Linux on Coinidea's Blog

[LNMP]Memory Optimization

Wed, 19 Dec 2018 03:01:19 +0000

Problem Description

Recently, my server ran into an issue where it would frequently kill my Tomcat process when memory ran low. Since the overall traffic isn’t very high, LNMP and Tomcat are currently running on the same machine, but this caused Tomcat-related Java applications to become unavailable.

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free -m

Checking the memory usage, I found the operating system had only about 60MB of memory left – almost completely used up. This situation persisted for about a month, and it was a constant source of frustration. I considered adding more memory but thought it was too expensive. When resources are insufficient, there are generally two approaches: the first is to add resources, and the second is to optimize existing resources. For someone on a budget like me, the second approach is usually my first choice.

Initially, I thought Tomcat itself or some other process was consuming too much memory. After running:

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top

I found that php-fpm processes were often the ones consuming the most memory.

To view the top 40 memory-consuming processes:

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ps auxw | head -1; ps auxw | sort -rn -k4 | head -40

I found that php-fpm dominated the top 40 processes, with memory usage ranging from 2% to 7%. The problem was now quite clear – the culprit was php-fpm.

Solution

You can control the number of php-fpm child processes by configuring the pm.max_children property. First, open the php-fpm configuration file:

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vim /etc/php-fpm.d/www.conf

pm.max_children defaults to 50. Each process uses 1%-2.5% of memory, which adds up to consuming most of the memory. Try reducing the value – I set mine to 25 (also referencing other blogs). At the same time, check the following two properties:

pm.max_spare_servers : This value represents the maximum number of idle processes. If idle processes exceed this value, they will be cleaned up.
pm.min_spare_servers : The minimum number of idle processes. If idle processes fall below this value, new child processes are created.

Neither of these values should exceed the pm.max_children value. Typically, pm.max_spare_servers is set to 60%-80% of the pm.max_children value.

Finally, restart php-fpm:

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service php-fpm restart

Check the memory again:

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free -m

I found there was about 600MB of free memory remaining. Even after starting Tomcat, there was still about 300MB to spare.

Reference: https://www.jb51.net/article/129528.htm

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Hope this helps! If you need anything else, feel free to let me know.

A Great Place to Download RPM Packages

Thu, 22 Nov 2018 07:17:03 +0000

In offline environments (specifically those where you can’t use wget or yum), we often need to compile and install Linux-based operating systems ourselves.

At that point, version dependencies become a real headache. Of course, we could use Docker to solve this.

But if your offline environment also restricts uploads and downloads, and the internal system versions are unpredictable, it becomes an even bigger pain. So I found a great place to download Linux RPM packages:

https://pkgs.org/

It solves the problems of operating system compatibility, system version dependencies, and software dependency management.

[CentOS]Upgrading OpenSSH and vsftpd on CentOS

Thu, 23 Nov 2017 08:05:04 +0000

Upgrading OpenSSH to Version 7.4 or Above

Background

Currently, OpenSSH versions below 7.4 on Linux have some critical vulnerabilities, and OpenSSH needs to be upgraded to version 7.4 or above. On a machine without external network access (no yum), this is quite troublesome. Online resources generally recommend backing up SSH and installing Telnet first, so you can still access the machine via Telnet if the upgrade fails.

Common Issues and Solutions

4. Dependencies required for manually compiling OpenSSH

zlib
pam
pam-devel

If you get the error configure: error: PAM headers not found, you can download pam-devel:

http://rpm.pbone.net/index.php3?stat=26&dist=74&size=208492&name=pam-devel-1.1.1-17.el6.x86_64.rpm

5. OpenSSL download

ftp://ftp.openssl.org/source/

6. Installing OpenSSL

References:

Notes

If SecureCRT cannot log in after upgrading OpenSSH, it may be because the SecureCRT version is too old to support certain encryption protocols:

