CPULeash: A tool for capping per-process and process group CPU utilisation from userspace

It’s now mid October, there are 0 posts in 2017. Therefore I think it’s time to loosen up posting constraints and try make a bit more casual posts. For a start I will dig up one of the unfinished works. I will start with a quick overview of an old piece of tool (unmaintained) I wrote quite a few years ago. The tool caps the maximum CPU utilisation of a process from userspace without requiring root access. One instance of the process can cap multiple processes individually, in a group or it can also cap an entire process sub-tree. I intended to post about it in great detail, but this time I will keep it brief, and dig deeper in the next opportunity. Let’s get into it.

Motivation (from agony)

A few years ago, I was used to run different CPU intensive experiments on my old laptop, which would sometime run weeks. The CPU temperature used to go up to 95 Deg C. Sometimes it went upto 99 Deg C and then it used to throttle down the cores to get the temperature down. This is definitely not a healthy temperature, therefore I feared of hardware damage. The laptop had a metal case, which became so hot that it left red marks on my thighs. One time an entire chocolate bar melted into a pool of liquid chocolate (someone left it near the exhaust fan). Therefore it was time that I figure something out, such that I can run the experiments, but keep the laptop cool (relatively).

The obvious idea was to cap the CPU usage for the processes. An easy solution is to install a virtual machine and run the code inside the virtual environment. Note that, in my case nice does not work, as it does not help control how much of the CPU a processes utilises when it acquires it. I could have used cgroups, but I didn’t want to (I don’t know why). At that point of time I was feeling the need for Solaris Zones.

Essentially what I wanted is something portable in any *nix system, works completely from userspace and also does not need root privilege. Therefore I made a small tool which does in. It was written in C and POSIX compliant libraries.

Also note that, there is a similiar kind of tool called cpulimit, which I didn’t know existed when I wrote this. This tool works well too.

Given this situation, it was time to write something in-house … drum rollCPULeash.

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Hacking (Reverse engineering): Gaining access with wrong password

In this post, I will quickly show how you can use a debugger to hack poorly written or packaged code. More specifically, how to enter a wrong password but still get access. Before proceeding, I should be clear, that this is just a demonstration, and the programs out there in production (these days) will definitely not vulnerable to this method (if it is, then it is a shitty program). This is to demonstrate how you can change the execution path as you wish.

First I will show a simple code which prompts for a password to be set, then encrypts it using MD5 sum hash and salt and stores the hash in a file. Then it asks for the same password, reads the hash from the stored file and compares if the two entered passwords are same or not. Then I will show how to reverse engineer the executable file and enter the wrong password, but still make it think that we the correct password was entered. Continue reading “Hacking (Reverse engineering): Gaining access with wrong password”

Using WordPress.com stats API

gimp_logoLong ago I posted about the WordPress.com stats API. WordPress.com Stats API. WordPress.com stores your blog’s raw visit stats which is available via your API key and the blog URL. Basically these stats are used to show the stats you see in your dashboard.

wordpress_default_dashboard_stats

The older wordpress.com dashboard did not have much visualisations of the stats, but the new dashboard has introduced some more ways to see how your blog is doing. When I posted WordPress.com Stats API, there was absolutely no trace or documentation about the wordpress.com API. It seemed that even the guys in wordpress.com was not well aware about if such an API existed :D . (Check the post to know why).

At the previous post I just provided some guides on how to fetch the data. Recently I planned to write a quick implementation to fetch the stats from wordpress.com and therefore I am going to share the script. This is a quick and dirty implementation to demonstrate the stuff. I have also made a few processing on the incoming data and made a few plots. Most of which might not (will not) make much sense (especially the box plots), but it’s just for the baseline. Let’s proceed with the code. Continue reading “Using WordPress.com stats API”

A wide character trie implementation

It’s been a long time since I posted here. Therefore I decided to dig out some quick and straightforward stuffs from my disk which I previously decided should’t be in this blog.

I have posted multiple trie and dictionary search based programs in C, C++ and Perl before Jumble Work Solver and Jumble Work Solver Again. This time (again!) it’s about a trie. Although this time there was a specific requirement from a group who needed to implement a trie based word distance counting for bangla language, therefore Unicode support. This was supposed to be modified more and plugged into a spelling correction for scanned OCR text in the Bengali language.

