Linux – How to check what process is blocking local linux server (Slackware)

blockinglinuxslackwareslow-connectionweb-server

I have a Slackware linux server at my work. This is network, database and web server. Our main web application is used as a backend application to administrate our public website (which is frequently updated) and for working with clients (mini ERP) and most of the employees use it as the only application during working hours so when it is not working the whole firm is blocked.

Lately it happens quite often that web server is blocking or really slowly processing requests so it makes easy and short tasks last for tens of minutes and even longer. I use putty to connect to the server. When this happens, I cannot connect to the server through putty and even have difficulties logging into the server machine directly in server room so I don't know how to check what is going on on the server when this happens so I can diagnose and fix the problem. I know that sometimes mysql is overloaded, but I can log into server and see that (I have enabled slow query logging).

My main problem is how to detect what's going on.

Here's server information:

Version: Slackware 11.0.0 (when I log in I get: Linux 2.4.33.3.)
Processor: Intel(R) Xeon(R) CPU 3050 @ 2.13GHz
Memory: 3.5GB (swap 4GB)
Disk space: 1TB (3 partitions)

Thanks.

Best Answer

You should have some kind of monitoring to help debug, but you don't require it.

start a screen and run a vmstat 30

the vmstat will output the CPU, IO and RAM usage each 30 seconds and the the screen will allow you to retrieve the running vmstat later to analyse it. Tune the seconds for each output if 30 is too fast or too slow

If its so slow that you cant even login, i suspect two issues:

heavy swap usage. Kernel managing the memory have priority over everything else, so under memory pressure everything waits for the swap IO. More ram and fine tune the apps might help.
heavy load. If you have many clients, you have probably heavy queries on the sql make the sites lockup for a few seconds, stalling current requests. Yet more and more requests arrive and people start to press reload, increasing the load very quickly.

You can also leave a putty open and a htop running (not top, as it is heavy, htop do more and is lighter). When the load increase, check the htop to see the current machine stats

Related Solutions

Linux – How to run a server on port 80 as a normal user on Linux

Short answer: you can't. Ports below 1024 can be opened only by root. As per comment - well, you can, using CAP_NET_BIND_SERVICE, but that approach, applied to java bin will make any java program to be run with this setting, which is undesirable, if not a security risk.

The long answer: you can redirect connections on port 80 to some other port you can open as normal user.

Run as root:

# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080

As loopback devices (like localhost) do not use the prerouting rules, if you need to use localhost, etc., add this rule as well (thanks @Francesco):

# iptables -t nat -I OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-ports 8080

NOTE: The above solution is not well suited for multi-user systems, as any user can open port 8080 (or any other high port you decide to use), thus intercepting the traffic. (Credits to CesarB).

EDIT: as per comment question - to delete the above rule:

# iptables -t nat --line-numbers -n -L

This will output something like:

Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:8080 redir ports 8088
2    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:80 redir ports 8080

The rule you are interested in is nr. 2, so to delete it:

# iptables -t nat -D PREROUTING 2

Linux – Check if port is open or closed on a Linux server

You can check if a process listens on a TCP or UDP port with netstat -tuplen.

To check whether some ports are accessible from the outside (this is probably what you want) you can use a port scanner like Nmap from another system. Running Nmap on the same host you want to check is quite useless for your purpose.

Best Answer

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Linux – How to run a server on port 80 as a normal user on Linux

Linux – Check if port is open or closed on a Linux server

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