Re: [Htmlparser-developer] Java Performance question

[Derrick O. <Der...@ro...>]

Somik,

I think there are a couple of reasons. First is your instanceof test is 
always immediately succeeding. The penalty for instanceof is when it has 
to walk the inheritance heirarchy (usually all the way up to Object) to 
determine failure, which would happen often if you were trying to 
determine what to do with an unknown node type.  Second, your getType() 
involves a virtual method call that would normally not be done more than 
once. That is, you would typically get the unknown type once and compare 
it to each of the final types you are aware of, which would effectively 
move line 45 of the second dissassembly below (generated by "javap -c 
org.htmlparser.tests.InstanceofPerformanceTest") out of the kernel loop 
and replace it with a "lload 8" probably:

Method void doInstanceofTest(long[], int, long)
<snip>
  35 lconst_0    // for (i = 0
  36 lstore 7
  38 goto 57
  41 aload_0    // this
  42 getfield #10 <Field org.htmlparser.HTMLNode node>    // get 
InstancofPerformanceTest 'node' member variable
  45 instanceof #21 <Class org.htmlparser.tags.HTMLTag>    // node 
instanceof HTMLTag
  48 ifeq 51    // { }
  51 lload 7    // i++
  53 lconst_1
  54 ladd
  55 lstore 7
  57 lload 7    // i < numTimes
  59 lload_3
  60 lcmp
  61 iflt 41    // repeat
</snip>


Method void doGetTypeTest(long[], int, long)
<snip>
  35 lconst_0    // for (i = 0
  36 lstore 7
  38 goto 59
  41 aload_0    // this
  42 getfield #10 <Field org.htmlparser.HTMLNode node>    // get 
InstancofPerformanceTest 'node' member variable
  45 invokevirtual #23 <Method java.lang.String getType()>    // 
getType() virtual method call
  48 ldc #24 <String "NODE">    // 'retrieve' String "NODE" from 
HTMLNode, but since it's final it's a local copy
  50 if_acmpne 53    // ==
  53 lload 7    // i++
  55 lconst_1
  56 ladd
  57 lstore 7
  59 lload 7    // i < numTimes
  61 lload_3
  62 lcmp
  63 iflt 41    // repeat
</snip>

A fairer test might be:

type = node.getType();
for (...
    if (type == "BOGUS")
    {}
    else if (type == "FAKE")
    {}
    else if (type == "NODE")
    {}

vs.

for (...
    if (node instanceof HTMLFrameTag)
    {}
    else if (node instanceof HTMLFormTag)
    {}
    else if (node instanceof HTMLNode)
    {}

Derrick


Somik Raha wrote:

> Hi Folks,
>     I was tinkering around with instanceof and under the impression 
> that it causes a performance hit, I tried replacing it with a 
> polymorphic mechanism - by which HTMLNode has a method getType(), and 
> so do the other basic nodes. A match is then attempted like so :
> if (node.getType()==HTMLTag.TYPE)
>  
> instead of
>  
> if (node instanceof HTMLTag)
>  
> I have taken care that getType does not do object creation - it is a 
> static object. One would expect the former to be faster.
> But in a performance test (InstanceofPerformanceTest in 
> org.htmlparser.tests) - I find the opposite behaviour.
> Here's a graph showing the response of instanceof in blue and 
> getType()==HTMLTag.TYPE in pink -
> http://htmlparser.sourceforge.net/design/pics/performance.gif
>  
> Does anyone have explanations ?
>  
> Regards,
> Somik