[infinispan-dev] To Optional or not to Optional?
Dan Berindei
dan.berindei at gmail.com
Tue May 23 09:58:37 EDT 2017
I wouldn't say I'm an extreme naysayer, but I do have 2 issues with
Optional:
1. Performance becomes harder to quantify: the allocations may or may not
be eliminated, and a change in one part of the code may change how
allocations are eliminated in a completely different part of the code.
2. My personal opinion is it's just ugly... instead of having one field
that could be null or non-null, you now have a field that could be null,
Optional.empty(), or Optional.of(something).
Cheers
Dan
On Tue, May 23, 2017 at 1:54 PM, Sebastian Laskawiec <slaskawi at redhat.com>
wrote:
> Hey!
>
> So I think we have no extreme naysayers to Optional. So let me try to sum
> up what we have achieved so:
>
> - In macroscale benchmark based on REST interface using Optionals
> didn't lower the performance.
> - +1 for using it in public APIs, especially for those using
> functional style.
> - Creating lots of Optional instances might add some pressure on GC,
> so we need to be careful when using them in hot code paths. In such cases
> it is required to run a micro scale benchamark to make sure the performance
> didn't drop. The microbenchmark should also be followed by macro scale
> benchamrk - PerfJobAck. Also, keep an eye on Eden space in such cases.
>
> If you agree with me, and there are no hard evidence that using Optional
> degrade performance significantly, I would like to issue a pull request and
> put those findings into contributing guide [1].
>
> Thanks,
> Sebastian
>
> [1] https://github.com/infinispan/infinispan/tree/
> master/documentation/src/main/asciidoc/contributing
>
> On Mon, May 22, 2017 at 6:36 PM Galder Zamarreño <galder at redhat.com>
> wrote:
>
>> I think Sanne's right here, any differences in such large scale test are
>> hard to decipher.
>>
>> Also, as mentioned in a previous email, my view on its usage is same as
>> Sanne's:
>>
>> * Definitely in APIs/SPIs.
>> * Be gentle with it internals.
>>
>> Cheers,
>> --
>> Galder Zamarreño
>> Infinispan, Red Hat
>>
>> > On 18 May 2017, at 14:35, Sanne Grinovero <sanne at infinispan.org> wrote:
>> >
>> > Hi Sebastian,
>> >
>> > sorry but I think you've been wasting time, I hope it was fun :) This
>> is not the right methodology to "settle" the matter (unless you want
>> Radim's eyes to get bloody..).
>> >
>> > Any change in such a complex system will only affect the performance
>> metrics if you're actually addressing the dominant bottleneck. In some
>> cases it might be CPU, like if your system is at 90%+ CPU then it's likely
>> that reviewing the code to use less CPU would be beneficial; but even that
>> can be counter-productive, for example if you're having contention caused
>> by optimistic locking and you fail to address that while making something
>> else "faster" the performance loss on the optimistic lock might become
>> asymptotic.
>> >
>> > A good reason to avoid excessive usage of Optional (and *excessive*
>> doesn't mean a couple dozen in a millions lines of code..) is to not run
>> out of eden space, especially for all the code running in interpreted mode.
>> >
>> > In your case you've been benchmarking a hugely complex beast, not least
>> over REST! When running the REST Server I doubt that allocation in eden is
>> your main problem. You just happened to have a couple Optionals on your
>> path; sure performance changed but there's no enough data in this way to
>> figure out what exactly happened:
>> > - did it change at all or was it just because of a lucky optimisation?
>> (The JIT will always optimise stuff differently even when re-running the
>> same code)
>> > - did the overall picture improve because this code became much *less*
>> slower?
>> >
>> > The real complexity in benchmarking is to accurately understand why it
>> changed; this should also tell you why it didn't change more, or less..
>> >
>> > To be fair I actually agree that it's very likely that C2 can make any
>> performance penalty disappear.. that's totally possible, although it's
>> unlikely to be faster than just reading the field (assuming we don't need
>> to do branching because of null-checks but C2 can optimise that as well).
>> > Still this requires the code to be optimised by JIT first, so it won't
>> prevent us from creating a gazillion of instances if we abuse its usage
>> irresponsibly. Fighting internal NPEs is a matter of writing better code;
>> I'm not against some "Optional" being strategically placed but I believe
>> it's much nicer for most internal code to just avoid null, use "final", and
>> initialize things aggressively.
>> >
>> > Sure use Optional where it makes sense, probably most on APIs and SPIs,
>> but please don't go overboard with it in internals. That's all I said in
>> the original debate.
