Linux内存管理与减少Swap使用的完整指南

# 查看内存和 swap 使用概况
free -h

# 查看各进程 swap 占用
for file in /proc/*/status ; do
    awk '/vmswap|name/{printf $2 " " $3}end{ print ""}' $file
done | sort -k 2 -n -r | head -10

# 实时监控 swap i/o
vmstat 1

# 查看详细内存统计
cat /proc/meminfo

              total        used        free      shared  buff/cache   available
mem:           15gi       8.2gi       1.1gi       345mi       6.3gi       6.7gi
swap:         2.0gi       1.8gi       200mi

# 查看当前 swappiness 值（默认通常是 60）
cat /proc/sys/vm/swappiness

# 临时生效
sudo sysctl vm.swappiness=1

# 永久生效（写入配置文件）
echo 'vm.swappiness=1' | sudo tee -a /etc/sysctl.conf
sudo sysctl -p

# 清理 pagecache
echo 1 > /proc/sys/vm/drop_caches

# 清理 dentries 和 inodes
echo 2 > /proc/sys/vm/drop_caches

# 清理所有（pagecache + dentries + inodes）
echo 3 > /proc/sys/vm/drop_caches

public class memoryfootprintdemo {
    public static void main(string[] args) throws exception {
        // 设置 jvm 参数：-xmx2g -xms2g -xx:maxmetaspacesize=256m
        system.out.println("jvm 最大堆内存: " + 
            runtime.getruntime().maxmemory() / 1024 / 1024 + " mb");
        system.out.println("jvm 总内存: " + 
            runtime.getruntime().totalmemory() / 1024 / 1024 + " mb");
        system.out.println("jvm 空闲内存: " + 
            runtime.getruntime().freememory() / 1024 / 1024 + " mb");
        // 创建一些对象模拟内存使用
        list<byte[]> list = new arraylist<>();
        for (int i = 0; i < 1000; i++) {
            list.add(new byte[1024 * 1024]); // 1mb each
            thread.sleep(10);
        }
        system.gc(); // 建议gc（不一定立即执行）
        thread.sleep(1000);
        system.out.println("gc后空闲内存: " + 
            runtime.getruntime().freememory() / 1024 / 1024 + " mb");
    }
}

javac memoryfootprintdemo.java
java -xmx2g -xms2g -xx:maxmetaspacesize=256m memoryfootprintdemo

总内存 ≈ xmx + maxmetaspacesize + (线程数 × 栈大小) + directmemory + 本地库 + 安全余量

-xmx4g
-xx:maxmetaspacesize=512m
-xx:threadstacksize=1m
线程数：200
-xx:maxdirectmemorysize=1g（默认等于 xmx）

估算：
4g (堆)
+ 0.5g (元空间)
+ 200 × 1m = 0.2g (栈)
+ 1g (直接内存)
+ 0.5g (jni/本地库/安全缓冲)

≈ 6.2 gb

# 不要贪心！留足空间给操作系统和其他进程
java -xmx3g -xms3g myapp

# 避免类加载过多撑爆内存
java -xx:maxmetaspacesize=256m myapp

# netty、nio 等框架常用 directbuffer，需显式限制
java -xx:maxdirectmemorysize=512m myapp

# 默认 1mb，对多数应用过大
java -xss256k myapp

cat /sys/kernel/mm/transparent_hugepage/enabled

[always] madvise never

# 临时关闭
echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/enabled
echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/defrag

