sysctl: Linux System Tweaking

sysctl: Linux System Tweaking

sysctl is a command-lin tool to modify kernel parameters at runtime in Linux.

ref:
http://man7.org/linux/man-pages/man8/sysctl.8.html

Usage

List All Parameters

$ sudo sysctl -a
$ sudo sysctl -a | grep tcp

The parameters available are those listed under /proc/sys/.

$ cat /proc/sys/net/core/somaxconn
1024

Show the Entry of a Specified Parameter

$ sudo sysctl net.core.somaxconn
net.core.somaxconn = 1024

### Show the Value of a Specified Parameter

```console
$ sysctl -n net.core.somaxconn
1024

Change a Specified Parameter

# Elasticsearch
$ sysctl -w vm.max_map_count = 262143

# Redis
$ sysctl -w vm.overcommit_memory = 1

ref:
https://www.elastic.co/guide/en/elasticsearch/reference/current/vm-max-map-count.html
https://redis.io/topics/admin

Persistence

`sysctl -w` only modify parameters at runtime, and they would be set to default values after the system is restarted. You must write those settings in `/etc/sysctl.conf` to persistent them.

# Do less swapping
vm.swappiness = 10
vm.dirty_ratio = 60
vm.dirty_background_ratio = 2

# Prevents SYN DOS attacks. Applies to ipv6 as well, despite name.
net.ipv4.tcp_syncookies = 1

# Prevents ip spoofing.
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.all.rp_filter = 1

# Only groups within this id range can use ping.
net.ipv4.ping_group_range=999 59999

# Redirects can potentially be used to maliciously alter hosts routing tables.
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 1
net.ipv6.conf.all.accept_redirects = 0

# The source routing feature includes some known vulnerabilities.
net.ipv4.conf.all.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0

# See RFC 1337
net.ipv4.tcp_rfc1337 = 1

# Enable IPv6 Privacy Extensions (see RFC4941 and RFC3041)
net.ipv6.conf.default.use_tempaddr = 2
net.ipv6.conf.all.use_tempaddr = 2

# Restarts computer after 120 seconds after kernel panic
kernel.panic = 120

# Users should not be able to create soft or hard links to files which they do not own. This mitigates several privilege escalation vulnerabilities.
fs.protected_hardlinks = 1
fs.protected_symlinks = 1

ref:
https://blog.runcloud.io/how-to-secure-your-linux-server/
https://www.percona.com/blog/2019/02/25/mysql-challenge-100k-connections/
https://www.nginx.com/blog/tuning-nginx/

Activate parameters from the configuration file.

$ sudo sysctl -p

Troubleshooting

OS error code 24: Too many open files

$ sudo vim /etc/sysctl.conf
fs.file-max = 601017

$ sudo sysctl -p

$ sudo vim /etc/security/limits.d/nofile.conf
* soft nofile 65535
* hard nofile 65535
root soft nofile 65535
root hard nofile 65535

$ ulimit -n 65535

OS error code 99: Cannot assign requested address

For MySQL. Because there's no available local network ports left. You might need to set `net.ipv4.tcp_tw_reuse = 1` instead of `net.ipv4.tcp_tw_recycle = 1`.

$ sudo vim /etc/sysctl.conf
net.ipv4.tcp_tw_reuse = 1

$ sudo sysctl -p

ref:
https://www.percona.com/blog/2014/12/08/what-happens-when-your-application-cannot-open-yet-another-connection-to-mysql/
https://stackoverflow.com/questions/6426253/tcp-tw-reuse-vs-tcp-tw-recycle-which-to-use-or-both

Parameters are missing from `sysctl -a` or `/proc/sys`

Sometimes you might find some parameters are not in `sysctl -a` or `/proc/sys`.

You can find them in `/sys`:

$ echo "never" > /sys/kernel/mm/transparent_hugepage/enabled
$ echo "never" > /sys/kernel/mm/transparent_hugepage/defrag

$ cat /sys/kernel/mm/transparent_hugepage/enabled

To persistent them:

$ vim /etc/rc.local
if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
   echo "never" > /sys/kernel/mm/transparent_hugepage/enabled
fi
if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
   echo "never" > /sys/kernel/mm/transparent_hugepage/defrag
fi

$ systemctl enable rc-local

