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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 Sun, 08 Feb 2015 19:25:34 GMT

    [ https://issues.apache.org/jira/browse/IO-468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14311504#comment-14311504
] 

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

The drawbacks of ThreadLocals are that they might leak memory in web application server, see
the blog here for example: http://niklasschlimm.blogspot.be/2012/04/threading-stories-threadlocal-in-web.html

As a developer of a general-purpose library one has to keep exactly an eye on that: general
purpose use. We can not optimize for one specific use-case, i.e. maximum performance, when
the utility method is mainly used in total different setups.

Furthermore, there is a way to get *maximum* performance: not using a ThreadLocal but providing
a local byte array when calling the method. This should be faster than going via a ThreadLocal.
Did you test this in your performance test? Also, the buffer size will highly depend on the
use-case. To get maximum performance you will want to adjust the buffer size, which is not
possible with the ThreadLocal solution either.

I can understand when someone is pissed because his performance patch is not accepted, but
if you take a step back you will realize that it really does not make any sense here.

> 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, monitoring_with_threadlocals.png, monitoring_without_threadlocals.png,
performancetest.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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