RFR: 8364532: G1: In liveness tracing, print more significant digits for the liveness value [v3]

[Albert Mingkun Yang]

On Tue, 5 Aug 2025 21:57:41 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this small change that adds some more significant digits (3 in total) for the "liveness" column in collection set group logging.
>> 
>> E.g. see the emphasized columns that lack precision (it's more irritating with lots of lines showing single-digit values):
>> 
>> 50.707s][trace][gc,liveness] GC(13) ### groud-id num-regions gc-eff liveness remset type
>> 50.707s][trace][gc,liveness] GC(13) ### (bytes/ms) % (bytes)
>> 50.707s][trace][gc,liveness] GC(13) ### 1 70       0.0 *100* 3199208 Y
>> 50.707s][trace][gc,liveness] GC(13) ### 2 51 5298129.5  *18* 1224072 M
>> 50.707s][trace][gc,liveness] GC(13) ### 3  5 3223983.6  *31*  176648 M
>> 50.707s][trace][gc,liveness] GC(13) ### 4  5 3022016.3  *32*  176648 M
>> 
>> 
>> (This depends on PR#26591, so please look at that one first).
>> 
>> The change makes sure that three significant digits are printed.
>> 
>> Testing: manual review
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
> 
>  - Merge branch 'master' into 8364532-liveness-significant-digits
>  - Merge branch '8364531-factor-out-liveness-code' into 8364532-liveness-significant-digits
>  - * use one line per parameter in log message
>  - * rename liveness() method to more accurate liveness_percent()
>  - 8364532
>    
>    Hi all,
>    
>      please review this small change that adds some more significant digits (3 in total) for the "liveness" column in collection set group logging
>    
>    Testing: manual testing
>    
>    Thanks,
>      Thomas
>  - 8364531
>    
>    Hi all,
>    
>      please review this refactoring, eliminating some copy&paste, in cset candidate group liveness logging.
>    
>    The output is mostly the same, except for the young cset group, where the "-" has been replaced to just print the default value (0.0) for efficiency. I did not see this important to have, the gc efficiency for young gen isn't interesting.
>    
>    Testing: local compilation, gha, visual inspection of output
>    
>    Thanks,
>      Thomas

Minor subjective issue; up to you.

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp line 2984:

> 2982: #define G1PPRL_LEN_H_FORMAT           "  %14s"
> 2983: #define G1PPRL_GID_GCEFF_FORMAT       "  %14.1f"
> 2984: #define G1PPRL_GID_LIVENESS_FORMAT    "  %#.3g"

Why requiring 3 sig digit in total? This can make it hard to parse, even `1.23e-03` can appear. Is `%.2f` enough for the increased precision?

-------------

Marked as reviewed by ayang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26595#pullrequestreview-3092186104
PR Review Comment: https://git.openjdk.org/jdk/pull/26595#discussion_r2256914889