Reference: http://blog.csdn.net/yangg1991/article/details/51755562

Steps to Upgrade OpenSSH

Reference: http://blog.csdn.net/u011080082/article/details/64503534

Upgrading vsftpd from 2.2.2 to 2.3.4

Download the vsftpd-2.3.4.tar.gz package. For 64-bit systems, note:

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sed -i 's/lib\/lib64\/g' vsf_findlibs.sh
sed -i 's/lib\//lib64\//g' vsf_findlibs.sh

Configuring vsftpd

Using active mode:

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vsftpd /etc/vsftpd/vsftpd.conf &

When starting in standard_alone mode, you need to add the following two lines to the top of the configuration file:

Reference: http://wingjang.blog.163.com/blog/static/47913442200811113104509/

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listen=YES
listen_port=21

After installation, only anonymous users are allowed to log in by default, so you need to modify the configuration:

Reference: http://blog.sina.com.cn/s/blog_7e16680c01018ox1.html

Handling TCP_Wrapper Issues

If there are issues, pay attention to removing spaces, or you can remove it entirely:

References:

vsftpd Download

http://pkgs.fedoraproject.org/repo/pkgs/vsftpd/

[Linux] Summary of Linux Commands

Sat, 17 Oct 2015 08:24:07 +0000

User Management

Recently I have been setting up a CDH cluster of about 10 nodes. The cluster uses 10 CentOS 6.7 servers, so I frequently need to use Linux user management commands.

Change password
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passwd
Enter the new password and confirm it. For security, the password should include uppercase and lowercase letters, numbers, and special characters. Linux will also remind you of the password rules.
Switch user
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su hdfs
This switches to the hdfs user.
Alternatively, without switching users, you can execute a command or access a file with a specific user’s permissions:
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su hdfs hadoop fs -mkdir /user/hdfs/test
This creates a test directory at /user/hdfs/ on the cluster using hdfs user permissions.

File Management

Create a directory

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mkdir [folder]

Create a file

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touch/vi [filename]

Timezone Selection

Follow the prompts to select a timezone, then enter the suggested command.
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tzselect

Modify the timezone in the clock file.

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vi /etc/sysconfig/clock
ZONE="Asia/Shanghai"

Delete the existing localtime and create a new one.

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rm /etc/localtime
ln -sf /usr/share/zoneinfo/Asia/Shanghai /etc/localtime

Run Commands at Startup

Edit the rc.local file

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vi /etc/rc.local

Start the NTP Service to Sync with a Time Server on the LAN

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service ntpd start
/usr/sbin/ntpdate 192.168.34.115

[To be continued…]

[Linux] Configuring DNS Addresses on Linux

Wed, 14 Oct 2015 13:28:18 +0000

DNS determines whether a server can reach the internet via domain names. There are two ways to configure DNS server IP addresses on Linux.

Verification method:

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ping www.baidu.com

Temporary method
a) Edit resolv.conf
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vi ./etc/resolv.conf
b) Enter
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nameserver 8.8.8.8
Save and you’re done.

Permanent method
a) Edit ifcfg-eth0

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vi /etc/sysconfig/network-scripts/ifcfg-eth0

b) Enter

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DNS1=8.8.8.8

Save and you’re done.

You can use

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ifconfig

to view the local network interfaces, where you can see the eth0 configuration information.

[Linux] Mounting an NTFS External Hard Drive on CentOS

Thu, 01 Oct 2015 06:26:08 +0000

The server runs CentOS and needs to mount two 2TB external hard drives.

The command is as follows:

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mount -t ntfs /dev/sdn1 /test1
mount： unknown filesystem type 'ntfs'

An error occurred, meaning the system does not recognize “NTFS.” The solution is as follows:

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wget http://tuxera.com/opensource/ntfs-3g_ntfsprogs-2013.1.13.tgz
tar zxvf ntfs-3g_ntfsprogs-2013.1.13.tgz
cd ntfs-3g_ntfsprogs-2013.1.13
./configure
make
make install
mkdir /mnt/window
mount -t ntfs-3g /dev/sdn1 /mnt/window

To unmount:

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umount /dev/sdn1