perf(ReplaySubject): slightly improved performance

[martinsik]

I was going through the ReplaySubject class and I think in some situations it can be simplified. Most importantly when I don't set the timeWindow parameter the buffer trimming method can be avoided completely and I can store just the values without wrapping them with ReplayEvent.

Also, I tried some performance tests to see whether splice() or shift() would be better. It seems that running shift() in a for loop is faster than splice() when removing < 25 items. However, splice() is more consistent even when removing a large portion of an array. See https://jsperf.com/splice-vs-shift-2

There were no benchmarks for Subjects in perf directory so I made a few myself but I'm not sure I'm testing it the right way. It shows that RxJS 4 implementation was faster than RxJS 5...

Before:

                                         |                     RxJS 4.1.0 |                     RxJS 5.4.1 |          factor |      % improved
---------------------------------------------------------------------------------------------------------------------------------------------------
    replaysubject_windowtime - immediate |                68,591 (±1.36%) |                17,151 (±2.57%) |           0.25x |          -75.0%
               replaysubject - immediate |                72,951 (±1.08%) |                17,099 (±2.69%) |           0.23x |          -76.6%
                replaysubject_windowtime |                39,104 (±0.85%) |                15,393 (±2.85%) |           0.39x |          -60.6%
                           replaysubject |                34,721 (±1.13%) |                15,393 (±2.30%) |           0.44x |          -55.7%

After:

                                        |                     RxJS 4.1.0 |                     RxJS 5.4.1 |          factor |      % improved
---------------------------------------------------------------------------------------------------------------------------------------------------
    replaysubject_windowtime - immediate |                63,306 (±1.14%) |                19,745 (±2.68%) |           0.31x |          -68.8%
               replaysubject - immediate |                63,870 (±3.35%) |                28,659 (±2.49%) |           0.45x |          -55.1%
                replaysubject_windowtime |                36,607 (±1.78%) |                16,646 (±2.83%) |           0.45x |          -54.5%
                           replaysubject |                36,058 (±1.29%) |                22,130 (±2.86%) |           0.61x |          -38.6%

[rxjs-bot]

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[coveralls]

Coverage decreased (-0.01%) to 97.723% when pulling a4e146e on martinsik:perf-replaySubject into 4694a27 on ReactiveX:master.

[kwonoj]

lol I need to update size diff output

[benlesh]

Please verify that the loop with shift is faster than the splice

[benlesh]

I'm actually shocked this would be faster. this section was O(1) now it's O(n).

[benlesh]

After reviewing perf, i think we should return it to splice because of it's consistent performance, even if shift is faster on smaller arrays.

[benlesh]

@martinsik the change looks fantastic, just one thing I'd like you to verify if possible.

I suspect that shift() is faster for 1 or 2 items, but splice is faster for more than that. If that's the case, we just need to accommodate the 80% case and move on (with a comment to note why it's a loop and not a splice)

[martinsik]

@benlesh I made this test to see which one is faster: https://jsperf.com/splice-vs-shift-2
It seems like even this:

for (var i = 0; i < 25; i++) {
    arr.shift();
}

is faster than this:

arr.splice(0, 25);

I don't know if there're any other aspects to consider. I guess size of the array shouldn't matter but maybe there are some specific cases when it does?

[benlesh]

Yeah, it looks like what I suspected. Which is that for small arrays shift() will be more efficient, but for larger arrays splice() wins out (the 50 vs 50 tests show a 2x perf increase for splice)

While there's some serious wins for the smaller arrays, I think they die off quickly enough that it's worth looking at the more consistent (and better for a larger range of sizes) performance of splice.

I think we should move it back to splice to keep performance consistent.

[coveralls]

Coverage decreased (-0.01%) to 97.723% when pulling 43f8bac on martinsik:perf-replaySubject into 4694a27 on ReactiveX:master.

[coveralls]

Coverage decreased (-0.01%) to 97.723% when pulling a4e146e on martinsik:perf-replaySubject into 4694a27 on ReactiveX:master.

[martinsik]

@benlesh I understand, I switched back to splice().

I'm not sure about the performance test I made. It shows that RxJS 4 is faster than RxJS 5 but is this even possible?

[martinsik]

@benlesh I just want to ask if this PR is still relevant or it needs updates for 5.5 or 6.0 :).

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.