什么是虛擬內存？
先直接摘抄一段 wikipedia 上的介紹。
虛擬內存是計算機系統內存管理的一種技術。它使得應用程序認為它擁有連續的可用的內存（一個連續完整的地址空間），而實際上，它通常是被分隔成多個物理內存碎片，還有部分暫時存儲在外部磁盤存儲器上，在需要時進行數據交換。
對于 C 語言里面的變量，我們可以使用 & 運算符來獲得其地址， 既然是虛擬地址，就是指這個地址是虛擬的。
虛擬地址機制不是必須的，在簡單的單片機中，編寫的代碼編譯時都需要指定物理 RAM 空間分布，不會有虛擬地址的概念，地址就是指在 RAM 中的物理地址。
介紹
在我們自己的購買的服務器環境中，一般是買的1g的內存，但是當服務器里面的東西裝的比較多的時候就會導致內存不夠用了，本文將模擬一個真實的內存不夠用的情況下，如何通過修改虛擬內存來讓系統正常運行，我們這里的環境是搭建一個ElasticSearch搜索的環境，但是我們的服務器內存只有1g，下面將演示如何在將1g的虛擬內存修改為4G。
搭建ElasticSearch環境
現在我們的服務器環境是空的，什么都沒有，我們這里先將ElasticSearch上傳到服務器，然后將jdk和ElasticSearch安裝好。
安裝jdk
安裝教程后面更新（該文章主要介紹設置虛擬內存，安裝這些東西主要是模擬一個內存不夠的狀態）
安裝ElasticSearch
安裝鏈接后面更新（該文章主要介紹設置虛擬內存，安裝這些東西主要是模擬一個內存不夠的狀態）
啟動ElasticSearch
啟動ElasticSearch，會發現啟動的時候報錯了，原因是我們的服務器現在的內存并不能滿足ElasticSearch需要的內存。
[esyonghu@localhost elasticsearch-6.4.0]$ ./bin/elasticsearch [1] 3228[esyonghu@localhost elasticsearch-6.4.0]$ Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x000000008a660000, 1973026816, 0) failed; error='Cannot allocate memory' (errno=12)## There is insufficient memory for the Java Runtime Environment to continue.# Native memory allocation (mmap) failed to map 1973026816 bytes for committing reserved memory.# An error report file with more information is saved as:# logs/hs_err_pid3228.log[esyonghu@localhost elasticsearch-6.4.0]$
查看我們的服務器的內存，使用命令 free,可以看到我們服務器的內存是1g, 這個時候就需要我們修改虛擬內存來解決該問題了。
[esyonghu@localhost elasticsearch-6.4.0]$ free -m total used free shared buffers cachedMem: 980 582 397 2 23 245-/+ buffers/cache: 313 667Swap: 0 0 0[esyonghu@localhost elasticsearch-6.4.0]$
創建swap文件
進入/usr目錄
[root@localhost usr]$ pwd/usr[root@localhost usr]$
創建swap文件夾,并進入該文件夾
[root@localhost usr]# mkdir swap[root@localhost usr]# cd swap/[root@localhost swap]# pwd/usr/swap[root@localhost swap]#
創建swapfile文件,使用命令dd if=/dev/zero of=/usr/swap/swapfile bs=1M count=4096
[root@localhost swap]# dd if=/dev/zero of=/usr/swap/swapfile bs=1M count=4096記錄了4096+0 的讀入記錄了4096+0 的寫出4294967296字節(4.3 GB)已復制，15.7479 秒，273 MB/秒[root@localhost swap]#
查看swap文件
使用命令du -sh /usr/swap/swapfile,可以看到我們創建的這個swap文件為4g
[root@localhost swap]# du -sh /usr/swap/swapfile4.1G /usr/swap/swapfile[root@localhost swap]#
將目標設置為swap分區文件
1、使用命令mkswap /usr/swap/swapfile將swapfile文件設置為swap分區文件
[root@localhost swap]# mkswap /usr/swap/swapfilemkswap: /usr/swap/swapfile: warning: don't erase bootbits sectors on whole disk. Use -f to force.Setting up swapspace version 1, size = 4194300 KiBno label, UUID=5bd241ff-5375-449d-9975-5fdd429df784[root@localhost swap]#
激活swap區，并立即啟用交換區文件
使用命令swapon /usr/swap/swapfile
[root@localhost swap]# swapon /usr/swap/swapfile[root@localhost swap]#
使用命令free -m 來查看現在的內存,可以看到里面的Swap分區變成了4095M，也就是4G內存。
[root@localhost swap]# free -m total used free shared buffers cachedMem: 980 910 70 3 8 575-/+ buffers/cache: 326 654Swap: 4095 0 4095[root@localhost swap]#
設置開機自動啟用虛擬內存，在etc/fstab文件中加入如下命令
1、使用vim編輯器打開/etc/fstab文件
2、在文件中加入如下內容
/usr/swap/swapfile2 swap swap defaults 0 0
使用reboot命令重啟服務器
1、輸入reboot 命令來重啟
[root@localhost swap]# reboot Broadcast message from liaocheng@localhost.localdomain (/dev/pts/1) at 3:56 ... The system is going down for reboot NOW! [root@localhost swap]# Connection to 192.168.136.142 closed by remote host. Connection to 192.168.136.142 closed. [進程已完成]
2、重啟完成過后使用free -m 命令來查看現在的內存是否掛在上了。
[root@localhost swap]# free -m total used free shared buffers cachedMem: 980 910 70 3 8 575-/+ buffers/cache: 326 654Swap: 4095 0 4095
再次啟動ElasticSearch看看是否還會報內存不足的錯誤
1、還是切換到esyonghu去啟動（這里為什么要使用es用戶啟動就先不介紹了，這是elasticsearch里面的知識，這里只是用elasticsearch來模擬內存不足的情況）,可以看到已經不會有內存不足的問題了。