If /etc/rc.local doesn't exist, create one and run chmod 644 /etc/rc.local.

ref:
https://redis.io/topics/admin
https://unix.stackexchange.com/questions/99154/disable-transparent-hugepages

Use Makefile as a task runner for arbitrary projects

Use Makefile as a task runner for arbitrary projects

Use the GNU make, Luke!

Some notes:

  • The first target will be executed by default when we call make without any subcommand.
  • The order of the targets does not matter.
  • Add an @ to suppress output of the command that is executed.

ref:
https://www.gnu.org/software/make/manual/make.html

.PHONY

Let's assume you have install target, which is a very common in Makefiles. If you do not add .PHONY, and a file or a directory named install exists in the same directory as the Makefile, then make install will do nothing.

ref:
https://stackoverflow.com/questions/2145590/what-is-the-purpose-of-phony-in-a-makefile

Automatic Variables

ref:
https://www.gnu.org/software/make/manual/make.html#toc-How-to-Use-Variables
https://www.gnu.org/software/make/manual/make.html#Automatic-Variables

Set Environment Variables From .env

  • export $(grep -v '^#' .env | xargs -0)
  • set -o allexport; source .env; set +o allexport
include .env
export $(shell sed 's/=.*//' .env)

run_web:
    poetry run python -m flask run -h 0.0.0.0 -p 8000 --reload

ref:
https://unix.stackexchange.com/questions/235223/makefile-include-env-file
https://stackoverflow.com/questions/19331497/set-environment-variables-from-file-of-key-pair-values

Also see: https://github.com/Tarrasch/zsh-autoenv

Run Another Target in the Same Makefile

Say that coverage depends on clean.

.PHONY: clean coverage

clean:
     find . -regex "\(.*__pycache__.*\|*.py[co]\)" -delete

coverage: clean
     docker exec -i -t streetvoice_django_1 python -m coverage run manage.py test --failfast
     docker exec -i -t streetvoice_django_1 python -m coverage html -i

ref:
https://stackoverflow.com/questions/13337727/how-do-i-make-a-target-in-a-makefile-invoke-another-target-in-the-makefile

Pass Arguments to make command

.PHONY: something

something:
ifeq ($(var),foo)
    @echo $(var) "bar"
else
    @echo "others"
endif
$ make something var=foo
foo bar

$ make something
others

ref:
https://stackoverflow.com/questions/2214575/passing-arguments-to-make-run

Detect OS

.PHONY: update

update:
ifeq ($(shell uname),Darwin)
    brew update
else
    apt-get update
endif

Check Whether a File or Directory Exists

.PHONY: up install

ifneq ($(wildcard /usr/local/HAL-9000/bin/hal),)
    UP_COMMAND = /usr/local/HAL-9000/bin/hal up
else
    UP_COMMAND = docker-compose up
endif

up:
    $(UP_COMMAND)

install:
    pip install -r requirements_dev.txt

ref:
https://stackoverflow.com/questions/20763629/test-whether-a-directory-exists-inside-a-makefile

Run Targets Parallelly

You could add MAKEFLAGS += --jobs=4 in your Makefile.

MAKEFLAGS += --jobs

.PHONY: task1 task2 task3 task4

task1:
    echo "hello [email protected]"

task2:
    echo "hello [email protected]"

task3:
    echo "hello [email protected]"

task4:
    echo "hello [email protected]"

tasks: task1 task2 task3 task4
$ make tasks

Or you could call make with -j 4 explicitly.

# allow 4 jobs at once
$ make -j 4 tasks

# allow infinite jobs with no arg
$ make -j tasks

ref:
https://stackoverflow.com/questions/10567890/parallel-make-set-j8-as-the-default-option

Simple Examples

Example 1:

include .env
export $(shell sed 's/=.*//' .env)

.PHONY: clean run_web run_worker

clean:
    find . \( -name \*.pyc -o -name \*.pyo -o -name __pycache__ \) -prune -exec rm -rf {} +

run_web:
    poetry run python -m flask run -h 0.0.0.0 -p 8000 --reload

run_worker:
    poetry run watchmedo auto-restart -d . -p '*.py' -R -- celery -A app:celery worker --pid= --without-gossip --prefetch-multiplier 1 -Ofair -l debug --purge -P gevent

