RFR: JDK-8266503: [ul] Make Decorations safely copy-able and reduce their size (first attempt, closed)

[Thomas Stuefe]

On Tue, 4 May 2021 15:05:43 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> In Universal Logging, class LogDecorations keeps resolved decorations as well as a lookup table of pointers to the start of each resolved decoration, by decorator. This is dangerous, since it makes object copy non-trivial (the pointers would need to be relocated). It is also wasteful since we spend 8 bytes per pointer per index.
> 
> Better would be to use a numerical index array of offset values, which could be safely copied. 
> 
> And since the resolve buffer is 256 char, we can easily make this index an 8-bit value, which reduces the size of a LogDecorations object down to 280 bytes (from 368). Size matters especially in the context of JDK-8229517.
> 
> The patch also adds a gtest, which tests that decorations are safely copy-able and that decorator resolution works as expected.
> 
> Testing:
> - manually jtreg runtime/logging
> - manually gtests
> - Will run more tests tonight

> > Really? It decreases your planned message buffer size, no?
> 
> Yes, I am happy to see it reduces the size of each payload. but sometimes smaller memory footprint doesn't change to runtime performance directly. From my measurement, LogDecoration doesn't impact that much.
> 
> Check out this performance comparison. I found that copying 20 bytes is almost identical to copying 256 bytes.
> [#3855 (comment)](https://github.com/openjdk/jdk/pull/3855#issuecomment-832324143)

Oh sure. But I never claimed this to be performance relevant. Its all about memory.

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PR: https://git.openjdk.java.net/jdk/pull/3855