Speedup PriorityQueue a little#13936
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Saving some field accesses.
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This results in a lot more code complexity, which makes maintenance difficult. Maybe the version of java you are testing with has a bug in its register allocator or something? seriously? I think we should take a step back before making all of our code more complex for a 1% benefit which might just be an upstream compiler bug. |
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I think the queue methods changed here in isolation get a far bigger improvement than 1% in many cases. Plus making methods like the ones adjusted here smaller and easier on the CPU cache tends to help the performance of "neighboring" code as well in many case (hence the across the board speedup in the luceneutil run). I don't think this is the result of a JVM bug, it's just something that is hard to optimize by the compiler with Java so dynamic. It's a combination of two things.
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| public void addAll(Collection<T> elements) { | ||
| if (this.size + elements.size() > this.maxSize) { | ||
| int s = size; |
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Can we at least rename "s" to "size" and use this.size as the right hand side of this assignment?
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Right that was a little weird sorry :) renamed now.
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| private final boolean upHeap(int origPos) { | ||
| private boolean upHeap(int origPos, T[] heap) { |
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I'd create a local heap variable (var heap = this.heap) locally in this method, not pass it as an argument. It is confusing why you'd want it as an argument. I agree with Robert here that we should perhaps see long-term maintenance as worth the tiny performance benefit (although I think assigning to a local variable within the method would yield the same result).
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although I think assigning to a local variable within the method would yield the same result
Not quite, the idea was that I already have heap in a local in the caller, so if I pass it as an argument I save a field read and as an added bonus get a smaller method that inlines better.
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long-term maintenance as worth the tiny performance benefit
With this class in particular I'm not sure the argument holds. Isn't the whole point of it the ability to mutate top and resort via updateTop as an optimization over the JDKs priority queue? If the implementation is slower than java.util.PriorityQueue, then what's the point? :) Also, I'm still not sure I agree with the "tiny" part :)
Granted there's limits to the benchmark data provided, but it's more likely than not that a couple things improved by 3%+ isn't it? Plus, I could see a possible compounding effect with further optimizations in the users of the PQ if those can be reduced in size enough to have lessThan inline and not be a megamorphic callsite here and there.
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Not quite, the idea was that I already have heap in a local in the caller, so if I pass it as an argument I save a field read and as an added bonus get a smaller method that inlines better.
I did understand the intention but I think the difference, if any, will be noticeable only if the loop doesn't hoist out the field read (which, I think it should?). My suggestion keeps the variables local, which helps in understanding of what it does. But anyway. I'm not entirely sold on these low-level optimizations that target c2/hotspot. There is so many moving parts here... operating system and CPU included. Eh.
Isn't the whole point of it the ability to mutate top and resort via updateTop as an optimization over the JDKs priority queue? If the implementation is slower than java.util.PriorityQueue, then what's the point? :)
I believe the differences were also functional - insertWithOverflow is one particular example that comes to mind and would require more complex logic in the JDK's PQ. Another is resigning from one level of indirection (method instead of Comparator) - these choices predate a lot of newer Java's offerings - perhaps it could be implemented in a different way now.
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Whether or not it's worth doing this kind of optimization for the observed gain is a tricky question. From the perspective of a user of a large (and read-heavy) ES, Opensearch or similar deployment, an O(1%) gain might translate into a lot of dollars saved and this kind of thing is well worth the effort.
Personally, an extra 10 lines of code for the observed speedups seems like a reasonable deal, but that's admittedly quite subjective. Maybe a stronger argument would be: optimizing this kind of thing in core hot code removes potential bottlenecks from the system, enabling other optimizations. If the core logic puts massive pressure on e.g. the CPU cache then optimizations (or regressions!) in higher-level code are masked on CPUs with smaller caches. So doing a 1% optimization and living with slightly more complicated code makes more sense here than a 1% gain would in more "peripheral" code. Also, you could use that same angle and argue that this code hardly ever gets touched, so the maintenance burden added matters less than it would elsewhere.
That said :) as far as the technical details go I don't think it can hoist out those reads and it's not an exclusively C2/hotstpot specific thing either. Since Java allows using reflection to update final field values (except for fields that are either static, on a record or on a hidden classs) the compiler can't hoist the field access out of the loop I think (maybe in some happy cases escape analysis helps here).
