Gatherer: spliterator characteristics are not propagated
Viktor Klang
viktor.klang at oracle.com
Mon Jan 22 21:06:27 UTC 2024
The flags are in sync with the implementation because the only way to create a Gatherer if through the factory methods and those factory methods (and only them) compute the proper combination of SEQUENTIAL | STATELESS | GREEDY. A user should not be able to set those flags. Only the flags KEEP_* are settable by a user.
But I presume this also requires to have a `int characteristics()`-method on the Gatherer interfacewhich means that users who are not using the factory methods will have full possibility of not only returning the flags, but returning any int.
The stream implementation has a whole mechanism in place to propagate/preverse flags like SIZED, DISTINCT or SORTED. For me, discussing about the merit of this mechanism seems a little off topic. I would prefer the Gatherer to be a good citizen and works seemlessly with the other intermediary operations.
I can see where you're coming from here, but to me, adding API surface needs to pull its weight.
In this case I wasn't convinced that it did, hence we're having this conversation. 🙂
Cheers,
√
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Monday, 22 January 2024 19:56
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>; Paul Sandoz <psandoz at openjdk.java.net>
Subject: [External] : Re: Gatherer: spliterator characteristics are not propagated
________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>
Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>, "Paul Sandoz" <psandoz at openjdk.java.net>
Sent: Monday, January 22, 2024 4:22:11 PM
Subject: Re: Gatherer: spliterator characteristics are not propagated
Hi Rémi,
Hello,
For instance, stateless is neither recessive nor dominant, since the composition of two stateless operations is only ever stateless if they both are greedy as well: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/Gatherers.java#L588<https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/Gatherers.java*L588__;Iw!!ACWV5N9M2RV99hQ!Lm52jd6kovd5t-cmrqqSLiRcIajBGXLxh85LO3eeiL6UxbKZuNPcUnO6z2i0FzMEoNr7U-cOBuWPCjo57FVW$>
Okay, so choosing SEQUENTIAL vs PARALELLIZABLE is not that important given that the combination is ad-hoc, reflecting the characterristics is enough.
So even if making it represented as ints (more like Spliterator, rather than Collector) makes things faster, it's still both work to track, propagate, and also becomes a side-channel that needs to remain in sync with the actual implementation of the logic.
The flags are in sync with the implementation because the only way to create a Gatherer if through the factory methods and those factory methods (and only them) compute the proper combination of SEQUENTIAL | STATELESS | GREEDY. A user should not be able to set those flags. Only the flags KEEP_* are settable by a user.
One could argue that logic such as: someCollection.stream().map(…).count() is a performance bug/inefficiency in an of itself as it would be faster to do someCollection.size().
The stream implementation has a whole mechanism in place to propagate/preverse flags like SIZED, DISTINCT or SORTED. For me, discussing about the merit of this mechanism seems a little off topic. I would prefer the Gatherer to be a good citizen and works seemlessly with the other intermediary operations.
Cheers,
√
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Monday, 22 January 2024 19:56
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>; Paul Sandoz <psandoz at openjdk.java.net>
Subject: [External] : Re: Gatherer: spliterator characteristics are not propagated
________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>
Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>, "Paul Sandoz" <psandoz at openjdk.java.net>
Sent: Monday, January 22, 2024 4:22:11 PM
Subject: Re: Gatherer: spliterator characteristics are not propagated
Hi Rémi,
Hello,
For instance, stateless is neither recessive nor dominant, since the composition of two stateless operations is only ever stateless if they both are greedy as well: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/Gatherers.java#L588<https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/Gatherers.java*L588__;Iw!!ACWV5N9M2RV99hQ!Lm52jd6kovd5t-cmrqqSLiRcIajBGXLxh85LO3eeiL6UxbKZuNPcUnO6z2i0FzMEoNr7U-cOBuWPCjo57FVW$>
Okay, so choosing SEQUENTIAL vs PARALELLIZABLE is not that important given that the combination is ad-hoc, reflecting the characterristics is enough.
So even if making it represented as ints (more like Spliterator, rather than Collector) makes things faster, it's still both work to track, propagate, and also becomes a side-channel that needs to remain in sync with the actual implementation of the logic.
The flags are in sync with the implementation because the only way to create a Gatherer if through the factory methods and those factory methods (and only them) compute the proper combination of SEQUENTIAL | STATELESS | GREEDY. A user should not be able to set those flags. Only the flags KEEP_* are settable by a user.
One could argue that logic such as: someCollection.stream().map(…).count() is a performance bug/inefficiency in an of itself as it would be faster to do someCollection.size().