Initially I suggested that a ternary tree would be more appropriate as the memory cost for standard trie (not compressed) would be huge. Although, finally the decision was to go with plain and simple trie. I know it is mad, but this is what it is :D.

Also, before going into the implementation, I should note that there is an efficient implementation of Radix Tree present in libcprops library. Though which we won’t be using for this one. I would recommend you people to have a look into this library if you already haven’t seen it yet.

Let’s say, we have a word “hello” and a node with an array of pointers of length 26 representing each character of the English language, each of which indicates that if the ith character follows the character represented by this node. Therefore for this example “hello”, the head node’s array of pointers will have the location 7 pointing to another node, which will have the 4th location of the pointer pointing to another node, whose pointer array’s 11 th location will point to another node and so on. Note here the indexing starts from zero. When the word ends, then the next pointer can be pointed to a special terminal marker node, which can be common to all, and the node is marked as a terminal node. This is essential as a valid word can be a substring of another valid word, in which case the shorter would need to be decided. When another new word like “help” comes in, we will follow the same path upto “hel” created by the word “hello”, and as we find there are no pointer for “p” pointing to any node, a new node will be created as explained before. Continue reading “A wide character trie implementation”

A generic Fisher-Yates Shuffle

It’s been a long time I have done any activity in this blog. I was going through some old stuffs and thought to post something. Today I will post a generic implementation for Fisher-Yates Shuffle.

Although you can get the Fisher-Yates algorithm from wiki, still I am briefly explaining it.

Let us assume that there is a array arr of length n. We need to find a uniformly random permutation of the elements of the array. One of the variations of the algorithm is as follows.

arr is an array of length n, where indexing starts from 0
i=n-1
while i>0
{
  r = generate a random number between 0 and i (both inclusive)
  swap the array elements arr[r] and arr[i]
  i = i - 1
}
arr is now uniformly permuted

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Respect the ~ files

It has been a long time since a post in this blog. I thought to break this silence with a short post.

Several text editors in Linux environment like KWrite and GEdit that will store automatic temporary files ending with a ~ (tilde) character. Generally it is a default setting in most of the major distributions. Initially this feature annoyed me, as now you have twice the number of files in your directory, and often used rm *~ to clear out the files. Although it was annoying at the beginning, there are several cases it saved me. Continue reading “Respect the ~ files”

Generate the process tree of a Linux system

This is a quick post on how to generate a process tree Linux (and *nix) operating systems.

The idea is the same, as in the previous posts: Finding overall and per core CPU utilization and Find process IDs of a running process by name. Read the information present in the /proc/ directory. To get which processes are running we can read the directories with numbers as their names in the /proc/ directory. To generate a process tree we need to establish a process child relationship within the running processes. Each process has a parent (the first generated process is an exception), and it is stored in the process table entry of that process. We need to fetch the parent process id for each running process inorder to establish the tree. Here’s the plan. Continue reading “Generate the process tree of a Linux system”

Find process IDs of a running process by name

In this post I will talk about a procedure to find the process IDs of a running process by name, which can then be used to send signals or do other stuffs. For example if you have multiple instances of bash opened, this should be able to get you the list of process IDs (PIDs) of the bash instances.

Firstly, a shell utility is already available called pidof which is a part of the sysvinit-tools package. There are a whole bunch of tools in this package which lets you query PID based on different requirements, send signals to set of processes, etc. Just check out the stuff.

I will only mention the outline of how this is done and post the sourcecodes to do it. After that this can be extended to have many features just like the tools of sysvinit-tools package or more.
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Finding overall and per core CPU utilization

Today I will post about how to monitor CPU usage by processor in Linux. As you might have expected this will simply access the CPU time information from /proc pseudo-filesystem and report the results.

The proc filesystem or the Procfs is a special filesystem which gives you a view into the kernel data. No files in procfs exists actually in the disk. There is no disk inodes and thus storage related to the files. Instead of going into procfs I will redirect you to wikipedia: http://en.wikipedia.org/wiki/Procfs. Let’s get in.
Continue reading “Finding overall and per core CPU utilization”

Find pairs of numbers in an array with difference ‘k’

The problem statement is, an long array is given with n elements, we need to find all the pairs of numbers in this long array which have a constant difference k. I will post three methods. The first method is the brute force method and has a runtime complexity O (n2), the next method has runtime complexity O (n*log (n)), and the last one will have the runtime complexity O (n).

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