用sysctl進行調整Linux系統性能
sysctl是一個允許您改變正在運行中的Linux系統的接口。它包含一些 TCP/IP 堆棧和虛擬內存系統的高級選項， 這可以讓有經驗的管理員提高引人注目的系統性能。用sysctl可以讀取設置超過五百個系統變量。基於這點，sysctl(8)提供兩個功能：讀取和修改系統設置。
查看所有可讀變量：
%sysctl -a
讀一個指定的變量，例如kern.maxproc：
%sysctl kern.maxprockern.maxproc: 1044
要設置一個指定的變量，直接用variable=value這樣的語法：
#sysctl kern.maxfiles=5000
kern.maxfiles: 2088 -> 5000
您可以使用sysctl修改系統變量，也可以通過編輯sysctl.conf文件來修改系統變量。sysctl.conf 看起來很像rc.conf。它用variable=value的形式來設定值。指定的值在系統進入多用戶模式之後被設定。並不是所有的變量都可以在這個模式下設定。
sysctl 變量的設置通常是字符串、數字或者布爾型。 (布爾型用1來表示'yes'，用0來表示'no')。
sysctl -w kernel.sysrq=0
sysctl -w kernel.core_uses_pid=1
sysctl -w net.ipv4.conf.default.accept_redirects=0
sysctl -w net.ipv4.conf.default.accept_source_route=0
sysctl -w net.ipv4.conf.default.rp_filter=1
sysctl -w net.ipv4.tcp_syncookies=1
sysctl -w net.ipv4.tcp_max_syn_backlog=2048
sysctl -w net.ipv4.tcp_fin_timeout=30
sysctl -w net.ipv4.tcp_synack_retries=2
sysctl -w net.ipv4.tcp_keepalive_time=3600
sysctl -w net.ipv4.tcp_window_scaling=1
sysctl -w net.ipv4.tcp_sack=1
配置sysctl
編輯此文件：
vi /etc/sysctl.conf
如果該文件為空，則輸入以下內容，否則請根據情況自己做調整：
# Controls source route verification
# Default should work for all interfaces
net.ipv4.conf.default.rp_filter = 1
# net.ipv4.conf.all.rp_filter = 1
# net.ipv4.conf.lo.rp_filter = 1
# net.ipv4.conf.eth0.rp_filter = 1
# Disables IP source routing
# Default should work for all interfaces
net.ipv4.conf.default.accept_source_route = 0
# net.ipv4.conf.all.accept_source_route = 0
# net.ipv4.conf.lo.accept_source_route = 0
# net.ipv4.conf.eth0.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1
# Increase maximum amount of memory allocated to shm
# Only uncomment if needed!
# kernel.shmmax = 67108864
# Disable ICMP Redirect Acceptance
# Default should work for all interfaces
net.ipv4.conf.default.accept_redirects = 0
# net.ipv4.conf.all.accept_redirects = 0
# net.ipv4.conf.lo.accept_redirects = 0
# net.ipv4.conf.eth0.accept_redirects = 0
# Enable Log Spoofed Packets, Source Routed Packets, Redirect Packets
# Default should work for all interfaces
net.ipv4.conf.default.log_martians = 1
# net.ipv4.conf.all.log_martians = 1
# net.ipv4.conf.lo.log_martians = 1
# net.ipv4.conf.eth0.log_martians = 1
# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 25
# Decrease the time default value for tcp_keepalive_time connection
net.ipv4.tcp_keepalive_time = 1200
# Turn on the tcp_window_scaling
net.ipv4.tcp_window_scaling = 1
# Turn on the tcp_sack
net.ipv4.tcp_sack = 1
# tcp_fack should be on because of sack
net.ipv4.tcp_fack = 1
# Turn on the tcp_timestamps
net.ipv4.tcp_timestamps = 1
# Enable TCP SYN Cookie Protection
net.ipv4.tcp_syncookies = 1
# Enable ignoring broadcasts request
net.ipv4.icmp_echo_ignore_broadcasts = 1
# Enable bad error message Protection
net.ipv4.icmp_ignore_bogus_error_responses = 1
# Make more local ports available
# net.ipv4.ip_local_port_range = 1024 65000
# Set TCP Re-Ordering value in kernel to 『5′
net.ipv4.tcp_reordering = 5
