On 05/27/2015 11:36 AM, Ivan Gerasimov wrote:
Hi Sherman!
Please take a look at my other webrev, that I provided in the first
message.
WEBREV: http://cr.openjdk.java.net/~igerasim/8058779/01/webrev/
I used StringJoiner there, which in some aspects seems to fit better
here, comparing to StringBuilder.
It does reduce the code, but of course runs slower.
In that version every part of the source string had to be converted to
a separate String, which add another redundant copying.
I still prefer the version, which deals with arrays. I don't think
it's too complex.
It surely adds some complexity to the original code, but it's only 70
lines of code in total.
Everything else are comments and empty lines.
Sincerely yours,
Ivan
Hi Ivan,
If I was to choose, I would take the 1st variant (with index arrays)
too. The speed does matter!
But anyway, every now and then one would like to write an algorithm that
gathers sub-strings and then either joins them or dumps them to some
buffer that eventually becomes another string or array. Since
String.substring() changed when offset/length fields were removed, there
is no official way to handle sub-sequences as "vews" of underlying
String. There was some debate in the past about using String's
implementation of CharSequence.subSequence() for that purpose - to
return a private CharSequence implementation that would represent a view
of underlying String's array, but that was not pursued because of
limiting API specification that says:
"Returns a character sequence that is a subsequence of this sequence.
An invocation of this method of the form
str.subSequence(begin, end)
behaves in exactly the same way as the invocation
str.substring(begin, end)"
My opinion is that this part of text did matter at the time when
substring() did return a view over underlying array, but when the
substring method changed, there was no reason to keep the specification
bound to that text. It would have been wiser to keep the behavior
unchanged and change the specification. Anyway, what's done is done. I
don't know if String.subSequence can be changed now. Maybe?
What could be done is introduce some new API. It could be in the form of
a new class or just a static method (on CharSequence perhaps?) that
would return a private implementation of CharSequence as a sub-sequence
view over underlying CharSequence or String's array in case the
passed-in argument is a String.
With such API one could gather sub-sequence view objects into a
LinkedList and then dump them into the correctly pre-sized
StringBuilder. There could even be a String counstructor with the
following signature:
public String(Iterable<CharSequence> sequences, int estimatedLength) { ...
..which would eliminate the last copying step too. Hopefully all
temporary view objects would be found as method-local and would be
allocated on stack / in registers by JIT (wishful thinking)...
Regards, Peter
On 27.05.2015 2:38, Xueming Shen wrote:
Ivan,
It might be worth trying String.index + StringBuilder, instead of
writing/handling everything yourself.
Yes, it inevitably adds an arraycopy at the end to convert the
StrinbBuilder to String, but it might have
better balance between performance and code complexity. The regex is
probably a little heavy for
literal string replacement, but StringBuilder should not be that bad ...
-Sherman
On 5/26/15 4:11 PM, Ivan Gerasimov wrote:
I updated the webrev:
http://cr.openjdk.java.net/~igerasim/8058779/02/webrev/
In the check at 2300-2301 and 2351-2352 I replaced MAX_ARRAY_SIZE
with Integer.MAX_VALUE, which seems to be more accurate here.
And I want to add that this proposed implementation is not only
faster, but also more memory efficient.
The following simple stress-test shows that the proposed version is
able to handle twice larger strings, comparing to the current
implementation.
----------------------------------------
public class C {
public static void main(String[] args) throws Throwable {
String s = "string";
for (int i = 1; i < Integer.MAX_VALUE; ++i) {
try {
s = s.replace("string", "stringstring");
} catch (OutOfMemoryError o) {
System.out.println(i + ") " + s.length());
break;
}
}
}
}
----------------------------------------
$ time ~/java9/jdk/bin/java -showversion -Xmx1g C
java version "1.9.0-ea"
Java(TM) SE Runtime Environment (build 1.9.0-ea-b63)
Java HotSpot(TM) 64-Bit Server VM (build 1.9.0-ea-b63, mixed mode)
25) 100663296
real 0m4.525s
user 0m4.402s
sys 0m1.189s
$ time ~/java9/jdk-build/bin/java -showversion -Xmx1g C
java version "1.9.0-internal"
Java(TM) SE Runtime Environment (build
1.9.0-internal-igerasim_2015_05_23_19_25-b00)
Java HotSpot(TM) 64-Bit Server VM (build
1.9.0-internal-igerasim_2015_05_23_19_25-b00, mixed mode)
26) 201326592
real 0m2.139s
user 0m1.960s
sys 0m0.461s
Sincerely yours,
Ivan
On 24.05.2015 23:17, Ivan Gerasimov wrote:
Hello everybody!
I know many people here like it when the performance is getting
better.
It was suggested to make the literal variant of String.replace()
faster.
Currently, this method is implemented as a few calls to regexp API,
so that the whole implementation takes only two lines of code.
I've created two versions of the fix.
In the first one, we scan the string and store indices of the found
substrings in an array.
Then, we allocate the precisely sized char array and fill it it.
The case with the empty target has to be handled separately.
BUGURL: https://bugs.openjdk.java.net/browse/JDK-8058779
WEBREV: http://cr.openjdk.java.net/~igerasim/8058779/00/webrev/
The second variant is much less verbose, however it's less
efficient too.
Here the StringJoiner is used as an intermediate storage.
WEBREV: http://cr.openjdk.java.net/~igerasim/8058779/01/webrev/
Here are the micro-benchmark results (in a string of ~300 chars do
~15 replacements).
0) Baseline
MyBenchmark.test thrpt 40 257'051.948 ± 4537.484 ops/s
1) Heavy-duty +308%
MyBenchmark.test thrpt 40 1'049'235.602 ± 15501.803 ops/s
2) StringJoiner +190%
MyBenchmark.test thrpt 40 746'000.629 ± 15387.036 ops/s
Personally, I like my first variant better, even though it adds
almost 300 lines of code.
But I'd like to hear what people think of it.
Sincerely yours,
Ivan
