I forgot to note hat i test the java sample and clojure sample code with
the same jvm options '-server'.
2014-03-01 20:03 GMT+08:00 dennis zhuang <killme2...@gmail.com>:
> The "String a=i+"another word";" is also compiled into using
> StringBuilder, see the byte code by javap -v:
>
> Code:
> stack=5, locals=5, args_size=1
> 0: invokestatic #2 // Method
> java/lang/System.nanoTime:()J
> 3: lstore_1
> 4: iconst_0
> 5: istore_3
> 6: iload_3
> 7: ldc #3 // int 10000000
> 9: if_icmpge 39
> 12: new #4 // class
> java/lang/StringBuilder
> 15: dup
> 16: invokespecial #5 // Method
> java/lang/StringBuilder."<init>":()V
> 19: iload_3
> 20: invokevirtual #6 // Method
> java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
> 23: ldc #7 // String another word
> 25: invokevirtual #8 // Method
> java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
> 28: invokevirtual #9 // Method
> java/lang/StringBuilder.toString:()Ljava/lang/String;
> 31: astore 4
> 33: iinc 3, 1
> 36: goto 6
> 39: getstatic #10 // Field
> java/lang/System.out:Ljava/io/PrintStream;
> 42: invokestatic #2 // Method
> java/lang/System.nanoTime:()J
> 45: lload_1
> 46: lsub
> 47: l2d
> 48: ldc2_w #11 // double 1.0E9d
> 51: ddiv
> 52: invokevirtual #13 // Method
> java/io/PrintStream.println:(D)V
>
>
> I think the performance hotspot in this simple example is the object
> allocate/gc and function calling overhead.The str function create
> an anonymous function every time to concat argument strings:
>
> (^String [x & ys]
> ((fn [^StringBuilder sb more]
> (if more
> (recur (. sb (append (str (first more)))) (next more))
> (str sb)))
> (new StringBuilder (str x)) ys)))
>
> And we all know that a function in clojure is a java object allocated in
> heap.And another overhead is calling the function,it's virtual method.
>
> By watching the gc statistics using 'jstat -gcutil <pid> 2000', i found
> that the clojure sample ran about 670 minor gc,but the java sample is only
> 120 minor gc.
>
> A improved clojure version,it's performance is closed to java sample:
>
> user=> (time (dotimes [n 10000000] (-> (StringBuilder.) (.append n)
> (.append "another word") (.toString))))
> "Elapsed time: 1009.942 msecs"
>
>
>
>
> 2014-03-01 18:02 GMT+08:00 bob <wee....@gmail.com>:
>
> Case :
>>
>> clojure verison:
>>
>> (time (dotimes [n 10000000] (str n "another word"))) ;; take about
>> 5000msec
>>
>> java version
>>
>> long time = System.nanoTime();
>>
>> for(int i=0 ; i<10000000 ;i++){
>> String a=i+"another word";
>> }
>> System.out.println(System.nanoTime()-time);
>>
>>
>> The java version take about 500 msecs, I thought it might be caused by
>> the str implementation which is using string builder, and it might not be
>> the best choice in the case of no much string to concat, and then I replace
>> "another word" with 5 long strings as the parameter, however no surprise.
>>
>> I just wonder what make the difference, or how to find the difference.
>>
>> Thanks
>>
>>
>>
>> On Saturday, March 1, 2014 1:26:38 PM UTC+8, Shantanu Kumar wrote:
>>>
>>> I have seen (and I keep seeing) a ton of Java code that performs poorly.
>>> Empirically, it's equally easy to write a slow Java app. You always need a
>>> discerning programmer to get good performance from any language/tool.
>>>
>>> Numbers like 1/4 or 1/10 can be better discussed in presence of the
>>> use-cases and perf test cases. Most of the problems you listed can be
>>> mitigated by `-server` JIT, avoiding reflection, transients, loop-recur,
>>> arrays, perf libraries and some Java code.
>>>
>>> Shantanu
>>>
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>
>
>
> --
> 庄晓丹
> Email: killme2...@gmail.com xzhu...@avos.com
> Site: http://fnil.net
> Twitter: @killme2008
>
>
>
--
庄晓丹
Email: killme2...@gmail.com xzhu...@avos.com
Site: http://fnil.net
Twitter: @killme2008
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