[esyonghu@localhost elasticsearch-6.4.0]$ ./bin/elasticsearch &[1] 2898[esyonghu@localhost elasticsearch-6.4.0]$ [2019-03-06T04:00:24,841][INFO ][o.e.n.Node ] [] initializing ...[2019-03-06T04:00:24,928][INFO ][o.e.e.NodeEnvironment ] [dMy5nR5] using [1] data paths, mounts [[/ (rootfs)]], net usable_space [7.6gb], net total_space [17.3gb], types [rootfs][2019-03-06T04:00:24,928][INFO ][o.e.e.NodeEnvironment ] [dMy5nR5] heap size [1.9gb], compressed ordinary object pointers [true][2019-03-06T04:00:25,018][INFO ][o.e.n.Node ] [dMy5nR5] node name derived from node ID [dMy5nR5fThaBb-Q2T0txdA]; set [node.name] to override[2019-03-06T04:00:25,018][INFO ][o.e.n.Node ] [dMy5nR5] version[6.4.0], pid[2898], build[default/tar/595516e/2018-08-17T23:18:47.308994Z], OS[Linux/2.6.32-696.el6.x86_64/amd64], JVM[Oracle Corporation/Java HotSpot(TM) 64-Bit Server VM/1.8.0_181/25.181-b13][2019-03-06T04:00:25,018][INFO ][o.e.n.Node ] [dMy5nR5] JVM arguments [-Xms2g, -Xmx2g, -XX:+UseConcMarkSweepGC, -XX:CMSInitiatingOccupancyFraction=75, -XX:+UseCMSInitiatingOccupancyOnly, -XX:+AlwaysPreTouch, -Xss1m, -Djava.awt.headless=true, -Dfile.encoding=UTF-8, -Djna.nosys=true, -XX:-OmitStackTraceInFastThrow, -Dio.netty.noUnsafe=true, -Dio.netty.noKeySetOptimization=true, -Dio.netty.recycler.maxCapacityPerThread=0, -Dlog4j.shutdownHookEnabled=false, -Dlog4j2.disable.jmx=true, -Djava.io.tmpdir=/tmp/elasticsearch.24Q3S9AE, -XX:+HeapDumpOnOutOfMemoryError, -XX:HeapDumpPath=data, -XX:ErrorFile=logs/hs_err_pid%p.log, -XX:+PrintGCDetails, -XX:+PrintGCDateStamps, -XX:+PrintTenuringDistribution, -XX:+PrintGCApplicationStoppedTime, -Xloggc:logs/gc.log, -XX:+UseGCLogFileRotation, -XX:NumberOfGCLogFiles=32, -XX:GCLogFileSize=64m, -Des.path.home=/home/esyonghu/elasticsearch-6.4.0, -Des.path.conf=/home/esyonghu/elasticsearch-6.4.0/config, -Des.distribution.flavor=default, -Des.distribution.type=tar][2019-03-06T04:00:28,022][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [aggs-matrix-stats][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [analysis-common][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [ingest-common][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [lang-expression][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [lang-mustache][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [lang-painless][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [mapper-extras][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [parent-join][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [percolator][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [rank-eval][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [reindex][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [repository-url][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [transport-netty4][2019-03-06T04:00:28,023][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [tribe][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-core][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-deprecation][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-graph][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-logstash][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-ml][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-monitoring][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-rollup][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-security][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-sql][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-upgrade][2019-03-06T04:00:28,024][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded module [x-pack-watcher][2019-03-06T04:00:28,025][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded plugin [analysis-ik][2019-03-06T04:00:28,025][INFO ][o.e.p.PluginsService ] [dMy5nR5] loaded plugin [analysis-pinyin][2019-03-06T04:00:31,315][INFO ][o.e.x.s.a.s.FileRolesStore] [dMy5nR5] parsed [0] roles from file [/home/esyonghu/elasticsearch-6.4.0/config/roles.yml][2019-03-06T04:00:32,017][INFO ][o.e.x.m.j.p.l.CppLogMessageHandler] [controller/2947] [Main.cc@109] controller (64 bit): Version 6.4.0 (Build cf8246175efff5) Copyright (c) 2018 Elasticsearch BV[2019-03-06T04:00:32,495][DEBUG][o.e.a.ActionModule ] Using REST wrapper from plugin org.elasticsearch.xpack.security.Security[2019-03-06T04:00:32,768][INFO ][o.e.d.DiscoveryModule ] [dMy5nR5] using discovery type [zen][2019-03-06T04:00:33,628][INFO ][o.e.n.Node ] [dMy5nR5] initialized[2019-03-06T04:00:33,628][INFO ][o.e.n.Node ] [dMy5nR5] starting ...[2019-03-06T04:00:33,860][INFO ][o.e.t.TransportService ] [dMy5nR5] publish_address {192.168.136.142:9300}, bound_addresses {[::]:9300}[2019-03-06T04:00:33,884][INFO ][o.e.b.BootstrapChecks ] [dMy5nR5] bound or publishing to a non-loopback address, enforcing bootstrap checks[2019-03-06T04:00:36,995][INFO ][o.e.c.s.MasterService ] [dMy5nR5] zen-disco-elected-as-master ([0] nodes joined)[, ], reason: new_master {dMy5nR5}{dMy5nR5fThaBb-Q2T0txdA}{ldgTZ1XZSfOpda9uP4treA}{192.168.136.142}{192.168.136.142:9300}{ml.machine_memory=1028210688, xpack.installed=true, ml.max_open_jobs=20, ml.enabled=true}[2019-03-06T04:00:37,003][INFO ][o.e.c.s.ClusterApplierService] [dMy5nR5] new_master {dMy5nR5}{dMy5nR5fThaBb-Q2T0txdA}{ldgTZ1XZSfOpda9uP4treA}{192.168.136.142}{192.168.136.142:9300}{ml.machine_memory=1028210688, xpack.installed=true, ml.max_open_jobs=20, ml.enabled=true}, reason: apply cluster state (from master [master {dMy5nR5}{dMy5nR5fThaBb-Q2T0txdA}{ldgTZ1XZSfOpda9uP4treA}{192.168.136.142}{192.168.136.142:9300}{ml.machine_memory=1028210688, xpack.installed=true, ml.max_open_jobs=20, ml.enabled=true} committed version [1] source [zen-disco-elected-as-master ([0] nodes joined)[, ]]])[2019-03-06T04:00:37,058][INFO ][o.e.x.s.t.n.SecurityNetty4HttpServerTransport] [dMy5nR5] publish_address {192.168.136.142:9200}, bound_addresses {[::]:9200}[2019-03-06T04:00:37,058][INFO ][o.e.n.Node ] [dMy5nR5] started[2019-03-06T04:00:37,177][INFO ][o.w.a.d.Monitor ] try load config from /home/esyonghu/elasticsearch-6.4.0/config/analysis-ik/IKAnalyzer.cfg.xml[2019-03-06T04:00:37,179][INFO ][o.w.a.d.Monitor ] try load config from /home/esyonghu/elasticsearch-6.4.0/plugins/ik/config/IKAnalyzer.cfg.xml[2019-03-06T04:00:37,888][INFO ][o.e.m.j.JvmGcMonitorService] [dMy5nR5] [gc][4] overhead, spent [486ms] collecting in the last [1.2s][2019-03-06T04:00:38,435][WARN ][o.e.x.s.a.s.m.NativeRoleMappingStore] [dMy5nR5] Failed to clear cache for realms [[]][2019-03-06T04:00:38,469][INFO ][o.e.l.LicenseService ] [dMy5nR5] license [c91cae39-79d7-4a0e-b40b-b1918a45f80c] mode [trial] - valid[2019-03-06T04:00:38,477][INFO ][o.e.g.GatewayService ] [dMy5nR5] recovered [5] indices into cluster_state[2019-03-06T04:00:38,902][WARN ][o.e.x.s.a.s.m.NativeRoleMappingStore] [dMy5nR5] Failed to clear cache for realms [[]][2019-03-06T04:00:39,106][INFO ][o.e.c.r.a.AllocationService] [dMy5nR5] Cluster health status changed from [RED] to [YELLOW] (reason: [shards started [[mynote2][2]] ...]).2、現在使用free -m來查看內存使用情況, 可以看到swap已經被使用了1.7G
[esyonghu@localhost elasticsearch-6.4.0]$ free -m total used free shared buffers cachedMem: 980 916 64 0 3 33-/+ buffers/cache: 880 100Swap: 4095 1735 2360[esyonghu@localhost elasticsearch-6.4.0]$
總結
以上就是這篇文章的全部內容了，希望本文的內容對大家的學習或者工作具有一定的參考學習價值，如果有疑問大家可以留言交流，謝謝大家對VEVB武林網的支持。
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