {6, 1783099882},
   {7, 1783099883}, {8, 1783099884}, {9, 1783099885},
   {10, 667842086}, {11, 667842087}, {12, 667842088},
   {13, 667842089}, {14, 667842090}, {15, 667842091},
   {16, 667842092}, {17, 667842093}, {18, 667842094},
   {19, 667842095}, {20, 667843323}}

You can see a big jump in the hash value at weirdness$10. That is  
exactly the point at which the slowdown occurs.

Andrzej Kozlowski


On 3 Jan 2005, at 21:55, Andrzej Kozlowski wrote:

> I can't see any siginificant slowdowns demonstrated by your code. The  
> differences in timings between different combinations seem  
> insignificant. Cna you suggest any other name but "weirdness" that  
> shows significant slowdown?
>
> Besides, using Module only obsures what is really happening. Module  
> simply renames local variables by appending $somenumber to their name   
> but the "number" that is used is different each time. You can do that  
> whiteout Module and see exactly which names produce the slowdown  
> effect. It seems that the names weirdness$1 to weirdness$9 do not.
>
> In[1]:=
> L=weirdness$1[];
>           Do[L=weirdness$1[L,i],{i,10^4}]//Timing
>
> Out[2]=
> {0.03 Second,Null}
>
> In[3]:=
> L=weirdness$9[];
>           Do[L=weirdness$9[L,i],{i,10^4}]//Timing
>
> Out[4]=
> {0.08 Second,Null}
>
> The problem seems to begin with weirdness$10
>
> In[5]:=
> L=weirdness$10[];
>           Do[L=weirdness$10[L,i],{i,10^4}]//Timing
>
> Out[6]=
> {13.2 Second,Null}
>
> and continues for higher values, as far as I have checked.
>
> When you run Module with a fresh Kernel Mathematica seems to always  
> begin by appending $17:
>
> In[1]:=
> Module[{weirdness, L},
>    L = weirdness[];
>    Do[L = weirdness[L, i], {i, 1}]
> ] // Trace
>
> Out[1]=
> {Module[{weirdness,L},L=weirdness[];Do[L=weirdness[L,i],{i,1}]],{L$17=\
> weirdness$17[];Do[L$17=weirdness$17[L$17,
>        
> i],{i,1}],{L$17=weirdness$17[],weirdness$17[]},{Do[L$17=weirdness$17[L$ 
> \
> 17,i],{i,1}],{{{L$17,weirdness$17[]},{i,1},weirdness$17[weirdness$17[],
>          
> 1]},L$17=weirdness$17[weirdness$17[],1],weirdness$17[weirdness$17[],1]\
> },Null},Null},Null}
>
> on subsequent runs this is increased:
>
> In[2]:=
> Module[{weirdness, L},
>    L = weirdness[];
>    Do[L = weirdness[L, i], {i, 1}]
> ] // Trace
>
> Out[2]=
> {Module[{weirdness,L},L=weirdness[];Do[L=weirdness[L,i],{i,1}]],{L$19=\
> weirdness$19[];Do[L$19=weirdness$19[L$19,
>        
> i],{i,1}],{L$19=weirdness$19[],weirdness$19[]},{Do[L$19=weirdness$19[L$ 
> \
> 19,i],{i,1}],{{{L$19,weirdness$19[]},{i,1},weirdness$19[weirdness$19[],
>          
> 1]},L$19=weirdness$19[weirdness$19[],1],weirdness$19[weirdness$19[],1]\
> },Null},Null},Null}
>
> in any case all these numbers are larger than 10 and produce slowdown,  
> but the numbers 10-16 which never occur inside Module also suffer form  
> this problem:
>
> In[3]:=
> L=weirdness$11[];
>           Do[L=weirdness$11[L,i],{i,10^4}]//Timing
>
> Out[4]=
> {13.1 Second,Null}
>
> In[5]:=
> L=weirdness$16[];
>           Do[L=weirdness$16[L,i],{i,10^4}]//Timing
>
> Out[6]=
> {14.62 Second,Null}
>
> However, I have not been able to find any other name but weirdness  
> that produces this effect. This seems weird.
>
> Andrzej Kozlowski
>
>
>
> On 3 Jan 2005, at 18:29, yehuda ben-shimol wrote:
>
>> Hi Roland,
>> I'm afraid this is not true. Just try to run the code I sent with my
>> post.  "weirdness" was time consuming while weirdness1 was not. In
>> addition it happened (not consistently) that function name of a SINGLE
>> character suffered from this behavior as well.
>> the code is given below for your convenience.
>>
>> t = CharacterRange["a", "z"];
>> Do[fname = StringJoin[t[[Table[Random[Integer, {1,26}], {i}]]]];
>> Print[i, "\t", fname, "\t",
>> 	Timing[
>> 	  Module[{fs = ToExpression[fname], L},
>>           L = fs[];
>>         Do[L = fs[L, j], {j, 104}]]]], {i, 1, 50}, {5}]
>>
>>
>> yehuda
>>
>> Roland Franzius wrote:
>>
>>> Maxim wrote:
>>>
>>>
>>>> Consider:
>>>>
>>>> In[1]:=
>>>> Module[{f, L},
>>>>   L = f[];
>>>>   Do[L = f[L, i], {i, 10^4}]
>>>> ] // Timing
>>>>
>>>> Module[{weirdness, L},
>>>>   L = weirdness[];
>>>>   Do[L = weirdness[L, i], {i, 10^4}]
>>>> ] // Timing
>>>>
>>>> Out[1]=
>>>> {0.015*Second, Null}
>>>>
>>>> Out[2]=
>>>> {3.063*Second, Null}
>>>>
>>>> Here the timings differ by a factor of 200. Besides, the timing  
>>>> grows
>>>> linearly in the first case and quadratically in the second  
>>>> (therefore, for
>>>> n=10^5 there will be an approximately 2000 times slowdown). We can  
>>>> only
>>>> guess that something goes wrong with the symbol name hashing.
>>>>
>>>>
>>>
>>> The timing difference occurs when the symbol "wierdness" exceeds 8
>>> characters. Test it for "wierdnes". That seems to be a consequence of
>>> the machine routine for string comparison. Up to 8 characters can be
>>> used without using a memory to memory compare. Of course it should be
>>> possible to write a compare routine that makes not such a bit step.
>>>
>>
>


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