Reduce the overhead of unconfigured loggers

[swebb2066]

AppenderAttachableImpl::appendLoopOnAppenders does not need to lock a mutex for any Logger in the path to the root Logger which has no appenders attached (This partly addressed #347).

[swebb2066]

On Windows, the benchmark\tools\compare.py benchmarks old.json new.json results are:

Benchmark	Time	CPU	Time Old	Time New	CPU Old	CPU New
Testing disabled logging request	+0.0034	+0.1569	4	4	1	1
Testing disabled logging request/threads:4	-0.2516	-0.4159	1	1	4	2
Logging 5 char string using MessageBuffer, pattern: %m%n	-0.1902	-0.2120	738	597	443	349
Logging 5 char string using MessageBuffer, pattern: %m%n/threads:4	+0.0410	-0.5887	437	455	1289	530
Logging 49 char string using MessageBuffer, pattern: %m%n	-0.0180	-0.1123	752	739	481	427
Logging 49 char string using MessageBuffer, pattern: %m%n/threads:4	-0.0485	-0.3203	462	439	1242	844
Logging int value using MessageBuffer, pattern: %m%n	-0.0725	-0.1875	1756	1629	1099	893
Logging int value using MessageBuffer, pattern: %m%n/threads:4	-0.1219	-0.3980	740	650	1744	1050
Logging int+float using MessageBuffer, pattern: %m%n	-0.1529	-0.3460	3535	2995	2288	1496
Logging int+float using MessageBuffer, pattern: %m%n/threads:4	-0.1841	-0.3902	1387	1132	3139	1914
Logging int value using MessageBuffer, pattern: [%d] %m%n	-0.1133	-0.2317	1868	1656	1144	879
Logging int value using MessageBuffer, pattern: [%d] [%c] [%p] %m%n	-0.0990	-0.2480	1876	1690	1308	984
Logging 49 char string using FMT, pattern: %m%n	-0.0972	-0.2988	796	719	572	401
Logging 49 char string using FMT, pattern: %m%n/threads:4	-0.1717	-0.4716	489	405	1135	600
Logging int value using FMT, pattern: %m%n	-0.0587	-0.0932	712	670	460	417
Logging int value using FMT, pattern: %m%n/threads:4	-0.1114	-0.3957	458	407	1062	642
Logging int+float using FMT, pattern: %m%n	-0.1281	-0.1353	1132	987	663	573
Logging int+float using FMT, pattern: %m%n/threads:4	-0.1281	-0.5153	544	475	1662	806
Async, int value using MessageBuffer, pattern: %m%n	-0.1272	-0.2650	1824	1592	1224	900
Async, int value using MessageBuffer, pattern: %m%n/threads:4	-0.0474	-0.3363	745	709	1883	1250

[rm5248]

What's the reasoning behind the lazy initialization of m_priv? It makes the code harder to read IMO since you now have an extra check to see if it is valid, instead of just knowing that it always exists.

[swebb2066]

What's the reasoning behind the lazy initialization of m_priv? It makes the code harder to read IMO since you now have an extra check to see if it is valid, instead of just knowing that it always exists.

Less memory and processor overhead (see the above benchmarks).

An application will create many loggers, but few (usually only the root logger) has attached appenders.

Lazy initialization reduces memory alloc's by avoiding allocating AppenderAttachableImpl::priv_data (and initializing a std::mutex + std::vector) for all those loggers that do not have attached appenders.

Throughput is improved because appendLoopOnAppenders does not acquire a lock for each decendent logger in the path to the root logger.

[swebb2066]

On Ubuntu (gcc 11), the benchmark/tools/compare.py --no-color benchmarks /tmp/old.json /tmp/new.json results are:

Benchmark	Time	CPU	Time Old	Time New	CPU Old	CPU New
Testing disabled logging request	-0.0358	-0.0354	0	0	0	0
Testing disabled logging request/threads:4	-0.2218	-0.2217	0	0	1	1
Logging 5 char string using MessageBuffer, pattern: %m%n	-0.0704	-0.0703	311	289	311	289
Logging 5 char string using MessageBuffer, pattern: %m%n/threads:4	-0.0560	-0.0531	553	522	2172	2056
Logging 49 char string using MessageBuffer, pattern: %m%n	-0.0133	-0.0133	318	314	318	314
Logging 49 char string using MessageBuffer, pattern: %m%n/threads:4	-0.2431	-0.2367	575	435	2262	1727
Logging int value using MessageBuffer, pattern: %m%n	-0.0474	-0.0475	489	466	489	466
Logging int value using MessageBuffer, pattern: %m%n/threads:4	-0.2029	-0.2014	593	473	2345	1872
Logging int+float using MessageBuffer, pattern: %m%n	-0.0000	-0.0000	941	941	941	941
Logging int+float using MessageBuffer, pattern: %m%n/threads:4	-0.2770	-0.2735	754	545	2997	2177
Logging int value using MessageBuffer, pattern: [%d] %m%n	-0.1074	-0.1074	627	559	627	559
Logging int value using MessageBuffer, pattern: [%d] [%c] [%p] %m%n	-0.1052	-0.1051	629	563	629	563
Logging 49 char string using FMT, pattern: %m%n	-0.0333	-0.0333	293	283	293	283
Logging 49 char string using FMT, pattern: %m%n/threads:4	-0.1931	-0.1891	537	433	2114	1714
Logging int value using FMT, pattern: %m%n	-0.0156	-0.0156	315	310	315	310
Logging int value using FMT, pattern: %m%n/threads:4	-0.1616	-0.1590	548	459	2162	1818
Logging int+float using FMT, pattern: %m%n	-0.0558	-0.0559	468	442	468	442
Logging int+float using FMT, pattern: %m%n/threads:4	+0.0794	+0.0839	429	463	1700	1843
Async, int value using MessageBuffer, pattern: %m%n	-0.0463	-0.0463	493	471	493	471
Async, int value using MessageBuffer, pattern: %m%n/threads:4	-0.3025	-0.2992	680	474	2683	1880