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Understanding free and top memory statistics [Update]

Posted on Friday, June 12 2015 at 12:15 | Category: Linux | 1 Comment(s)

Both top and free can be used to gather basic information about memory usage, but each of them reports the statistics in a slightly different way which might not be directly obvious. An example output of free, using the -m switch to report numbers in MiB instead of KiB, is as follows:

Lets ignore the last line (Swap - it simply shows the total swap space and how much from that swap space is allocated and how much is still free) and focus on physical memory:

The first three numbers in the Mem: line are straight forward: the "total" column shows the total physical memory available (most likely, this system has 8 GiB installed and uses a part of it for its graphics device, hence the "total" column shows less than 8 GiB). The "used" column shows the amount of memory which is currently in use, and the "free" column shows the amount which is still available.

Then, there are the "buffers" and "cached" columns - they show how much from the "used" memory is really used for buffers and caches. Buffers and caches is memory which the kernel uses for temporary data - if an application requires more memory, and there is no memory "free" anymore, the kernel can still use this temporary memory and assign it to application processes (probably resulting in lower I/O performance since there is not as much cache memory available now).

Finally, there is the "+/- buffers/cache" line: This might look strange first, but what it does is that it also reports the "used" and "free" memory, without the buffers and caches - as said above, buffer and cache memory is dynamic and can be assigned to an application process, if required. Hence, the "+/- buffers/cache" line actually shows the memory which is used by and available for processes.

The following diagram shows the memory allocation from the sample above:

top returns almost the same information, in a slightly different layout (note that the numbers are somewhat different since some time has elapsed between the execution of the two commands):

The main difference is that it does not directly show the "used" and "free" memory without the buffers - but this can be easily calculated.

Another thing which looks strange is that the amount of "cached" memory is shown in the "Swap" line - however, it has nothing to do with swap, probably it has been put there to use the available screen area as efficient as possible.

Update: procps >= 3.3.10

Starting with procps 3.3.10, the output of free has changed which might cause some confusion. I came across this through a question on StackOverflow: Linux "free -m": Total, used and free memory values don't add up. Essentially, free does not show the "+/- buffers/cache" line anymore, but instead shows an "available" column which is taken from the MemAvailable metric which has been introduced with kernel 3.14. See https://www.kernel.org/doc/Documentation/filesystems/proc.txt for a complete description:

MemAvailable: An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone. The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

The new format of the free output looks like this:

The main difference is that the "buff/cache" values are not part of "Used" anymore, but counted separately. Hence, the total memory is calculated as "used + buff/cache + free":

Since the "available" value is an estimation which considers some system specific factors, it can not directly be calculated from the other values which are shown by free.


Windows Clipboard Inspector

Posted on Tuesday, June 09 2015 at 12:38 | Category: News | 0 Comment(s)

When working on copy&paste features, it is useful to be able to inspect the current contents of the Clipboard. Unfortunately, at least Windows 7 does not include a clipboard viewer which shows the current clipboard data in all available formats, but Free Clipboard Viewer seems to do the trick. It automatically updates whenever new content is copied into the clipboard and updates the list of available formats accordingly on the left side, while the content itself is displayed on the right side. The following screen shot shows how the tool displays a part of its own web page after some content has been marked and copied into the clipboard with CTRL-C:


This tutorial shows how to expose a plain Java Bean as a Data Control, and how to create a user interface from this Data Control. The sample use case is to show the current time depending on a timezone which can be selected from a drop down list.


In JDeveloper, the integrated weblogic server creates its runtime domain in a directory like $HOME/.jdeveloper/system12.1.3.0.41.140521.1008/DefaultDomain or $HOME/.jdeveloper/system/11.1.1.7.40.65.57.5/DefaultDomain.

While $HOME and the fixed DefaultDomain are well known, it is not so easy to determine the actual name of the system directory - one could assume that there is only one and use system* as the name, but there could be more than one, especially if more than one JDeveloper version is installed at the same time and each of them had been creating its own WLS domain. What we actually need is to get the JDeveloper version since that is what is used in the system directory name.

Assumed that $JDEV_HOME is the installation directory of the JDeveloper installation for which we would like to get its domain home directory (like /opt/Oracle/Middleware/Oracle_Home/jdeveloper), then the file $JDEV_HOME/jdev/bin/version.properties contains the full version number of JDeveloper, which is identical to the version number used in the system directory name:

$ grep VER_FULL $JDEV_HOME/jdev/bin/version.properties
VER_FULL=12.1.3.0.41.140521.1008

To extract the version number, we can use a simple sed script like

$ sed -n 's/^VER_FULL=\([0-9.]*\).*$/\1/p' $JDEV_HOME/jdev/bin/version.properties
12.1.3.0.41.140521.1008

(we need to make sure not to include the terminating line feed, hence we only collect digits and dots after the =).

Finally we can now generate the complete directoy path to the domain directory:

$ JDEV_VER=`sed -n 's/^VER_FULL=\([0-9.]*\).*$/\1/p' $JDEV_HOME/jdev/bin/version.properties`
$ DOMAIN_HOME="${HOME}/.jdeveloper/system${JDEV_VER}/DefaultDomain"
$ ls $DOMAIN_HOME/servers
AdminServerTag  DefaultServer

This can be useful for any kind of automation tasks.


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