1、JVM的啟動參數
我是這樣設置的：
java -Xmx1024m -Xms1024m -Xss128k -XX:NewRatio=4 -XX:SurvivorRatio=4 -XX:MaxPermSize=16m
啟動tomcat之后，使用 jmap -heap `pgrep -u root java`，得到如下信息：
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 1073741824 (1024.0MB)
NewSize = 1048576 (1.0MB)
MaxNewSize = 4294901760 (4095.9375MB)
OldSize = 4194304 (4.0MB)
NewRatio = 4
SurvivorRatio = 4
PermSize = 12582912 (12.0MB)
MaxPermSize = 16777216 (16.0MB)
Heap Usage:
New Generation (Eden + 1 Survivor Space):
capacity = 178913280 (170.625MB)
used = 51533904 (49.14656066894531MB)
free = 127379376 (121.47843933105469MB)
28.80384508070055% used
Eden Space:
capacity = 143130624 (136.5MB)
used = 51533904 (49.14656066894531MB)
free = 91596720 (87.35343933105469MB)
36.00480635087569% used
From Space:
capacity = 35782656 (34.125MB)
used = 0 (0.0MB)
free = 35782656 (34.125MB)
0.0% used
To Space:
capacity = 35782656 (34.125MB)
used = 0 (0.0MB)
free = 35782656 (34.125MB)
0.0% used
tenured generation:
capacity = 859045888 (819.25MB)
used = 1952984 (1.8625106811523438MB)
free = 857092904 (817.3874893188477MB)
0.22734338494383202% used
Perm Generation:
capacity = 12582912 (12.0MB)
used = 6656024 (6.347679138183594MB)
free = 5926888 (5.652320861816406MB)
52.897326151529946% used
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-Xmx1024m -Xms1024m -Xss128k -XX:NewRatio=4 -XX:SurvivorRatio=4 -XX:MaxPermSize=16m
-Xmx1024m 最大堆內存為 1024M
-Xms1024m 初始堆內存為 1024M
-XX:NewRatio=4
則 年輕代:年老代=1:4 1024M/5=204.8M
故 年輕代=204.8M 年老代=819.2M
-XX:SurvivorRatio=4
則年輕代中 2Survivor:1Eden=2:4 204.8M/6=34.13333333333333M
故 Eden=136.5333333333333M 1Suivivor=34.13333333333333M
用 jmap -heap <pid>
查看的結果 與我們計算的結果一致
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3、編寫測試頁面
在網站根目錄里新建頁面perf.jsp，內容如下：
<%intsize = (int)(1024 * 1024 * m);byte[] buffer = new byte[size];Thread.sleep(s);%>
注：m值用來設置每次申請內存的大小，s 表示睡眠多少ms
4、使用jstat來監控內存變化
這里使用 jstat -gcutil `pgrep -u root java` 1500 10
再解釋一下，這里有三個參數：
·pgrep -u root java --> 得到java的進程ID號
·1500 --> 表示每隔1500ms取一次數據
·10 --> 表示一共取10次數據
5、用ab來進行壓測
壓測的命令：[root@CentOS ~]# ab -c150 -n50000 "http://localhost/perf.jsp?m=1&s=10"
注：這里使用150個線程并發訪問，一共訪問50000次。
默認情況下你可以使用 http://localhost:8080/perf.jsp?m=1&s=10 來訪問。
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下面開始進行實驗：
·先啟動Java內存的監聽：
[root@CentOS ~]# jstat -gcutil 8570 1500 10
·在開啟一個終端，開始壓測：
[root@CentOS ~]# ab -c150 -n50000 "http://localhost/perf.jsp?m=1&s=10"
兩個命令結束之后的結果如下：
jstat：
[root@CentOS ~]# jstat -gcutil 8570 1500 10
S0 S1 E O P YGC YGCT FGC FGCT GCT
0.06 0.00 53.15 2.03 67.18 52 0.830 1 0.218 1.048
0.00 0.04 18.46 2.03 67.18 55 0.833 1 0.218 1.052
0.03 0.00 28.94 2.03 67.18 56 0.835 1 0.218 1.053
0.00 0.04 34.02 2.03 67.18 57 0.836 1 0.218 1.054
0.04 0.00 34.13 2.03 67.18 58 0.837 1 0.218 1.055
0.00 0.04 38.62 2.03 67.18 59 0.838 1 0.218 1.056
0.04 0.00 8.39 2.03 67.18 60 0.839 1 0.218 1.058
0.04 0.00 8.39 2.03 67.18 60 0.839 1 0.218 1.058
0.04 0.00 8.39 2.03 67.18 60 0.839 1 0.218 1.058
0.04 0.00 8.39 2.03 67.18 60 0.839 1 0.218 1.058
結果簡單解析：
可以看到JVM里S0和S1始終有一個是空的，Eden區達到一定比例之后就會產生Minor GC，由于我這里的Old Generation 區設置的比較大，所以沒有產生Full GC。
ab
[root@CentOS ~]# ab -c150 -n50000 "http://localhost/perf.jsp?m=1&s=10"
This is ApacheBench, Version 2.0.40-dev <$Revision: 1.146 $> apache-2.0
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Copyright 2006 The Apache Software Foundation, http://www.apache.org/
Benchmarking localhost (be patient)
Completed 5000 requests
Completed 10000 requests
Completed 15000 requests
Completed 20000 requests
Completed 25000 requests
Completed 30000 requests
Completed 35000 requests
Completed 40000 requests
Completed 45000 requests
Finished 50000 requests
Server Software: Apache/2.2.3
Server Hostname: localhost
Server Port: 80
Document Path: /perf.jsp?m=1&s=10
Document Length: 979 bytes
Concurrency Level: 150
Time taken for tests: 13.467648 seconds
Complete requests: 50000
Failed requests: 0
Write errors: 0
Non-2xx responses: 50005
Total transferred: 57605760 bytes
HTML transferred: 48954895 bytes
Requests per second: 3712.60 [#/sec] (mean)
Time per request: 40.403 [ms] (mean) #平均請求時間
Time per request: 0.269 [ms] (mean, across all concurrent requests)
Transfer rate: 4177.05 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 1 46.5 0 3701
Processing: 10 38 70.3 36 6885
Waiting: 3 35 70.3 33 6883
Total: 10 39 84.4 37 6901
Percentage of the requests served within a certain time (ms)
50% 37
66% 38
75% 39
80% 39
90% 41
95% 43
98% 50
99% 58
100% 6901 (longest request)