As I understand, the size of the virtual memory that a process uses is listed as &quot;total-vm&quot;. Part of it is really mapped into the RAM itself (allocated and used). This is &quot;RSS&quot;.

Part of the RSS is allocated in real memory blocks (other than mapped into a file or device). This is anonymous memory (&quot;anon-rss&quot;) and there is also RSS memory blocks that are mapped into devices and files (&quot;file-rss&quot;). 

So, if you open a huge file in vim, the file-rss would be high, on the other side, if you malloc() a lot of memory and really use it, your anon-rss would be high also. 

On the other side, if you allocate a lot of space (with malloc()), but nevers use it, the total-vm would be higher, but no real memory would be used (due to the memory overcommit), so, the rss values would be low.

Using [this solution][1], I tried to use COALESCE as part of a MySQL query that outputs to a csv file using SELECT As to name the column names when exporting the data.

    SELECT FirstName AS First_Name
         , LastName AS Last_Name
         , ContactPhoneAreaCode1
         , ContactPhoneNumber1
         , COALESCE(ContactPhoneAreaCode1, ContactPhoneNumber1) AS Contact_Phone 
      FROM TABLE1

I wanted 3 columns: First_Name, Last_Name and Contact_Phone

I am getting 5 columns: First_Name, Last_Name, ContactPhoneAreaCode1, ContactPhoneNumber1 and Contact_Phone

How do I hide the merging of ContactPhoneAreaCode1, ContactPhoneNumber1 into a single column for Contact_Phone from within the query?




  [1]: https://stackoverflow.com/questions/16062825/combine-two-columns-in-one-column

MySQL SELECT AS combine two columns into one

I have this problem where I have set an image to display another image when the mouse hovers over, however the first image still appears and the new one doesn&#39;t change height and width and overlaps the other one. I&#39;m still pretty new to HTML/CSS so I may have missed something simple. Here is the code:


    &lt;img src=&quot;LibraryTransparent.png&quot; id=&quot;Library&quot;&gt;

&lt;!-- language: lang-css --&gt;

    #Library {
		height: 70px;
		width: 120px;
	}
		
	#Library:hover {
		background-image: url(&#39;LibraryHoverTrans.png&#39;);
		height: 70px;
		width: 120px;
	}

Changing image on hover with CSS/HTML

Recently, tomcat process on my Linux machine was killed abruptly. After investigation I found below error message in  /var/log/messages file:

&lt;pre&gt;
kernel: [1799319.246494] Out of memory: Kill process 28536 (java) score 673 or sacrifice childSep 
kernel: [1799319.246506] Killed process 28536 (java) total-vm:1271568kB, anon-rss:426528kB, file-rss:0kB
&lt;/pre&gt;

Now, can someone please tell me that what all is included in `total-vm` and how is `anon-rss` different from `rss`?

what does anon-rss and total-vm mean

Recently, tomcat process on my Linux machine was killed abruptly. After investigation I found below error message in /var/log/messages file:
<pre>kernel: [1799319.246494] Out of memory: Kill process 28536 (java) score 673 or sacrifice childSep
kernel: [1799319.246506] Killed process 28536 (java) total-vm:1271568kB, anon-rss:426528kB, file-rss:0kB
</pre>
Now, can someone please tell me that what all is included in <code>total-vm</code> and how is <code>anon-rss</code> different from <code>rss</code>?

I have Nginx installed on a Docker container, and am trying to run it like this:

    docker run -i -t -p 80:80 mydockerimage /usr/sbin/nginx

The problem is that the way Nginx works, is that the initial process immediately spawns a master Nginx process and some workers, and then quits. Since Docker is only watching the PID of the original command, the container then halts.

How do I prevent the container from halting? I need to be able to tell it to bind to the first child process, or stop Nginx&#39;s initial process from exiting.

How to run Nginx within a Docker container without halting?

I want to extract the value pair from a key-value pair syntax but I can not.   
Example I tried:  

    echo employee_id=1234 | sed &#39;s/employee_id=\([0-9]+\)/\1/g&#39;

But this gives `employee_id=1234` and not `1234` which is actually the capture group.  

