JVM GC日志分析之日志参数

　　不同的垃圾收集器(参考JVM 垃圾收集器分类)，输出的日志格式各不相同，但也有一些相同的特征。熟悉各个常用垃圾收集器的GC日志，是进行JVM调优的必备一步。解析GC日志，首先需要收集日志(参考https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html#enable-logging-with-the-jvm-unified-logging-framework)，通过-Xlog:help打印日志相关帮助信息如下：

　　从上图看首先日志配置的使用语法是：-Xlog[:[selections][:[output][:[decorators][:output-options]]]]

　　　　'selections' are combinations of tags and levels of the form tag1[+tag2...][*][=level][,...]

　　而tag的内容可以是(Available log tags)：

　　　　add, age, alloc, annotation, arguments, attach, barrier, biasedlocking, blocks, bot, breakpoint, bytecode, cds, census, class, classhisto, cleanup, codecache, compaction, compilation, condy, constantpool, constraints, container, coops, cpu, cset, data, datacreation, dcmd, decoder, defaultmethods, director, dump, dynamic, ergo, event, exceptions, exit, fingerprint, free, freelist, gc, handshake, hashtables, heap, humongous, ihop, iklass, indy, init, inlining, install, interpreter, itables, jfr, jit, jni, jvmci, jvmti, lambda, library, liveness, load, loader, logging, malloc, map, mark, marking, membername, memops, metadata, metaspace, methodcomparator, methodhandles, mirror, mmu, module, monitorinflation, monitormismatch, nestmates, nmethod, nmt, normalize, numa, objecttagging, obsolete, oldobject, oom, oopmap, oops, oopstorage, os, owner, pagesize, parser, patch, path, perf, periodic, phases, plab, placeholders, preorder, preview, promotion, protectiondomain, ptrqueue, purge, record, redefine, ref, refine, region, reloc, remset, resolve, safepoint, sampling, scavenge, sealed, setting, smr, stackbarrier, stackmap, stacktrace, stackwalk, start, startup, startuptime, state, stats, streaming, stringdedup, stringtable, subclass, survivor, suspend, sweep, symboltable, system, table, task, thread, throttle, time, timer, tlab, tracking, unload, unshareable, update, valuebasedclasses, verification, verify, vmmutex, vmoperation, vmthread, vtables, vtablestubs, workgang

　　　　如果使用all而不是tag的组合会将所有tag组合日志打印出来。

　　tag为必需项，仅提供tag即可打印日志了。例如：