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

Dhanji R. Prasanna dhanji at gmail.com
Sat Dec 26 21:42:00 EST 2009

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.


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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