What am I doing wrong here? I also tried:

    echo employee_id=1234| egrep -o employee_id=([0-9]+)

but no success.

Can not extract the capture group with either sed or grep

In crontab does the Day of the Week field run from `0 - 6` or `1 -7`?

I am seeing conflicting information on this.  wikipedia states `0-6` and other sites I have seen are `1-7`.

Also what would be the implication or either using `0` or `7` incorrectly?  i.e. would the cron still run?

Crontab Day of the Week syntax

I have a **text file containing a specific date and time.** 
I want to be able to **run a script at the time specified in that file.**
How would you achieve that?
Create another script that runs in background (sort of a deamon) and checks every second if the current time is matching the time in the file?
Is there another way?
The machine is a **linux** server , Debian wheezy.
Thanks in advance

How to run a script at a certain time on Linux?

I&#39;m running an instance of Debian on Amazon EC2 with Node.js installed. If I run the code below:

    http = require(&#39;http&#39;);
    
    http.createServer(function (request, response){
      response.writeHead(200, {&#39;Content-Type&#39;:&#39;text/plain&#39;});
      response.end(&#39;Hello World\n&#39;);
    }).listen(80);
    console.log(&quot;Running server at port 80&quot;);

I get the output below which tells me there&#39;s another process listening at port 80:

    Running server at port 80
    
    events.js:72
            throw er; // Unhandled &#39;error&#39; event
                  ^
    Error: listen EACCES
        at errnoException (net.js:901:11)
        at Server._listen2 (net.js:1020:19)
        at listen (net.js:1061:10)
        at Server.listen (net.js:1127:5)
        at Object.&lt;anonymous&gt; (/home/admin/nodetests/nodetest.js:6:4)
        at Module._compile (module.js:456:26)
        at Object.Module._extensions..js (module.js:474:10)
        at Module.load (module.js:356:32)
        at Function.Module._load (module.js:312:12)
        at Function.Module.runMain (module.js:497:10)

Now when I check to see if there&#39;s a process (as root in case anything is hidden) listening on port 80 using: 

    netstat -tupln

I get the below output, which tells me theres nothing listening at port 80: 

    Active Internet connections (only servers)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1667/sshd       
    tcp6       0      0 :::22                   :::*                    LISTEN      1667/sshd

I should note that the debian has port 80 open as an inbound rule if that makes a difference. 

My question is: What am I doing wrong? How come I can&#39;t identify the process listening to port 80? Why is it blocked in Debian? What steps should I take to get the code running correctly?   

Node.js app can&#39;t run on port 80 even though there&#39;s no other process blocking the port

I have not used Redis yet, but I heard about it and plan to try it as cache storing.

I heard Redis using memory as cache store database, so what&#39;s the difference if I use a variable as an object or dictionary datatype to store data? like:

    var cache = {
        key: {
        
        },
        key: {
        
        }
        ...
    }

What&#39;s the advantage Redis have?

Redis cache vs using memory directly

How can I monitor the memory usage of Node.js?

How to monitor the memory usage of Node.js?

I understand that each thread has its own `stack`. Primitive types and references are kept on the stack, and that no object is kept on the stack. 


My questions are:

 - How much can a stack grow? (like with the paramters -`Xms` and -`Xmx`)
 - Can we limit its growth? 
 - Does the stack have a default minimum value and maximum value?
 - How does garbage collection work on the stack?


What is the default stack size, can it grow, how does it work with garbage collection?

I tested the speed of `memcpy()` noticing the speed drops dramatically at i*4KB. The result is as follow: the Y-axis is the speed(MB/second) and the X-axis is the size of buffer for `memcpy()`, increasing from 1KB to 2MB. Subfigure 2 and Subfigure 3 detail the part of 1KB-150KB and 1KB-32KB.

