来源：https://www.ardanlabs.com/blog/2019/05/garbage-collection-in-go-part2-gctraces.html

开关GC的对比

有一个从不同的新闻提供商下载RSS并搜索的应用程序

在关闭GC的情况下测试并发请求应用程序耗时情况

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$ go build
$ GOGC=off ./project > /dev/null

# 10k requests using 100 connections
hey -m POST -c 100 -n 10000 "http://localhost:5000/search?term=topic&cnn=on&bbc=on&nyt=on"

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处理10k个请求需要4188ms，每秒处理约2387个请求

在开启GC的情况下呢，

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$ GODEBUG=gctrace=1 ./project > /dev/null
gc 3 @3.182s 0%: 0.015+0.59+0.096 ms clock, 0.19+0.10/1.3/3.0+1.1 ms cpu, 4->4->2 MB, 5 MB goal, 12 P
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gc 2553 @8.452s 14%: 0.004+0.33+0.051 ms clock, 0.056+0.12/0.56/0.94+0.61 ms cpu, 4->4->2 MB, 5 MB goal, 12 P

共需要2551次gc（省略前两次还没有开始正式测试的gc数据）

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gc 2553 @8.452s 14%: 0.004+0.33+0.051 ms clock, 0.056+0.12/0.56/0.94+0.61 ms cpu, 4->4->2 MB, 5 MB goal, 12 P

gc 2553     : The 2553 GC runs since the program started
@8.452s     : Eight seconds since the program started
14%         : Fourteen percent of the available CPU so far has been spent in GC

// wall-clock
0.004ms     : STW        : Write-Barrier - Wait for all Ps to reach a GC safe-point.
0.33ms      : Concurrent : Marking
0.051ms     : STW        : Mark Term     - Write Barrier off and clean up.

// CPU time
0.056ms     : STW        : Write-Barrier
0.12ms      : Concurrent : Mark - Assist Time (GC performed in line with allocation)
0.56ms      : Concurrent : Mark - Background GC time
0.94ms      : Concurrent : Mark - Idle GC time
0.61ms      : STW        : Mark Term

4MB         : Heap memory in-use before the Marking started
4MB         : Heap memory in-use after the Marking finished
2MB         : Heap memory marked as live after the Marking finished
5MB         : Collection goal for heap memory in-use after Marking finished

// Threads
12P         : Number of logical processors or threads used to run Goroutines.
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Requests            : 10,000
------------------------------------------------------
Requests/sec        : 1,882 r/s   - Hey
Total Duration      : 5,311ms     - Hey
Percent Time in GC  : 14%         - GC Trace
Total Collections   : 2,551       - GC Trace
------------------------------------------------------
Total GC Duration   : 744.54ms    - (5,311ms * .14)
Average Pace of GC  : ~2.08ms     - (5,311ms / 2,551)
Requests/Collection : ~3.98 r/gc  - (10,000 / 2,511)

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开启GC后耗费的时间增加了1.1s的延迟

减少分配

首先使用go的pprof工具查看内存使用情况

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import _ "net/http/pprof"

go func() {
    http.ListenAndServe("localhost:5000", http.DefaultServeMux)
}()

---
$ go tool pprof http://localhost:5000/debug/pprof/allocs

使用top命令查看前6个分配内存最多的函数

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(pprof) top 6 -cum
Showing nodes accounting for 0.56GB, 5.84% of 9.56GB total
Dropped 80 nodes (cum <= 0.05GB)
Showing top 6 nodes out of 51
      flat  flat%   sum%        cum   cum%
         0     0%     0%     4.96GB 51.90%  net/http.(*conn).serve
    0.49GB  5.11%  5.11%     4.93GB 51.55%  project/service.handler
         0     0%  5.11%     4.93GB 51.55%  net/http.(*ServeMux).ServeHTTP
         0     0%  5.11%     4.93GB 51.55%  net/http.HandlerFunc.ServeHTTP
         0     0%  5.11%     4.93GB 51.55%  net/http.serverHandler.ServeHTTP
    0.07GB  0.73%  5.84%     4.55GB 47.63%  project/search.rssSearch
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(pprof) list rssSearch
Total: 9.56GB
ROUTINE ======================== project/search.rssSearch in project/search/rss.go
   71.53MB     4.55GB (flat, cum) 47.63% of Total

         .          .    117:	// Capture the data we need for our results if we find ...
         .          .    118:	for _, item := range d.Channel.Items {
         .     4.48GB    119:		if strings.Contains(strings.ToLower(item.Description), strings.ToLower(term)) {
   48.53MB    48.53MB    120:			results = append(results, Result{
         .          .    121:				Engine:  engine,
         .          .    122:				Title:   item.Title,
         .          .    123:				Link:    item.Link,
         .          .    124:				Content: item.Description,
         .          .    125:			})

strings.ToLower会在堆上面创建新的字符串，循环调用是不需要的，可以放在循环外

改正之后重新测试一下

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$ go build
$ GODEBUG=gctrace=1 ./project > /dev/null
gc 3 @6.156s 0%: 0.011+0.72+0.068 ms clock, 0.13+0.21/1.5/3.2+0.82 ms cpu, 4->4->2 MB, 5 MB goal, 12 P
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gc 1404 @8.808s 7%: 0.005+0.54+0.059 ms clock, 0.060+0.47/0.79/0.25+0.71 ms cpu, 4->5->2 MB, 5 MB goal, 12 P
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With Extra Allocations              Without Extra Allocations
======================================================================
Requests            : 10,000        Requests            : 10,000
----------------------------------------------------------------------
Requests/sec        : 1,882 r/s     Requests/sec        : 3,631 r/s
Total Duration      : 5,311ms       Total Duration      : 2,753 ms
Percent Time in GC  : 14%           Percent Time in GC  : 7%
Total Collections   : 2,551         Total Collections   : 1,402
----------------------------------------------------------------------
Total GC Duration   : 744.54ms      Total GC Duration   : 192.71 ms
Average Pace of GC  : ~2.08ms       Average Pace of GC  : ~1.96ms
Requests/Collection : ~3.98 r/gc    Requests/Collection : 7.13 r/gc

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相比上一次减少了1149次GC，GC时间占比从14%下降到7%，程序运行速度提高了48%，collection时间减少74%

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