In fact I've found you need to pursue both the 2x type gains and also
the many smaller ones, to reach good performance. And it requires
alot of ongoing vigilence to keep good performance. You lose 3-4%
here and there and very quickly, very easily you're 2X slower.
These tests are very real. I'm indexing Wikipedia content, using
StandardAnalyzer, running under contrib/benchmark. It's true, in a
real app more time will be spent pulling documents from the source,
but I'm intentionally trying to minimize that in order to measure just
the indexing time. Getting a 4% gain by replacing mergesort with
quicksort is real.
If the profiler found other 4% gains, with such a small increase in
code complexity, I would passionately argue for those as well. So
far it hasn't.
Robert if you have some concrete ideas for the 2X type gains, I'm all
ears :)
I certainly agree there is a point where complexity cost doesn't
offset the performance gain, but I think this particular change is
well before that point.
Lucene's indexing throughput is an important metric in its
competitiveness with other search engines. And I want Lucene to be
the best.
Mike
eks dev wrote:
again, as long as you do not make one step forward into actual
code, we will continue to have what we have today, as this is the
best what we have.
you made your statement:
"Clear code will allow for more radical improvements as more eyes
will be able to easily understand the inner workings and offer
better algorithms",
Not a single person here would ever dispute this statement, but
unfortunately there is no compiler that executes such statements.
Make a patch that utilizes this "clear-code" paradigm, show us
these better algorithms on actual example and than say: "without
LUCENE-1172 I was able to improve XYZ feature by using ABC
algorithm". That would work smooth.
Anyhow, I am not going to write more on this topic, sorry for the
noise...
And Robert, please do not get this wrong, I see your point and I
respect it! I just felt slight unfairness to the people that make
the hands dirty writing as clear and fast code as possible.
----- Original Message ----
From: robert engels <[EMAIL PROTECTED]>
To: java-dev@lucene.apache.org
Sent: Monday, 11 February, 2008 9:55:02 AM
Subject: Re: [jira] Created: (LUCENE-1172) Small speedups to
DocumentsWriter
I am not disputing that there is a speed improvement. I am disputing
that the performance gain of many of these patches is not worth the
additional complexity in the code. Clear code will allow for more
radical improvements as more eyes will be able to easily understand
the inner workings and offer better algorithms, not just micro
improvements that the JVM (eventually) can probably figure out on its
own.
It is a value judgement, and regretfully I don't have another 30
years to pass down the full knowledge behind my reasoning.
Luckily, however, there are some very good books available on the
subject...
It's not the fault of the submitter, but many of these timings are
suspect due to difficulty in measuring the improvements accurately.
Here is a simple example:
You can configure the JVM to not perform aggressive garbage
collection, and write a program that generates a lot garbage - but it
runs very fast (not GCing), until the GC eventually occurs (if the
program runs long enough). It may be overall much slower than an
alternative that runs slower as it executes, but has code to manage
the objects as they are created, and rarely if ever hits a GC cycle.
But then, the JVM (e.g. generational GC) can implement improvements
that makes choice A faster (and the better choice)... and the cycle
continues...
Without detailed timings and other metrics (GC pauses, IO, memory
utilization, native compilation, etc.) most benchmarks are not very
accurate or useful. There are a lot of variables to consider - maybe
more so than can reasonably be considered. That is why a 4% gain is
highly suspect. If the gain was 25%, or 50% or 100%, you have a
better chance of it being an innate improvement, and not just the
interaction of some other factors.
On Feb 11, 2008, at 2:32 AM, eks dev wrote:
Robert,
you may or may not be right, I do not know. The only way to prove
it would be to show you can do it better, no?
If you are so convinced this is wrong, you could, much better than
quoting textbooks:
a) write better patch, get attention with something you think is
"better bottleneck"
b) provide realistic "performance tests" as you dispute the
measurement provided here
It has to be that concrete, academic discussions are cool, but at
the end of a day, it is the code that executes that counts.
cheers,
eks
----- Original Message ----
From: robert engels <[EMAIL PROTECTED]>
To: java-dev@lucene.apache.org
Sent: Sunday, 10 February, 2008 9:15:30 PM
Subject: Re: [jira] Created: (LUCENE-1172) Small speedups to
DocumentsWriter
I am not sure these numbers matter. I think they are skewed because
you are probably running too short a test, and the index is in memory
(or OS cache).
Once you use a real index that needs to read/write from the disk, the
percentage change will be negligible.
This is the problem with many of these "performance changes" - they
just aren't real world enough. Even if they were, I would argue that
code simplicity/maintainability is worth more than 6 seconds on a
operation that takes 4 minutes to run...
There are many people that believe micro benchmarks are next to
worthless. A good rule of thumb is that if the optimization doesn't
result in 2x speedup, it probably shouldn't be done. In most cases
any efficiency gains are later lost in maintainability issues.
See http://en.wikipedia.org/wiki/Optimization_(computer_science)
Almost always there is a better bottleneck somewhere.
On Feb 10, 2008, at 1:37 PM, Michael McCandless wrote:
Yonik Seeley wrote:
I wonder how well a single generic quickSort(Object[] arr, int low,
int high) would perform vs the type-specific ones? I guess the
main
overhead would be a cast from Object to the specific class to do
the
compare? Too bad Java doesn't have true generics/templates.
OK I tested this.
Starting from the patch on LUCENE-1172, which has 3 quickSort
methods
(one per type), I created a single quickSort method on Object[] that
takes a Comparator, and made 3 Comparators instead.
Mac OS X 10.4 (JVM 1.5):
original patch --> 247.1
simplified patch --> 254.9 (3.2% slower)
Windows Server 2003 R64 (JVM 1.6):
original patch --> 440.6
simplified patch --> 452.7 (2.7% slower)
The times are best in 10 runs. I'm running all tests with these JVM
args:
-Xms1024M -Xmx1024M -Xbatch -server
I think this is a big enough difference in performance that it's
worth keeping 3 separate quickSorts in DocumentsWriter.
Mike
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