`ByteArrayOutput` should have a customizable initial capacity

[pm47]

It should be possible to define a custom initial size for the backing array of ByteArrayOutput, like the jdk impl. The initial size of 32B is tiny and results in dozens of reallocation+memcopies in the lightning-kmp serialization use case (and probably is similar within bitcoin-kmp). That can't be good for performance, I wonder how setting a reasonable capacity would reduce the test time.

[sstone]

Yes we should allow it, but a quick test with a default value of 32 Kb (instead of 32 bytes) did not speed things up at all.

[pm47]

Is this on the jvm ? Is there a chance the difference is more notable on native iOS ?

[sstone]

I ran linuxTest (on linux) and performance did not improve at all. I don't think it would be much different on iOS (but not completely sure).
The test I'm using is:

./gradlew :cleanLinuxTest :linuxTest --tests "fr.acinq.lightning.channel.states.NormalTestsCommon"

which should imho highlight serialization-related performance issues. I noticed too that lightning-kmp tests are slower now with the new serialization but I'm not sure it is I/O related. We had severe performance issues on bitcoin-kmp (#72) and the root cause was not I/O but the poor performance of kotlin collections, though I don't think it's the same problem with lightning-kmp because the performance difference between native and jvm tests is not as significant.

[pm47]

I noticed too that lightning-kmp tests are slower now with the new serialization

We do serialize (bin+json) at all state transitions now (compared to only bin for some transitions), might explain the longer test time. I guess, if there is no measurable performance improvement, this can be left as-is.