The stream implementation has a whole mechanism in place to propagate/preverse flags like SIZED, DISTINCT or SORTED. For me, discussing about the merit of this mechanism seems a little off topic. I would prefer the Gatherer to be a good citizen and works seemlessly with the other intermediary operations.
Cheers,
√
regards,
Rémi
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Saturday, 20 January 2024 17:40
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>; Paul Sandoz <psandoz at openjdk.java.net>
Subject: [External] : Re: Gatherer: spliterator characteristics are not propagated
________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>
Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>, "Paul Sandoz" <psandoz at openjdk.java.net>
Sent: Thursday, January 18, 2024 5:14:38 PM
Subject: Re: Gatherer: spliterator characteristics are not propagated
And for A.andThen(B), A.flags & B.flags should work, the stream is sorted if both gatherers keep it sorted.
That is unfortunately not the case. That would presume that you can implement the composition such that it can preserve all the common flags. Some flags could be "dominant" and some "recessive" to use genetics nomenclature.
Some flags of the stream pipeline are "recessive", mainly SHORT_CIRCUIT, but not the characteristics of the Gatherer which can have the corresponding "dominant" flag, GREEDY, in this case.
And the same for sequential, the flag should be PARALELIZABLE and not SEQUENTIAL.
The idea is that the Gatherer characteristics can have the same bit set at the same position as the stream op flags (as defined by StreamOpFlag).
So KEEP_DISTINCT is in position 0, KEEP_SORTED in in position 2 and KEEP_SIZED is in position 3.
For GREEDY, we use the same position as SHORT_CIRCUIT and we will flip the bit (using XOR) when we want to extract the stream op flags from the characteristics
All the factory methods call the generic of() with a combination of PARALELIZABLE and STATELESS and the user can adds the characteristics GREEDY, KEEP_DISTINCT, KEEP_SORTED and KEEP_SIZED (otherwise an exception should be raised).
In StreamOpFlag, there are two unused positions (14 and 15), that's perfect for our two new states PARALELIZABLE and STATELESS, so no problem here (technically we can also reuse positions of the Spliterator characteristic given that those flags are masked before being sent to the GathererOp super constructor).
The way to transform a Gatherer characteristics op to a stream flags op is to flip the bits corresponding to SHORT_CIRCUIT, add the highter bit of all flags but SHORT-CIRCUIT (because stream op flags are encoded using 2 bits) and mask to only retain SHORT_CIRCUIT, KEEP_DISTINCT, KEEP_SORTED and KEEP_SIZED.
public static int toOpFlags(int characteristics) {
return ((characteristics ^ SHORT_CIRCUIT_MASK) | HIGHER_BITS) & STREAM_OP_MASK;
}
see below for a full script.
I suppose that if those flags already exist, it's because they have a purpose and i do not understand how it can make the other operations slower.
Extra invocations, extra storage, and extra composition overhead is not free. Since Stream is one-shot you need to include the construction cost with the execution cost. For something like an empty Stream construction cost scan be 90+% of the total costs.
If you create a Gatherer, the characteristics is a constant (so the validity check is removed, it's just a mask and a test) so the result of calling toOpFlags() is a constant too.
If the factory method is not inlined, the cost is 3 bitwise operations which is I believe faster than the "instanceof Greedy" used in the current code.
Cheers,
√
regards,
Rémi
---
public class GathererFlag {
// cut and paste from StreamOpFlag
/**
* The bit pattern for setting/injecting a flag.
*/
private static final int SET_BITS = 0b01;
/**
* The bit pattern for clearing a flag.
*/
private static final int CLEAR_BITS = 0b10;
/**
* The bit pattern for preserving a flag.
*/
private static final int PRESERVE_BITS = 0b11;
private static int position(int opFlagSet) {
return Integer.numberOfTrailingZeros(opFlagSet) >> 1;
}
private static final int DISTINCT_POSITION = position(StreamOpFlag.IS_DISTINCT);
private static final int SORTED_POSITION = position(StreamOpFlag.IS_SORTED);
private static final int SIZED_POSITION = position(StreamOpFlag.IS_SIZED);
private static final int SHORT_CIRCUIT_POSITION = position(StreamOpFlag.IS_SHORT_CIRCUIT);
private static final int STATELESS_POSITION = 14;
private static final int PARELLIZABLE_POSITION = 15;
public static final int PARELLIZABLE = SET_BITS << (PARELLIZABLE_POSITION << 1);
public static final int STATELESS = SET_BITS << (STATELESS_POSITION << 1);
public static final int GREEDY = SET_BITS << (SHORT_CIRCUIT_POSITION << 1);
public static final int KEEP_DISTINCT = SET_BITS << (DISTINCT_POSITION << 1);
public static final int KEEP_SORTED = SET_BITS << (SORTED_POSITION << 1);
public static final int KEEP_SIZED = SET_BITS << (SIZED_POSITION << 1);
private static final int SHORT_CIRCUIT_MASK = SET_BITS << (SHORT_CIRCUIT_POSITION << 1);
// no GREEDY here
private static final int HIGHER_BITS = (PARELLIZABLE | STATELESS | KEEP_DISTINCT | KEEP_SORTED | KEEP_SIZED) << 1;
private static final int STREAM_OP_MASK =
((GREEDY | KEEP_DISTINCT | KEEP_SORTED | KEEP_SIZED) << 1) |
GREEDY | KEEP_DISTINCT | KEEP_SORTED | KEEP_SIZED;
public static String toString(int characteristics) {
return Stream.of(characteristics)
.<String>mapMulti((f, consumer) -> {
if ((f & PARELLIZABLE) == PARELLIZABLE) {
consumer.accept("PARELLIZABLE");
}
if ((f & STATELESS) == STATELESS) {
consumer.accept("STATELESS");
}
if ((f & GREEDY) == GREEDY) {
consumer.accept("GREEDY");
}
if ((f & KEEP_DISTINCT) == KEEP_DISTINCT) {
consumer.accept("KEEP_DISTINCT");
}
if ((f & KEEP_SORTED) == KEEP_SORTED) {
consumer.accept("KEEP_SORTED");
}
if ((f & KEEP_SIZED) == KEEP_SIZED) {
consumer.accept("KEEP_SIZED");
}
})
.collect(Collectors.joining(", "));
}
public static int toOpFlags(int characteristics) {
return ((characteristics ^ SHORT_CIRCUIT_MASK) | HIGHER_BITS) & STREAM_OP_MASK;
}
public static String toOpFlagsString(int opFlags) {
return Arrays.stream(StreamOpFlag.values())
.map(op -> {
if (op.isPreserved(opFlags)) {
return "preserved " + op;
}
if (op.isCleared(opFlags)) {
return "cleared " + op;
}
if (op.isKnown(opFlags)) {
return "set " + op;
}
return "?? " + op;
})
.collect(Collectors.joining(", "));
}
void main() {
var characteristics = PARELLIZABLE | STATELESS | GREEDY | KEEP_DISTINCT | KEEP_SORTED | KEEP_SIZED;
System.out.println(toOpFlagsString(toOpFlags(characteristics)));
var characteristics2 = STATELESS | KEEP_DISTINCT | KEEP_SIZED;
System.out.println(toOpFlagsString(toOpFlags(characteristics2)));
}
}
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Thursday, 18 January 2024 16:17
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>; Paul Sandoz <psandoz at openjdk.java.net>
Subject: [External] : Re: Gatherer: spliterator characteristics are not propagated
________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>
Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>, "Paul Sandoz" <psandoz at openjdk.java.net>
Sent: Thursday, January 18, 2024 3:36:07 PM
Subject: Re: Gatherer: spliterator characteristics are not propagated
I suspect that it is a rather slippery slope, once KEEP-flags are added, then others will want to be able to have INJECT-flags, and then people might have different opinions w.r.t. the default should be to clear all flags etc.
And that's even before one looks at the composition-part of it, what are the flags for A.andThen(B)? (then extrapolate to N compositions and the available set of flags always approaches 0)
I spent quite a bit of time on this and in the end tracking all this info, and making sure that the flags of implementations correspond to the actual behavior, just ended up costing performance for most streams and introduced an extra dimension to creation and maintenance which I had a hard time justifying.
It can be a slippery slope if we were designing from the ground up but the stream implementation already exists and SORTED, DISTINCT and SIZED are the flags that are already tracked by the current implementation.
Currently only SHORT_CIRCUIT is set (if not greedy),
see https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/GathererOp.java#L209<https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/GathererOp.java*L209__;Iw!!ACWV5N9M2RV99hQ!PhMxqlDzLWPRuwYc7ECRKNPVs0BtnoE-RdT-Jdkng7S-iFuERAHYcWvJ-OMKGLrkPdSrUl3xj1R9ypyeqeWI$>
And for A.andThen(B), A.flags & B.flags should work, the stream is sorted if both gatherers keep it sorted.
Making specific, rare, combinations of operations faster at the expense of making 99% of all others slower is a hard pill for most to swallow.
I suppose that if those flags already exist, it's because they have a purpose and i do not understand how it can make the other operations slower.