>> >
>> > In case you want to benchmark the impact of Optional make a JMH based
>> microbenchmark - that's interesting to see what C2 is capable of - but even
>> so that's not going to tell you much on the impact it would have to patch
>> thousands of code all around Infinispan. And it will need some peer review
>> before it can tell you anything at all ;)
>> >
>> > It's actually a very challenging topic, as we produce libraries meant
>> for "anyone to use" and don't get to set the hardware specification
>> requirements it's hard to predict if we should optimise the system for
>> this/that resource consumption. Some people will have plenty of CPU and
>> have problems with us needing too much memory, some others will have the
>> opposite.. the real challenge is in making internals "elastic" to such
>> factors and adaptable without making it too hard to tune.
>> >
>> > Thanks,
>> > Sanne
>> >
>> >
>> >
>> > On 18 May 2017 at 12:30, Sebastian Laskawiec <slaskawi at redhat.com>
>> wrote:
>> > Hey!
>> >
>> > In our past we had a couple of discussions about whether we should or
>> should not use Optionals [1][2]. The main argument against it was
>> performance.
>> >
>> > On one hand we risk additional object allocation (the Optional itself)
>> and wrong inlining decisions taken by C2 compiler [3]. On the other hand we
>> all probably "feel" that both of those things shouldn't be a problem and
>> should be optimized by C2. Another argument was the Optional's doesn't give
>> us anything but as I checked, we introduced nearly 80 NullPointerException
>> bugs in two years [4]. So we might consider Optional as a way of fighting
>> those things. The final argument that I've seen was about lack of higher
>> order functions which is simply not true since we have #map, #filter and
>> #flatmap functions. You can do pretty amazing things with this.
>> >
>> > I decided to check the performance when refactoring REST interface. I
>> created a PR with Optionals [5], ran performance tests, removed all
>> Optionals and reran tests. You will be surprised by the results [6]:
>> >
>> > Test case
>> > With Optionals [%] Without Optionals
>> > Run 1 Run 2 Avg Run 1 Run 2 Avg
>> > Non-TX reads 10 threads
>> > Throughput 32.54 32.87 32.71 31.74 34.04 32.89
>> > Response time -24.12 -24.63 -24.38 -24.37 -25.69 -25.03
>> > Non-TX reads 100 threads
>> > Throughput 6.48 -12.79 -3.16 -7.06 -6.14 -6.60
>> > Response time -6.15 14.93 4.39 7.88 6.49 7.19
>> > Non-TX writes 10 threads
>> > Throughput 9.21 7.60 8.41 4.66 7.15 5.91
>> > Response time -8.92 -7.11 -8.02 -5.29 -6.93 -6.11
>> > Non-TX writes 100 threads
>> > Throughput 2.53 1.65 2.09 -1.16 4.67 1.76
>> > Response time -2.13 -1.79 -1.96 0.91 -4.67 -1.88
>> >
>> > I also created JMH + Flight Recorder tests and again, the results
>> showed no evidence of slow down caused by Optionals [7].
>> >
>> > Now please take those results with a grain of salt since they tend to
>> drift by a factor of +/-5% (sometimes even more). But it's very clear the
>> performance results are very similar if not the same.
>> >
>> > Having those numbers at hand, do we want to have Optionals in
>> Infinispan codebase or not? And if not, let's state it very clearly (and
>> write it into contributing guide), it's because we don't like them. Not
>> because of performance.
>> >
>> > Thanks,
>> > Sebastian
>> >
>> > [1] http://lists.jboss.org/pipermail/infinispan-dev/2017-
>> March/017370.html
>> > [2] http://lists.jboss.org/pipermail/infinispan-dev/2016-
>> August/016796.html
>> > [3] http://vanillajava.blogspot.ro/2015/01/java-lambdas-and-
>> low-latency.html
>> > [4] https://issues.jboss.org/issues/?jql=project%20%3D%
>> 20ISPN%20AND%20issuetype%20%3D%20Bug%20AND%20text%20%7E%
>> 20%22NullPointerException%22%20AND%20created%20%3E%3D%
>> 202015-04-27%20AND%20created%20%3C%3D%202017-04-27
>> > [5] https://github.com/infinispan/infinispan/pull/5094
>> > [6] https://docs.google.com/a/redhat.com/spreadsheets/d/
>> 1oep6Was0FfvHdqBCwpCFIqcPfJZ5-5_YYUqlRtUxEkM/edit?usp=sharing
>> > [7] https://github.com/infinispan/infinispan/pull/5094#
>> issuecomment-296970673
>> > --
>> > SEBASTIAN ŁASKAWIEC
>> > INFINISPAN DEVELOPER
>> > Red Hat EMEA
>> >
>> >
>> > _______________________________________________
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>> >
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>>
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>
> --
>
> SEBASTIAN ŁASKAWIEC
>
> INFINISPAN DEVELOPER
>
> Red Hat EMEA <https://www.redhat.com/>
> <https://red.ht/sig>
>
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