[concurrency-interest] good design vs performance (object creation)

Ramesh Mandaleeka ramesh.mandaleeka at gmail.com
Sun Dec 27 19:59:53 EST 2009


Hi Dhanji,

>> A classic example of this is calling clear() on an expanding array list
and forgetting to call trimToSize()

Can you elaborate what do you mean? Is there any side effect by calling
clear() method? Do we need to call trimToSize() method after clear() method?

Thanks,
Ramesh

On Sat, Dec 26, 2009 at 9:42 PM, Dhanji R. Prasanna <dhanji at gmail.com>wrote:

> In general you want to write your code and prove its correctness first, and
> then optimize for performance. If your code is readable and maintainable
> this becomes easier to do after the fact. You should also never optimize
> code without being able to establish a clear, reproduceable performance
> bottleneck that can be *described* (not merely observed).
>
> As you note creation of thousands or even millions of objects is not often
> a serious concern if they are short lived and the GC is tuned accordingly.
> Particularly for things like iterators and so on.
>
> In my experience the real problems arise in two very different scenarios:
> - Premature optimations: growable caches, pools, SoftReferences etc. This
> is when you accidentally make objects long lived (for instance, to prevent
> their number from getting out of hand). This can really hurt GC performance.
> - Code immuaturity: This is the opposite end of the spectrum where you just
> assume Java takes care of memory management for you altogether. A classic
> example of this is calling clear() on an expanding array list and forgetting
> to call trimToSize(). Or the symmetrical action with j.u.HashMap. Creating
> large numbers of objects in an inner loop is similarly a problem.
>
> A good library should go through a thorough memory complexity analysis for
> each algorithm or utility introduced. This, almost always, will point you
> directly to a leak or potential bottleneck. When writing a concurrent data
> structure, I typically set up an expectation of computational complexity and
> walk through the various ways in which I can trade correctness for
> performance (a concurrent LRU cache is a good example).
>
> Like everything else in engineering, you need to set up a bunch of sliders
> that represent tradeoffs and move them around till you get a good balance
> between API ease-of-use, performance, development time and long term
> maintainability of your code internals.
>
> Dhanji.
>
>
> On Sun, Dec 27, 2009 at 11:00 AM, Peter Veentjer <alarmnummer at gmail.com>wrote:
>
>> Hi Guys,
>>
>> although the question is not directly concurrency related, I want to
>> place it on this mailinglist because here are a lot of guys that
>> encounter the issue from time to time.
>>
>> I'm currently working on a STM implementation and one of the things
>> that I have found out is that object creation can cause quite a big
>> slowdown, especially if done
>> millions of times per second (e.g. 1M to 20M times a second)  If I
>> look at the code of java.util.concurrent I see a lot of
>> nodes/iterators being created for example and this can
>> be a great thing for good oo design but my experience is that it can
>> cause a considerable slow down.
>>
>> My question is: how much should the design of a class be influenced by
>> preventing object creation? I'm at the point where I need to rewrite
>> the same algorithm
>> multiple times for different datastructures (e.g. array based versus
>> map based.. where an array is faster for very small collections). I
>> could create an ArrayIterator
>> so that the algorithm is able to work with iterators and it doesn't
>> matter which data structure is used, but it requires an additional
>> iterator object. I also know that
>> garbage collectors are quite smart and that very short lived objects
>> are very cheap to be garbage collected. I also know that some modern
>> jvm's are able
>> to allocate objects in the stack instead of the heap, so even less
>> worries about gc overhead.
>>
>> I need some feedback/guidelines from developers that have experienced
>> this in the trenches and really know what they are talking about. How
>> for should one go?
>> I would be very happy to stop ruining my code... but on the other side
>> I also want a good performance
>>
>> Peter
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>
>
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