On Oct 28, 2008, at 8:30 AM, Jerry Krinock wrote:
Although the documentation on message forwarding [1] explains that alot of stuff needs to happen, it does not say "Warning: Don't do this in performance-critical applications". So I made a test tool which forwarded a simple message with one integer argument to a class which would add it to its 'sum', an instance variable.Sending this message 10,000 times and comparing the difference in elapsed time with a similar task that sends a similar message directly, I found that the average overhead for forwarding one message on my Intel Core 2 Duo Mac Mini was about 20 microseconds.That would be unacceptably slow for an iterated operation in a my application, and I decided to not use message forwarding in this particular case.
That would not surprise me. An absolute microseconds overhead isn't a terribly useful measure without knowing the total # of microseconds. In general, measuring as a factor of speed -- 1.2x 20x 200x is more widely applicable (tends to be more consistent across different CPUs, for example).
It is highly atypical to employ a design pattern that involves message forwarding in the midst of a tight loop. While your test case does test the absolute overhead, it doesn't test what is typically found in the real world.
How many times does your iterated operation actually iterate? Human perception generally has a response granularity somewhere around about 120-160 msecs. Your users being above average, let's call it 100msecs. Slight apples and oranges here; "response time" is literally "time to respond" and we are really considering "perception of how long something took". Same ballpark and we are going on the highly optimistic side.
Given 20 microseconds overhead, your loop would have to iterate 5000 times to incur a delay that the user might perceive.
And that, of course, assumes that 20 microseconds of overhead even matters in your loop. If your actual operation is, say, 50 microseconds then -- sure -- that 33% overhead will add up. At 500 microseconds, you are looking 4% overhead for that one operation and, quite likely, a fraction of percentage of total CPU time.
So... sure... message forwarding is slow. But does it matter in your application?
b.bum
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