# 永久生效（加入 rc.local 或 systemd 服务）
echo 'echo madvise > /sys/kernel/mm/transparent_hugepage/enabled' | sudo tee -a /etc/rc.local
echo 'echo madvise > /sys/kernel/mm/transparent_hugepage/defrag' | sudo tee -a /etc/rc.local

docker run -it --memory=4g --memory-swap=4g openjdk:17 java -xmx3g myapp

resources:
  limits:
    memory: "4gi"
  requests:
    memory: "3gi"

java -xx:+usecontainersupport -xx:maxrampercentage=75.0 myapp

java -xx:+useg1gc -xx:maxgcpausemillis=200 myapp

java -xx:+usezgc -xmx4g myapp

java -xx:+useshenandoahgc -xmx4g myapp

public class gctest {
    private static final int size = 10000;
    private static list<byte[]> cache = new arraylist<>();
    public static void main(string[] args) throws interruptedexception {
        system.out.println("开始内存压力测试...");
        for (int i = 0; i < 100; i++) {
            simulateworkload();
            thread.sleep(1000);
        }
    }
    private static void simulateworkload() {
        // 分配大量临时对象
        for (int i = 0; i < size; i++) {
            cache.add(new byte[1024 * 10]); // 10kb each
        }
        // 偶尔保留一些对象模拟缓存
        if (cache.size() > size * 50) {
            cache.sublist(0, size * 30).clear();
        }
        system.out.println("当前缓存大小: " + cache.size());
    }
}

# parallel gc（传统，易波动）
java -xx:+useparallelgc gctest

# g1 gc（较平稳）
java -xx:+useg1gc gctest

# zgc（最平稳，适合大堆）
java -xx:+usezgc gctest

#!/bin/bash
# check_swap.sh
swap_used=$(free | grep swap | awk '{print $3}')
swap_total=$(free | grep swap | awk '{print $2}')
if [ $swap_total -eq 0 ]; then
    echo "no swap configured."
    exit 0
fi
swap_percent=$((swap_used * 100 / swap_total))
if [ $swap_percent -gt 30 ]; then
    echo "🚨 高 swap 使用率: ${swap_percent}%"
    # 可集成邮件、企业微信、钉钉等告警
    # send_alert "swap usage is ${swap_percent}% on $(hostname)"
fi

* * * * * /path/to/check_swap.sh >> /var/log/swap_monitor.log 2>&1

// ❌ 不推荐：在 jvm 堆内缓存大量数据
map<string, userdata> usercache = new concurrenthashmap<>();
// ✅ 推荐：使用 redis/memcached
redistemplate<string, userdata> redistemplate;

import org.apache.commons.pool2.basepooledobjectfactory;
import org.apache.commons.pool2.pooledobject;
import org.apache.commons.pool2.impl.defaultpooledobject;
import org.apache.commons.pool2.impl.genericobjectpool;
import org.apache.commons.pool2.impl.genericobjectpoolconfig;
public class bytebufferpool {
    private genericobjectpool<bytebuffer> pool;
    public bytebufferpool(int maxsize) {
        genericobjectpoolconfig<bytebuffer> config = new genericobjectpoolconfig<>();
        config.setmaxtotal(maxsize);
        config.setblockwhenexhausted(true);
        config.setmaxwaitmillis(3000);
        pool = new genericobjectpool<>(new bytebufferfactory(), config);
    }
    public bytebuffer borrow() throws exception {
        return pool.borrowobject();
    }
    public void release(bytebuffer buffer) {
        buffer.clear();
        pool.returnobject(buffer);
    }
    static class bytebufferfactory extends basepooledobjectfactory<bytebuffer> {
        @override
        public bytebuffer create() {
            return bytebuffer.allocatedirect(1024); // 1kb direct buffer
        }
        @override
        public pooledobject<bytebuffer> wrap(bytebuffer buffer) {
            return new defaultpooledobject<>(buffer);
        }
    }
}

ab -n 10000 -c 100 http://localhost:8080/api/heavy

sudo apt install zram-config
sudo systemctl restart zram-config

# 创建 2gb zram 设备
echo 2147483648 > /sys/class/zram-control/hot_add
zram_dev=/dev/zram$(cat /sys/class/zram-control/hot_add)

# 初始化并启用
mkswap $zram_dev
swapon $zram_dev

# 查看状态
zramctl