Environment：

CPU : Intel(R) Xeon(R) CPU E5620  @ 2.40GHz 

OS :  2.6.35-22-generic #33-Ubuntu

GCC compiler flags : -O3 -msse4 -DINTEL_SSE4 -Wall -std=c99
 
![Graphs of memcpy speed showing troughs every 4k][1]

I guess it must be related to caches, but I can&#39;t find a reason from the following cache-unfriendly cases:

 * https://stackoverflow.com/questions/12264970/why-is-my-program-slow-when-looping-over-exactly-8192-elements

 * https://stackoverflow.com/questions/11413855/why-is-transposing-a-matrix-of-512x512-much-slower-than-transposing-a-matrix-of

Since the performance degradation of these two cases are caused by unfriendly loops which read scattered bytes into the cache, wasting the rest of the space of a cache line.

Here is my code:

    void memcpy_speed(unsigned long buf_size, unsigned long iters){
        struct timeval start,  end;
        unsigned char * pbuff_1;
        unsigned char * pbuff_2;

        pbuff_1 = malloc(buf_size);
        pbuff_2 = malloc(buf_size);

        gettimeofday(&amp;start, NULL);
        for(int i = 0; i &lt; iters; ++i){
            memcpy(pbuff_2, pbuff_1, buf_size);
        }   
        gettimeofday(&amp;end, NULL);
        printf(&quot;%5.3f\n&quot;, ((buf_size*iters)/(1.024*1.024))/((end.tv_sec - \
        start.tv_sec)*1000*1000+(end.tv_usec - start.tv_usec)));
        free(pbuff_1);
        free(pbuff_2);
    }

UPDATE
=====
Considering suggestions from @usr, @ChrisW and @Leeor, I redid the test more precisely and the graph below shows the results. The buffer size is from 26KB to 38KB, and I tested it every other 64B(26KB, 26KB+64B, 26KB+128B, ......, 38KB). Each test loops 100,000 times in about 0.15 second. The interesting thing is the drop not only occurs exactly in 4KB boundary, but also comes out in 4*i+2 KB, with a much less falling amplitude.


![More graphs showing performance drops][2]


PS
==
@Leeor offered a way to fill the drop, adding a 2KB dummy buffer between `pbuff_1` and `pbuff_2`. It works, but I am not sure about Leeor&#39;s explanation.


![enter image description here][3] 


  [1]: http://i.stack.imgur.com/TJ4Xx.png
  [2]: http://i.stack.imgur.com/qto0R.png
  [3]: http://i.stack.imgur.com/NL3uy.png

Why does the speed of memcpy() drop dramatically every 4KB?

Sometimes data at memory address 0x0 is quite valuable -- take x86 real mode IVT as a more known example: it starts at 0x0 and contains pointers to interrupt handlers: a dword at 0x00 is a pointer to division by zero error handler.

However, C11 language standard prohibits dereferencing null pointers [\[WG14 N1570][1] 6.5.3.2], which are defined as pointers initialized with 0 or pointers initialized with a null pointer [\[WG14 N1570][1] 6.3.2.3], effectively banning the very first byte.

How do people actually use 0x0 when it&#39;s needed?


  [1]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

Dereferencing a pointer to 0 in C

With Android 4.4 KitKat, Google is upgrading linux kernel to version 3.8 which is the required kernel version for Docker.

I don&#39;t know the AUFS part but will there be a way to run docker containers on android with this update?

Running docker on Android

I did `sudo cat /proc/1/maps -vv`

I am attempting to make sense of the output.I can see a lot of shared libraries being mapped to the memory mapping segment as expected.

    7f3c00137000-7f3c00179000 r-xp 00000000 08:01 21233923                   /lib/x86_64-linux-gnu/libdbus-1.so.3.5.8
    7f3c00179000-7f3c00379000 ---p 00042000 08:01 21233923                   /lib/x86_64-linux-gnu/libdbus-1.so.3.5.8
    7f3c00379000-7f3c0037a000 r--p 00042000 08:01 21233923                   /lib/x86_64-linux-gnu/libdbus-1.so.3.5.8
    7f3c0037a000-7f3c0037b000 rw-p 00043000 08:01 21233923                   /lib/x86_64-linux-gnu/libdbus-1.so.3.5.8
    7f3c0037b000-7f3c00383000 r-xp 00000000 08:01 21237216                   /lib/x86_64-linux-gnu/libnih-dbus.so.1.0.0
    7f3c00383000-7f3c00583000 ---p 00008000 08:01 21237216                   /lib/x86_64-linux-gnu/libnih-dbus.so.1.0.0
    7f3c00583000-7f3c00584000 r--p 00008000 08:01 21237216                   /lib/x86_64-linux-gnu/libnih-dbus.so.1.0.0
    7f3c00584000-7f3c00585000 rw-p 00009000 08:01 21237216                   /lib/x86_64-linux-gnu/libnih-dbus.so.1.0.0
    7f3c00585000-7f3c0059b000 r-xp 00000000 08:01 21237220                   /lib/x86_64-linux-gnu/libnih.so.1.0.0
    7f3c0059b000-7f3c0079b000 ---p 00016000 08:01 21237220                   /lib/x86_64-linux-gnu/libnih.so.1.0.0
    7f3c0079b000-7f3c0079c000 r--p 00016000 08:01 21237220                   /lib/x86_64-linux-gnu/libnih.so.1.0.0