You can make the JIT hoist these things via -XX:+TrustFinalNonStaticFields which gives me a result like (main vs main with that flag set).
results
TaskQPS baseline StdDevQPS my_modified_version StdDev Pct diff p-value
HighTermTitleBDVSort 25.72 (7.0%) 25.77 (6.5%) 0.2% ( -12% - 14%) 0.897
BrowseRandomLabelSSDVFacets 3.37 (6.0%) 3.40 (4.2%) 1.0% ( -8% - 11%) 0.409
OrHighMed 214.06 (3.7%) 216.37 (3.8%) 1.1% ( -6% - 8%) 0.206
OrHighNotHigh 353.55 (8.4%) 358.52 (8.9%) 1.4% ( -14% - 20%) 0.475
AndHighHigh 111.32 (4.9%) 113.08 (5.5%) 1.6% ( -8% - 12%) 0.179
OrNotHighHigh 567.88 (4.8%) 577.94 (4.9%) 1.8% ( -7% - 12%) 0.108
PKLookup 241.21 (2.1%) 245.53 (2.1%) 1.8% ( -2% - 6%) 0.000
HighTerm 455.94 (6.6%) 464.35 (7.5%) 1.8% ( -11% - 17%) 0.250
MedTerm 590.06 (6.5%) 601.24 (6.0%) 1.9% ( -9% - 15%) 0.182
AndHighMed 156.22 (3.1%) 159.19 (2.9%) 1.9% ( -3% - 8%) 0.005
LowTerm 750.87 (4.6%) 765.45 (4.2%) 1.9% ( -6% - 11%) 0.052
BrowseRandomLabelTaxoFacets 4.48 (8.6%) 4.57 (3.9%) 2.0% ( -9% - 15%) 0.182
OrNotHighMed 479.29 (4.6%) 489.00 (5.4%) 2.0% ( -7% - 12%) 0.074
HighTermMonthSort 1515.68 (6.4%) 1546.97 (7.0%) 2.1% ( -10% - 16%) 0.171
OrHighHigh 85.48 (4.6%) 87.32 (5.3%) 2.2% ( -7% - 12%) 0.055
MedTermDayTaxoFacets 19.13 (3.0%) 19.55 (4.1%) 2.2% ( -4% - 9%) 0.007
MedIntervalsOrdered 28.59 (6.3%) 29.23 (4.7%) 2.2% ( -8% - 14%) 0.079
OrHighLow 610.70 (5.0%) 624.94 (5.0%) 2.3% ( -7% - 13%) 0.040
OrHighNotMed 474.52 (5.5%) 485.78 (5.7%) 2.4% ( -8% - 14%) 0.061
Fuzzy2 66.51 (3.2%) 68.09 (3.0%) 2.4% ( -3% - 8%) 0.001
BrowseDateSSDVFacets 1.24 (7.7%) 1.27 (8.1%) 2.4% ( -12% - 19%) 0.181
MedSpanNear 119.05 (4.4%) 121.94 (4.4%) 2.4% ( -6% - 11%) 0.016
HighTermTitleSort 76.83 (4.8%) 78.72 (3.7%) 2.5% ( -5% - 11%) 0.011
AndHighHighDayTaxoFacets 14.60 (3.8%) 14.96 (3.5%) 2.5% ( -4% - 10%) 0.003
BrowseMonthTaxoFacets 11.04 (38.5%) 11.32 (40.3%) 2.5% ( -55% - 132%) 0.778
OrNotHighLow 1089.24 (4.0%) 1117.30 (4.0%) 2.6% ( -5% - 10%) 0.004
TermDTSort 188.79 (4.6%) 193.74 (4.9%) 2.6% ( -6% - 12%) 0.015
Wildcard 426.59 (4.2%) 437.79 (4.2%) 2.6% ( -5% - 11%) 0.006
MedPhrase 78.10 (3.4%) 80.38 (3.2%) 2.9% ( -3% - 9%) 0.000
Prefix3 1068.70 (7.7%) 1100.07 (7.7%) 2.9% ( -11% - 19%) 0.094
AndHighLow 1546.10 (5.3%) 1591.97 (6.0%) 3.0% ( -7% - 15%) 0.020
LowIntervalsOrdered 134.11 (6.2%) 138.10 (5.0%) 3.0% ( -7% - 15%) 0.019
MedSloppyPhrase 47.07 (4.5%) 48.49 (3.7%) 3.0% ( -5% - 11%) 0.001
AndHighMedDayTaxoFacets 65.36 (2.3%) 67.38 (2.3%) 3.1% ( -1% - 7%) 0.000
LowSpanNear 175.93 (3.7%) 181.36 (4.7%) 3.1% ( -5% - 11%) 0.001
HighPhrase 131.54 (7.2%) 135.70 (5.8%) 3.2% ( -9% - 17%) 0.033
Fuzzy1 108.08 (3.4%) 111.62 (2.1%) 3.3% ( -2% - 9%) 0.000
BrowseDayOfYearSSDVFacets 4.52 (7.7%) 4.67 (7.9%) 3.4% ( -11% - 20%) 0.056
OrHighNotLow 550.21 (7.0%) 569.01 (7.9%) 3.4% ( -10% - 19%) 0.043
HighTermDayOfYearSort 380.03 (7.6%) 393.27 (6.6%) 3.5% ( -9% - 19%) 0.030
HighSpanNear 11.37 (4.5%) 11.77 (6.0%) 3.5% ( -6% - 14%) 0.004
Respell 54.77 (1.6%) 56.69 (1.7%) 3.5% ( 0% - 6%) 0.000
HighSloppyPhrase 30.28 (5.2%) 31.40 (4.8%) 3.7% ( -5% - 14%) 0.001
LowPhrase 76.63 (5.6%) 79.65 (5.6%) 3.9% ( -6% - 16%) 0.002
OrHighMedDayTaxoFacets 6.78 (6.2%) 7.05 (6.9%) 4.0% ( -8% - 18%) 0.007
IntNRQ 78.26 (6.5%) 81.38 (7.2%) 4.0% ( -9% - 18%) 0.010
LowSloppyPhrase 65.45 (6.6%) 68.14 (6.0%) 4.1% ( -7% - 17%) 0.004
HighIntervalsOrdered 9.16 (6.5%) 9.59 (5.9%) 4.6% ( -7% - 18%) 0.001
BrowseMonthSSDVFacets 4.48 (10.4%) 4.70 (12.4%) 4.8% ( -16% - 30%) 0.062
BrowseDateTaxoFacets 5.38 (10.8%) 5.67 (12.7%) 5.4% ( -16% - 32%) 0.043
BrowseDayOfYearTaxoFacets 5.44 (10.6%) 5.74 (12.7%) 5.5% ( -16% - 32%) 0.039