Cheers,
√
regards,
Rémi
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: forax at univ-mlv.fr <forax at univ-mlv.fr>
Sent: Thursday, 18 January 2024 10:28
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net>; Paul Sandoz <psandoz at openjdk.java.net>
Subject: [External] : Re: Gatherer: spliterator characteristics are not propagated
________________________________
From: "Viktor Klang" <viktor.klang at oracle.com>
To: "Remi Forax" <forax at univ-mlv.fr>, "core-libs-dev" <core-libs-dev at openjdk.java.net>
Sent: Wednesday, January 17, 2024 8:48:07 PM
Subject: Re: Gatherer: spliterator characteristics are not propagated
Hi Rémi,
You can find some of my benches here: https://github.com/openjdk/jdk/tree/master/test/micro/org/openjdk/bench/java/util/stream/ops/ref<https://urldefense.com/v3/__https://github.com/openjdk/jdk/tree/master/test/micro/org/openjdk/bench/java/util/stream/ops/ref__;!!ACWV5N9M2RV99hQ!JJy6F9NoL6wKZQK5158up_fTRvH8X7F6JK8T7Euuf8vzbSQbr23eWa9S_yb61ksONVrLrdesCF_au5zyje2l$>
Initially I had Characteristics such as ORDERED etc on Gatherer but it just didn't end up worth it when looking at the bench results over a wide array of stream sizes and number of operations.
I think there are 3 gatherer characteristics that make sense: KEEP_SORTED, KEEP_DISTINCT and KEEP_SIZED,
all of them say that if the stream was sorted/distinct/sized then the stream returned by a call to gather() is still sorted (with the same comparator), distinct or sized.
As examples, map() is KEEP_SIZED, filter() is KEEP_SORTED | KEEP_DISTINCT and windowFixed is KEEP_DISTINCT.
[CC Paul, so he can correct me if i'm saying something stupid]
Now for the benchmarks, it depends what you want to measure, benchmarking streams is tricky. This is what i know about benchmarking streams.
First, the JIT has two ways to profile types at runtime,
Either a method takes a function as parameter
void map(Function function) {
function.apply(...)
}
and when map is called with a subtype of Function, the JIT will propagate the exact type when map is inlined,
Or a method use a field
class Op {
Function function;
void map() {
function.apply(...)
}
}
in that case, the VM records the profile of function.apply() and if there are more than two different profiles, the VM declare profile poluttion and do not try to optimize.
The Stream implementation tries very hard to use only parameters instead of fields, that's why it does not use classical Iterator that are pull iterator (a filter iterator requires a field) but a Spliterator which is a push iterator, the element is sent as parameter of the consumer.That's also why collect does not use the builder pattern (that accumulate values in fields) but a Collector that publish the functions to be called as parameter.
Obvisously, this is more complex than that, a Collector stores the functions in fields so it should not work well but the implementation uses a record that plays well with escape analysis. Escape analysis see the fields of an instance as parameters so the functions of a Collector are correctly propagated (if the escape analysis works). And lambdas are using invokedynamic, and the VM tries really hard to inline invokedynamic, so lambdas (that captures value or not) are routinely fully inlined with the intermediate operation of a stream.
In your tests, i've not seen comparaisons between an existing method like map() or filter() followed by a sorted()/distinct()/count()/toList(), i.e. operations where the characteristics KEEP_* have an impact and their equivalent using a Gatherer.
Cheers,
√
regards,
Rémi
Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Remi Forax <forax at univ-mlv.fr>
Sent: Wednesday, 17 January 2024 16:48
To: core-libs-dev <core-libs-dev at openjdk.java.net>
Subject: Gatherer: spliterator characteristics are not propagated
While doing some benchmarking of the Gatherer API, i've found that the characteristics of the spliterator was not propagated by the method Stream.gather() making the stream slower than it should.
As an example, there is no way when reimplementing map() using a Gatherer to say that this intermediate operation keep the size, which is important if the terminal operation is toList() because if the size is known, toList() will presize the List and avoid the creation of an intermediary ArrayList.
See https://github.com/forax/we_are_all_to_gather/blob/master/src/main/java/com/gihtub/forax/wearealltogather/bench/MapGathererBenchmark.java<https://urldefense.com/v3/__https://github.com/forax/we_are_all_to_gather/blob/master/src/main/java/com/gihtub/forax/wearealltogather/bench/MapGathererBenchmark.java__;!!ACWV5N9M2RV99hQ!JJy6F9NoL6wKZQK5158up_fTRvH8X7F6JK8T7Euuf8vzbSQbr23eWa9S_yb61ksONVrLrdesCF_auzwTY8aB$>
I think that adding a way to propagate the spliterator characteristics through a Gatherer would greatly improve the performance of commons streams (at least all the ones that end with a call to toList).
I have some idea of how to do that, but I prefer first to hear if i've overlook something and if improving the performance is something worth changing the API.
regards,
Rémi
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