Towards the end there is something like

    7f3c0165b000-7f3c0177e000 rw-p 00000000 00:00 0                          [heap]
    7fff97863000-7fff97884000 rw-p 00000000 00:00 0                          [stack]
    7fff97945000-7fff97946000 r-xp 00000000 00:00 0                          [vdso]
    ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

What does `vdso` and `vsyscall` mean? is vsyscall the kernel portion of the memory? It would be great if anyone can throw some light on the issue.



What are vdso and vsyscall?

While debugging `C++` code in Qt creator I get the following error

    ptrace: Operation not permitted.
    
    Could not attach to the process. Make sure no other debugger traces this process.
    Check the settings of
    /proc/sys/kernel/yama/ptrace_scope
    For more details, see /etc/sysctl.d/10-ptrace.conf

Here a temporary solution is found: [Receiving error while trying to debug in QtProject][1]

 - temporary solution (won&#39;t survive a reboot):

     `echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope`


But it is difficult to run the same code in terminal every time when I start my PC to use Qt. 

    What is the permanent solution for this?

  [1]: https://askubuntu.com/questions/244336/receiving-error-while-trying-to-debug-in-qtproject


Qt Creator, ptrace: Operation not permitted. What is the permanent solution?

What is the difference between them?

I know that u-boot needs a kernel in uImage format.

The system I use first boots from stage 1 loader and then it calls u-boot. I want to discard u-boot and directly boot from stage 1 loader. Which type of kernel image do I have to use? 

Image vs zImage vs uImage

I need to write an SPI Linux character device driver for omap4 from scratch.
I know some basics of writing device drivers. But, I don&#39;t know how to start writing platform specific device driver from scratch.

I&#39;ve written some basic char drivers, and I thought writing SPI device driver would be similar to it. Char drivers have a structure `file_operations` which contains the functions implemented in the driver.

    struct file_operations Fops = {
    	.read = device_read,
    	.write = device_write,
    	.ioctl = device_ioctl,
    	.open = device_open,
    	.release = device_release,	/* a.k.a. close */
    };

Now, I am going through [spi-omap2-mcspi.c][1] code as a reference to get an idea to start developing SPI driver from scratch.

But, I don&#39;t see functions such as open, read, write etc.
Don&#39;t know from where the program starts.


  [1]: http://lxr.free-electrons.com/source/drivers/spi/spi-omap2-mcspi.c

How to write a simple Linux device driver?

Getting the following output from running this:

    ps aux | grep Python

Output:


    user_name  84487   0.0  0.0        0      0   ??  Z    12:15PM   0:00.00 (Python)
    user_name  84535   0.0  0.0        0      0   ??  Z    12:16PM   0:00.00 (Python)

I want to terminate all Python processes currently running on a machine....

How to kill all processes with the same name using OS X Terminal

The MySQL database hangs, due to some queries.

How can I find the processes and kill them?

Content Type	Original Author	Original Content on Stackoverflow
Question	UW20989	View Question on Stackoverflow
Solution 1 - Linux	Breno Leitão	View Answer on Stackoverflow

what does anon-rss and total-vm mean

Linux Problem Overview

Linux Solutions

Solution 1 - Linux

Changing image on hover with CSS/HTML

MySQL SELECT AS combine two columns into one

Attributions