Linux: Nagle's Algorithm and How to Disable it

 Nagle's algorithm is a TCP optimization in the kernel stack that waits to aggregate small chunks of bytes before sending packets on a TCP connection. This approach optimizes the amount of frame overhead spent in sending very small packets over the network. However, when the data is fairly sporadic, this could also lead to an increase in the average delay experienced.

Nagle's algorithm running on a host can be disabled by:

echo 1 > /proc/sys/net/ipv4/tcp_low_latency

Linux: No entries seen in /proc/modules on loading new kernel

Issue: No entries are seen in /proc/modules on loading a new kernel (typically built from scratch¹).
Solution: The solution is pretty simple. This is happening because the kernel has not loaded the module for which you would like to see the symbols. You can double confirm this by running:

$lsmod  > modulelist.txt
$cat modulelist.txt

If you do not see your module here, set the following while rebuilding your kernel:

make LSMOD="modulelist.txt" localmodconfig

Using this, reboot into your new kernel and your module should get loaded.

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1. Quick ref on rebuilding kernel from source↩︎

Linux: Why is the buddy system needed? - To prevent fragmentation

The buddy system is a mechanism for page management in Linux. It is needed to make sure that the free memory does not get fragmented and unusable. For an overview of the buddy system including a simple example of how it works, see this page [2]. From the same page, "In comparison to other simpler techniques such as dynamic allocation, the buddy memory system has little external fragmentation, and allows for compaction of memory with little overhead. The buddy method of freeing memory is fast, with the maximal number of compactions required equal to log2(highest order). Typically the buddy memory allocation system is implemented with the use of a binary tree to represent used or unused split memory blocks. The "buddy" of each block can be found with an exclusive OR of the block's address and the block's size."

An alternative to the buddy system would be to use the memory management unit (MMU) support to rewire or re-arrange blobs of free pages together to construct larger contiguous pages. However, this will not work for DMA systems which bypass the MMU. Also, modifying the virtual address on a continual basis would make the paging process slow.

Debugging on the buddy system can be done by printing the current stats. This is supported under the /proc/buddyinfo file. As described in the guide from centos.org, fragmentation issues can be debugged. A sample output from the same site is as shown below:

cat /proc/buddyinfo