On Mar 12, 2009, at 8:39 AM, Bill Bumgarner wrote:
On Mar 12, 2009, at 6:04 AM, John Engelhart wrote:
[.... way too many words deleted.... ... please try to succinctly
state issues in the future ....]
You have created a micro benchmark that demonstrates a significant
bit of overhead from GC vs. non-GC.
He extracted a real-world example of a performance problem into an
isolated test case. This is a good thing.
While micro benchmarks are certainly useful, they must be taken with
a grain of salt. Specifically, a real world app is not generally
going to go do, say, 10 bazillion of the operations in the micro
benchmark one after the other with zero feedback to the user.
Since the reported performance is relative, the scale-factor in front
is quite irrelevant. If you run it 10 times or 10 bazillion times,
2.8x slower is still 2.8x slower. However, having a large scale-
factor is convenient for taking meaningful measurements of running time.
Typically, a real world app would have progress bar(s), live button
(s), and displays that are updated.
So things that are not impacted by the GC dilute the effect?
GC was optimized for the real world situation.
This is not an 'optimization'. This is just saying that computers
today are fast enough that for many non-data-intensive applications,
there will be enough idle-time for the machine to catch up that people
won't notice wether operations are 2.8x slower.
In Leopard, the GC overhead is generally less than 10% CPU
overhead and 10% memory footprint overhead for the whole application.
While such sweeping generalizations can be useful, they must be taken
with a grain of salt.
In some specific measurements [of Xcode, for example, comparing GC
vs. non-GC performance], GC is actually significantly more efficient
than non-GC.
Is that actually true?
In others, non-GC is still quite a bit more efficient. But,
overall and for the general user experience, GC on Leopard beats the
general 10%/10% mark.
Is that for operations that actually use GC, or does that also include
operations that do not involve the GC at all?
And, just like every other component of Mac OS X, the goal is to
make the collector faster, more efficient, easier to use, and more
powerful with each successive release of Mac OS X.
Excellent!
Marcel
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