Example 2:

MAKEFLAGS += --jobs=4

INDICES = music_group music_album music_recording music_composition

.PHONY: prepare up stop get_indices $(INDICES)

prepare:
    mkdir -p ../Muzeum-Node-Data

up: prepare
    docker-compose up

stop:
    docker-compose stop

ipfs:
    docker-compose exec ipfs sh

$(INDICES):
    mkdir -p ../Muzeum-Node-Data/ipfs/export/soundscape/[email protected]
    docker-compose exec ipfs ipfs get /ipns/ipfs.soundscape.net/[email protected]/index.json -o soundscape/[email protected]/index.json

get_indices: $(INDICES)
Observe system metrics, status, and logs on Linux

Observe system metrics, status, and logs on Linux

Linux commands that DevOps engineers (or SysAdmin) should know.

ref:
https://peteris.rocks/blog/htop/
http://techblog.netflix.com/2015/11/linux-performance-analysis-in-60s.html
http://techblog.netflix.com/2015/08/netflix-at-velocity-2015-linux.html

總覽

$ top

$ sudo apt-get install htop
$ htop

# 每 1 秒輸出一次資訊
$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 1580104 171620 4287208    0    0     0    11    2    2  9  0 90  0  0
 0  0      0 1579832 171620 4287340    0    0     0     0 2871 2414 13  2 85  0  0
 0  0      0 1578688 171620 4287344    0    0     0    40 2311 1700 18  1 82  0  0
 1  0      0 1578640 171620 4287348    0    0     0    48 1302 1020  5  0 95  0  0
...

查 CPU

$ uptime

Load average: 0.03 0.11 0.19
Load average: 一分鐘 五分鐘 十五分鐘內的平均負載
單核心,如果 Load average 是 1 表示負載 100%
多核心的話,因為 Load average 是所有 CPU 數加起來,所以數值可能會大於 1

$ sudo apt-get install sysstat

# 每個 CPU 的使用率
$ mpstat -P ALL 1
Linux 3.13.0-49-generic (titanclusters-xxxxx)  07/14/2015  _x86_64_ (32 CPU)
07:38:49 PM  CPU   %usr  %nice   %sys %iowait   %irq  %soft  %steal  %guest  %gnice  %idle
07:38:50 PM  all  98.47   0.00   0.75    0.00   0.00   0.00    0.00    0.00    0.00   0.78
07:38:50 PM    0  96.04   0.00   2.97    0.00   0.00   0.00    0.00    0.00    0.00   0.99
07:38:50 PM    1  97.00   0.00   1.00    0.00   0.00   0.00    0.00    0.00    0.00   2.00
07:38:50 PM    2  98.00   0.00   1.00    0.00   0.00   0.00    0.00    0.00    0.00   1.00
...

# 每個 process 的 CPU 使用率
$ pidstat 1
Linux 3.13.0-49-generic (titanclusters-xxxxx)  07/14/2015    _x86_64_    (32 CPU)
07:41:02 PM   UID       PID    %usr %system  %guest    %CPU   CPU  Command
07:41:03 PM     0         9    0.00    0.94    0.00    0.94     1  rcuos/0
07:41:03 PM     0      4214    5.66    5.66    0.00   11.32    15  mesos-slave
07:41:03 PM     0      4354    0.94    0.94    0.00    1.89     8  java
07:41:03 PM     0      6521 1596.23    1.89    0.00 1598.11    27  java
...

查 Memory

$ free –m
             total       used       free     shared    buffers     cached
Mem:          7983       6443       1540          0        167       4192
-/+ buffers/cache:       2083       5900
Swap:            0          0          0

查 Disk

$ iostat -xz 1
Linux 3.13.0-49-generic (titanclusters-xxxxx)  07/14/2015  _x86_64_ (32 CPU)
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          73.96    0.00    3.73    0.03    0.06   22.21
Device:   rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda        0.00     0.23    0.21    0.18     4.52     2.08    34.37     0.00    9.98   13.80    5.42   2.44   0.09
xvdb        0.01     0.00    1.02    8.94   127.97   598.53   145.79     0.00    0.43    1.78    0.28   0.25   0.25
xvdc        0.01     0.00    1.02    8.86   127.79   595.94   146.50     0.00    0.45    1.82    0.30   0.27   0.26

查 Disk Usage

# show whole disk
$ df -h

# show every folder under the directory
$ du -h /data

# show the top directory only
$ du -hs /var/lib/influxdb/data
77.4G    /var/lib/influxdb/data

# show largest top 10 files
$ du -hsx * | sort -rh | head -10

ref:
https://www.codecoffee.com/tipsforlinux/articles/22.html
https://www.cyberciti.biz/faq/how-do-i-find-the-largest-filesdirectories-on-a-linuxunixbsd-filesystem/

