perf + Flame Graph火焰图分析程序性能

1、perf命令简要介绍

性能调优时，我们通常需要分析查找到程序百分比高的热点代码片段，这便需要使用 perf record 记录单个函数级别的统计信息，并使用 perf report 来显示统计结果；

usage: perf [--version] [--help] [OPTIONS] COMMAND [ARGS]

 The most commonly used perf commands are:
   annotate        Read perf.data (created by perf record) and display annotated code
   archive         Create archive with object files with build-ids found in perf.data file
   bench           General framework for benchmark suites
   buildid-cache   Manage build-id cache.
   buildid-list    List the buildids in a perf.data file
   data            Data file related processing
   diff            Read perf.data files and display the differential profile
   evlist          List the event names in a perf.data file
   inject          Filter to augment the events stream with additional information
   kmem            Tool to trace/measure kernel memory properties
   kvm             Tool to trace/measure kvm guest os
   list            List all symbolic event types
   lock            Analyze lock events
   mem             Profile memory accesses
   record          Run a command and record its profile into perf.data
   report          Read perf.data (created by perf record) and display the profile
   sched           Tool to trace/measure scheduler properties (latencies)
   script          Read perf.data (created by perf record) and display trace output
   stat            Run a command and gather performance counter statistics
   test            Runs sanity tests.
   timechart       Tool to visualize total system behavior during a workload
   top             System profiling tool.
   trace           strace inspired tool
   probe           Define new dynamic tracepoints

 See 'perf help COMMAND' for more information on a specific command.

举例

perf record -e cpu-clock -g -p 222

-g 选项是告诉perf record额外记录函数的调用关系

-e cpu-clock 指perf record监控的指标为cpu周期

-p 指定需要record的进程pid

程序运行完之后，perf record会生成一个名为perf.data的文件，如果之前已有，那么之前的perf.data文件会被覆盖

获得这个perf.data文件之后，就需要perf report工具进行查看

perf report -i perf.data

-i 指定要查看的文件

以诊断fluentd为例,report结果：

$perf report  -i perf.data

这种格式很不直观

 2、使用火焰图展示结果

1、Flame Graph项目位于GitHub上：https://github.com/brendangregg/FlameGraph

2、可以用git将其clone下来：git clone https://github.com/brendangregg/FlameGraph.git

 注意：git clone之后，下面用到的*.pl文件先给+x可执行权限，注意路径

我们以perf为例，看一下flamegraph的使用方法：

1、第一步

$perf record -e cpu-clock -g -p 28591

Ctrl+c结束执行后，在当前目录下会生成采样数据perf.data.

2、第二步

用perf script工具对perf.data进行解析

perf script -i perf.data &> perf.unfold

3、第三步

将perf.unfold中的符号进行折叠：

./stackcollapse-perf.pl perf.unfold &> perf.folded

注意：该命令可能有错误，错误提示在perf.folded

4、最后生成svg图：

./flamegraph.pl perf.folded > perf.svg