On 7/21/2010 12:58 PM, Gregg Wonderly wrote:
...
When I write code of this nature, attempting to remove all contention, I
try
to list every "step" that changes the "view" of the world, and think about
how that "view" can be made atomic by using explicit ordering of statements
rather than synchronized{} blocks. ...
I would like to discuss how to approach performance improvement, and
especially scaling improvement. We seem to have different philosophies,
and I'm interested in understanding other people's approaches to
programming.
I try to first find the really big wins, which are almost always data
structure and algorithm changes. That should result in code that is
efficient in terms of total CPU time and memory. During that part of the
process, I prefer to keep the concurrency design as simple as possible,
which in Java often means using synchronization at a coarse level, such
as synchronization on a TaskManager instance.
Once that is done, I review the performance. If it is fast and scalable
I stop there. If that is not the case, I look for the bottlenecks, and
consider whether parallelism, or some other strategy, will best improve
them. Any increase in concurrency complication has to be justified by a
demonstrated improvement in performance.
My big picture objective is to find the simplest implementation that
meets the performance requirements (or cannot reasonably be made
significantly faster, if the requirement is just "make it fast"). I
value simplicity in concurrency design over simplicity in data
structures or algorithms for two reasons:
1. Making the code more parallel does nothing to reduce the total
resources is uses. Better algorithms, on the other hand, can
significantly reduce total resources.
2. Reasoning about data structures and algorithms is generally easier
than reasoning about concurrency.
It sounds as though you are advocating almost the opposite approach -
aim for maximum concurrency from the start, without analysis or
measurement to see what it gains, or even having a baseline
implementation for comparison. Is that accurate? If so, could you
explain the thinking and objectives behind your approach? Or maybe I'm
misunderstanding, and you can clarify a bit?
Thanks,
Patricia
