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From "Thomas Neidhart (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (IO-468) Avoid allocating memory for method internal buffers, use threadlocal memory instead
Date Wed, 18 Feb 2015 09:13:12 GMT

    [ https://issues.apache.org/jira/browse/IO-468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14325626#comment-14325626
] 

Thomas Neidhart commented on IO-468:
------------------------------------

The Error is the score inside the 99.9% confidence interval.

It is indeed surprising that the ThreadLocal variant is sometimes faster, but I had the same
with my own (simpler) benchmark setup. It probably shows the general instability of such micro-benchmarks.
The memory array test is indeed not very meaningful as it does not really reflect normal usage,
but it is useful as stress test for the cost of array allocation as we will not run into general
IO blocking issues.

In my previous benchmark I also compared the common use-case of copying a file into memory,
where the differences quickly disappeared as expected (the cost of the allocation is negligible
to the cost of the stream copying from harddisk).

Testing different buffer sizes would also makes sense, but at some point we have to stop as
we have already proven that micro-optimization in this case does not make much sense.

> Avoid allocating memory for method internal buffers, use threadlocal memory instead
> -----------------------------------------------------------------------------------
>
>                 Key: IO-468
>                 URL: https://issues.apache.org/jira/browse/IO-468
>             Project: Commons IO
>          Issue Type: Improvement
>          Components: Utilities
>    Affects Versions: 2.4
>         Environment: all environments
>            Reporter: Bernd Hopp
>            Priority: Minor
>              Labels: newbie, performance
>             Fix For: 2.5
>
>         Attachments: PerfTest.java, micro-benchmark.zip, monitoring_with_threadlocals.png,
monitoring_without_threadlocals.png, performancetest.ods, performancetest_weakreference.ods
>
>   Original Estimate: 12h
>  Remaining Estimate: 12h
>
> In a lot of places, we allocate new buffers dynamically via new byte[]. This is a performance
drawback since many of these allocations could be avoided if we would use threadlocal buffers
that can be reused. For example, consider the following code from IOUtils.java, ln 2177:
> return copyLarge(input, output, inputOffset, length, new byte[DEFAULT_BUFFER_SIZE]);
> This code allocates new memory for every copy-process, that is not used outside of the
method and could easily and safely reused, as long as is is thread-local. So instead of allocating
new memory, a new utility-class could provide a thread-local bytearray like this:
> byte[] buffer = ThreadLocalByteArray.ofSize(DEFAULT_BUFFER_SIZE);
> return copyLarge(input, output, inputOffset, length, buffer);
> I have not measured the performance-benefits yet, but I would expect them to be significant,
especially when the streams itself are not the performance bottleneck. 
> Git PR is at https://github.com/apache/commons-io/pull/6/files



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