# Lower syn retry rates
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 3
# Set Max SYN Backlog to 『2048′
net.ipv4.tcp_max_syn_backlog = 2048
# Various Settings
net.core.netdev_max_backlog = 1024
# Increase the maximum number of skb-heads to be cached
net.core.hot_list_length = 256
# Increase the tcp-time-wait buckets pool size
net.ipv4.tcp_max_tw_buckets = 360000
# This will increase the amount of memory available for socket input/output queues
net.core.rmem_default = 65535
net.core.rmem_max = 8388608
net.ipv4.tcp_rmem = 4096 87380 8388608
net.core.wmem_default = 65535
net.core.wmem_max = 8388608
net.ipv4.tcp_wmem = 4096 65535 8388608
net.ipv4.tcp_mem = 8388608 8388608 8388608
net.core.optmem_max = 40960
如果希望屏蔽別人 ping 你的主機，則加入以下代碼：
# Disable ping requests
net.ipv4.icmp_echo_ignore_all = 1
編輯完成後，請執行以下命令使變動立即生效：
/sbin/sysctl -p
/sbin/sysctl -w net.ipv4.route.flush=1
我們常常在 Linux 的 /proc/sys 目錄下,手動設定一些 kernel 的參數或是直接 echo 特定的值給一個 proc下的虛擬檔案,俾利某些檔案之開啟,常見的例如設定開機時自動啟動 IP Forwarding:
echo 「1」 > /proc/sys/net/ipv4/ip_forward
其實,在 Linux 我們還可以用 sysctl command 便可以簡易的去檢視、設定或自動配置 特定的 kernel 設定。我們可以在系統提示符號下輸入「sysctl -a」,摘要如後:abi.defhandler_coff = 117440515
dev.raid.speed_limit_max = 100000
net.ipv4.conf.default.send_redirects = 1
net.ipv4.conf.default.secure_redirects = 1
net.ipv4.conf.default.accept_redirects = 1
net.ipv4.conf.default.mc_forwarding = 0
net.ipv4.neigh.lo.delay_first_probe_time = 5
net.ipv4.neigh.lo.base_reachable_time = 30
net.ipv4.icmp_ratelimit = 100
net.ipv4.inet_peer_gc_mintime = 10
net.ipv4.igmp_max_memberships = 20
net.ipv4.ip_no_pmtu_disc = 0
net.core.no_cong_thresh = 20
net.core.netdev_max_backlog = 300
net.core.rmem_default = 65535
net.core.wmem_max = 65535
vm.kswapd = 512 32 8
vm.overcommit_memory = 0
vm.bdflush = 30 64 64 256 500 3000 60 0 0
vm.freepages = 351 702 1053
kernel.sem = 250 32000 32 128
kernel.panic = 0
kernel.domainname = (none)
kernel.hostname = pc02.shinewave.com.tw
kernel.version = #1 Tue Oct 30 20:11:04 EST 2001
kernel.osrelease = 2.4.9-13
kernel.ostype = Linux
fs.dentry-state = 1611 969 45 0 0 0
fs.file-nr = 1121 73 8192
fs.inode-state = 1333 523 0 0 0 0 0
從上述的語法我們大概可看出 sysctl 的表示法乃把目錄結構的「/」以「.」表示,一層一層的連結下去。當然以echo 特定的值給一個 proc下的虛擬檔案也是可以用 sysctl加以表示,例如:
#sysctl –w net.ipv4.ip_forward =」1」
或是直接在 /etc/sysctl.conf 增刪修改特定檔案的 0,1值亦可:
# Enables packet forwarding
net.ipv4.ip_forward = 1
# Enables source route verification
net.ipv4.conf.default.rp_filter = 1
# Disables the magic-sysrq key
kernel.sysrq = 0
當然如果考慮 reboot 後仍有效, 直接在 /etc/sysctl.conf 增刪修改特定檔案的 0,1值才可使之保留設定(以RedHat 為例,每次開機系統啟動後, init 會執行 /etc/rc.d/rc.sysinit,便會使用 /etc/sysctl.conf 的預設值去執行 sysctl)。
相關參考檔案:
/sbin/sysctl
/etc/sysctl.conf
sysctl 及sysctl.conf manpage
/usr/src/linux-x.y.z/Documentation/sysctl/*
/usr/share/doc/kernel-doc-x.y.z/sysctl/* (RedHat)