查 IO

$ sudo apt-get install dstat iotop

# 可以顯示哪些 process 在進行 io 操作
$ dstat --top-io --top-bio

# with –only option to see only processes or threads actually doing I/O
$ sudo iotop --only

ref:
https://www.cyberciti.biz/hardware/linux-iotop-simple-top-like-io-monitor/

查 CPU bound 或 IO bound

$ iostat -c | head -3 ; iostat -c 1 20

ref:
https://serverfault.com/questions/72209/cpu-or-network-i-o-bound
https://askubuntu.com/questions/1540/how-can-i-find-out-if-a-process-is-cpu-memory-or-disk-bound

iotop cannot is not working inside a container.

查 Process

$ ps aux
$ pstree -a

# attach to a process to find out system calls the process calls
# -t -- absolute timestamp
# -T -- print time spent in each syscall
# -s strsize -- limit length of print strings to STRSIZE chars (default 32)
# -f -- follow forks
# -e -- filtering expression: `option=trace,abbrev,verbose,raw,signal,read,write,fault`
# -u username -- run command as username handling setuid and/or setgid
$ strace -t -T -f -s 2048 -p THE_PID

# find out which files that nginx accesses
# you could try to find something related to the error message first:
# write(1, "Ign http://192.168.212.136 trusty Release\n", 62) = 62
# writev(12, [{"HTTP/1.1 500 Internal Server Error"..., 256}, {...}, {...}, {...}, 4]) = 276
$ strace -f -e trace=file service nginx start

# 顯示 PID 3001 的 process 是用什麼指令和參數啟動的
$ tr '\0' '\n' < /proc/3001/cmdline

# only on macOS
$ top -c a -p 1537

ref:
https://mp.weixin.qq.com/s/Sf79W5dqUFx7rUYRrtx88Q
https://blogs.oracle.com/linux/strace-the-sysadmins-microscope-v2
https://zwischenzugs.com/2011/08/29/my-favourite-secret-weapon-strace/

查 Kernel Logs

# 顯示最近的 15 筆 system messages
$ dmesg | tail -fn 15

# 顯示有關 killed process 的 logs
$ dmesg | grep -E -i -B50 'killed process'

ref:
https://stackoverflow.com/questions/726690/what-killed-my-process-and-why

查 Network

$ sar -n TCP,ETCP 1

查 DNS

Resolve a domain name using dig:

$ apt-get install curl dnsutils iputils-ping
# or
$ apk add --update bind-tools

$ dig +short october-api.default.svc.cluster.local
10.32.1.79

$ dig +short redis-broker.default.svc.cluster.local
10.60.32.20
10.60.33.15

$ dig +short redis-broker-0.redis-broker.default.svc.cluster.local
10.60.32.20

ref:
https://blog.longwin.com.tw/2013/03/dig-dns-query-debug-2013/

Resolve a domain name using nslookup:

$ apt-get install dnsutils

$ nslookup redis-broker.default.svc.cluster.local
Server:    10.3.240.10
Address 1: 10.3.240.10 kube-dns.kube-system.svc.cluster.local

Name:      redis-broker.default.svc.cluster.local
Address 1: 10.0.69.46 redis-broker-0.redis-broker.default.svc.cluster.local

Find specific types of DNS records:

$ nslookup -q=TXT codetengu.com
Server:     1.1.1.1
Address:    1.1.1.1#53

Non-authoritative answer:
codetengu.com    text = "zoho-verification=xxx.zmverify.zoho.com"

Authoritative answers can be found from:

nslookup could return multiple A records for a domain which is commonly known as round-robin DNS.

ref:
https://serverfault.com/questions/590277/why-does-nslookup-return-two-or-more-ip-address-for-yahoo-com-or-microsoft-com

查 Nginx

# 顯示各個 status code 的數量
$ cat access.log | cut -d '"' -f3 | cut -d ' ' -f2 | sort | uniq -c | sort -rn

# 顯示哪些 URL 的 404 數量最多
$ awk '($9 ~ /404/)' access.log | awk '{print $7}' | sort | uniq -c | sort -rn

# 顯示 2016/10/01 的 16:00 ~ 18:00 的 log
$ grep "01/Oct/2016:1[6-8]" access.log

# 顯示 2016/10/01 的 09:00 ~ 12:00 的 log
$ egrep "01/Oct/2016:(0[8-9]|1[0-2])" access.log

ref:
http://stackoverflow.com/questions/7575267/extract-data-from-log-file-in-specified-range-of-time
http://superuser.com/questions/848971/unix-command-to-grep-a-time-range

如果 status code 是 502 Bad Gateway
通常表示是 load balancer / nginx 的 upstream server 掛了或連不到
如果是 Kubernetes service 的話
可能是 Service spec.selector 跟 pod 匹配不起來