So to me it feels like manually hoisting field access is a generally valid optimization in a world that has reflective writes to final fields. To me, reducing field access is not in the same category as e.g. extracting cold paths artifically to make a method inline or other such tricks that are specific to C2 and hardware. This is just giving the compiler input that it cannot practically work out with the constraints imposed by the language and the JIT's runtime cost needing
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the compiler can't hoist the field access out of the loop I think (maybe in some happy cases escape analysis helps here).
I don't think there's anything in the spec preventing it from doing so. The final keyword is indeed for the java compiler, not for the jvm, but... you know - it's easy to show that c2 can happily hoist out field reads, try it.
public final class SuperSoft {
private static boolean ready;
public static void startThread() {
new Thread() {
public void run() {
try {
sleep(2000);
} catch (Exception e) { /* ignore */ }
System.out.println("Marking loop exit.");
ready = true;
}
}.start();
}
public static void main(String[] args) {
startThread();
System.out.println("Entering the loop...");
while (!ready) {
// Do nothing.
}
System.out.println("Done, I left the loop!");
}
}
This aside, I am not rejecting the change - I just suggested to rename one local variable (s) and to remove method parameter in favor of a single local variable read - this should result in identical code to what your 1% gain was producing, if my gut feeling is right.
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Whether or not it's worth doing this kind of optimization for the observed gain is a tricky question
We've done such optimizations in the past for very hot hotspots in Lucene, e.g. readVInt, all the carefully gen'd code for decoding int[] blocks in different bit widths, etc. But it clearly is a tricky judgement call in each case...
mikemccand
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The gains measured by luceneutil are quite surprising ... Lucene's PQ is clearly a hot hotspot.
| */ | ||
| public void addAll(Collection<T> elements) { | ||
| if (this.size + elements.size() > this.maxSize) { | ||
| int size = this.size; |
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Could you add comments explaining that the local variable assignment is done on purpose for performance reasons? We don't want a future refactoring to "simplify" this code and cut back to this.size.
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| private final void downHeap(int i) { | ||
| private void downHeap(int i, T[] heap, int size) { |
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Why are we removing final on upHeap and downHeap? Does that somehow help performance?
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| private final boolean upHeap(int origPos) { | ||
| private boolean upHeap(int origPos, T[] heap) { |
There was a problem hiding this comment.
Whether or not it's worth doing this kind of optimization for the observed gain is a tricky question
We've done such optimizations in the past for very hot hotspots in Lucene, e.g. readVInt, all the carefully gen'd code for decoding int[] blocks in different bit widths, etc. But it clearly is a tricky judgement call in each case...
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This PR has not had activity in the past 2 weeks, labeling it as stale. If the PR is waiting for review, notify the dev@lucene.apache.org list. Thank you for your contribution! |
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This PR has not had activity in the past 2 weeks, labeling it as stale. If the PR is waiting for review, notify the dev@lucene.apache.org list. Thank you for your contribution! |
4 participants
Saving some field accesses results in small but visible savings: