I see.
Actually we didn't even need the side effects thing - I did notice that your
example stopped doing GC when setting "shouldLog = false" always, so I tried to
introduce your "if (shouldLog) { shouldLog = false; }" into my example, but was
still unable to reproduce GC. Then changing
- System.out.println("don't optimise me out");
+ System.out.println(format + args);
worked to re-introduce GC.
Well, that sucks. I guess the escape analyzer isn't sophisticated enough to see
that ar2.toString() does not cause ar2 to escape - and removing that from your
log() method does result in no GC.
I still think it's negligible though :p - for my test, I'm seeing only ~8ms
lost to GC after 20 million calls, and this appears to not change by much even
if I increase the number of arguments (and objects created) to log_info.
On 24/03/12 03:02, Zlatin Balevsky wrote:
> Your test has nothing to do with stack allocation because you never
> use the objects that you pass to the log_info method so JIT removes
> them. Apply the following patch and see yourself create garbage in
> both testWith and testWithout.
>
> --- Test2.java 2012-03-23 23:01:05.540475497 -0400
> +++ Test.java 2012-03-23 23:01:47.304477562 -0400
> @@ -6,8 +6,11 @@
> final Object existing = new Object();
> Object tmp = null;
>
> + long sideEffect;
> public void log_info(String format, Object ... args) {
> if (log_level) {
> + for (Object o : args)
> + sideEffect += o.hashCode();
> System.out.println("don't optimise me out");
> }
> }
> @@ -88,11 +91,8 @@
> printSummary("with alloc", res[0], "w/o alloc", res[1]);
> printSummary("with alloc", res[2], "w/o alloc", res[3]);
> printSummary("with alloc", res[4], "w/o alloc", res[5]);
> - System.out.println("---- with stack allocation and \"real use case\"");
> - res = testWithOtherCode(z);
> - printSummary("log only", res[0], "log+work", res[1]);
> - printSummary("log only", res[2], "log+work", res[3]);
> - printSummary("log only", res[4], "log+work", res[5]);
> +
> + System.out.println(sideEffect);
> }
>
> public static void main(String[] args) {
>
>
> On Fri, Mar 23, 2012 at 10:48 PM, Ximin Luo <infini...@gmx.com> wrote:
>> On 24/03/12 02:44, Zlatin Balevsky wrote:
>>> Wrong. Extracting the log statement into its own method and calling
>>> that method inside the while loop still creates garbage. Stack frames
>>> are different.
>>>
>>
>> How do you explain the results obtained from running my test code, then? Turn
>> on -verbose:gc. testWithStackAllocation results in no GC,
>> testWithoutStackAllocation results in lots of GC.
>>
>>> Read again exactly how Escape Analysis works. The objects it prevents
>>> from creating do not appear in heap histograms. Take a "jmap -histo
>>> <pid>" and you will see a huge number of java.lang.Integer objects in
>>> the heap, not on the stack.
>>>
>>
>> If you're still talking about your example, this is exactly consistent with
>> my
>> explanation, i.e. that escape analysis is NOT occurring, properly anyway.
>>
>>> The difference in run times of your example could be due to many
>>> things. How many iterations did you run? It may be still in
>>> interpreted mode. Enable -XX:+PrintCompilation and discard any
>>> results until JIT stops printing.
>>
>> Done that, JIT stops printing after the 3rd iteration and results are the
>> same
>> thereafter.
>>
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>>
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