Visualizing Linux Memory Compaction

Tags: linux memory

Intro

On Linux, memory is allocated by the buddy allocator. It allocates memory in chunks of sizes ranging from 4 kiB (a single page) to 4 MiB

We can inspect the available memory by chunk size with the pseudo-file /proc/buddyinfo. The first two lines indicate the size of each chunk, while the remaining lines indicate the number of available chunks:

$ cat /proc/buddyinfo 
power of 2               2^0    2^1    2^3    2^4    2^5    2^6    2^7    2^8    2^9   2^10   2^11
size (kiB)                 4      8     16     32     64    128    256    512   1024   2048   4096

Node 0, zone      DMA      0      0      0      0      0      0      0      0      0      1      3 
Node 0, zone    DMA32   6584   4336   2566   1463    770    435    231    125     83    168    407 
Node 0, zone   Normal  61346  38894  16324   4993   1241    353    150     80     47      3    112

When a process claims memory, it is best if the buddy allocator can find a large enough chunks that fits the request.

Over time, the memory may become fragmented, and memory allocation will be less efficient: more small chunks will be available, at the expense of bigger chunks.

It is possible to defragment memory. Dropping caches first will improve compaction efficiency.

This is done by writing to two pseudo files. Here "3" will drop all caches.

# echo 1 > /proc/sys/vm/compact_memory
# echo 3 > /proc/sys/vm/drop_caches

Demo

VM specs

Server is a virtual machine running in VirtualBox with 8192 "MB" of memory.

$ free -m
               total        used        free      shared  buff/cache   available
Mem:            7484        3284        4096          12         379        4200
Swap:           1999          39        1960

$ sudo cat /proc/iomem | grep RAM
00001000-0009fbff : System RAM
00100000-dffeffff : System RAM
100000000-21fffffff : System RAM

Visual illustration

The following images were generated with the tool processes2png from my Github.

  • Each image shows the physical memory of the server
  • Black pixels on the right side are outside of RAM due to aspect ratio
  • Each pixel represents 1 page (1 kiB)
  • Cache is not visible. Kernel memory is not visible. Shared memory is visible if at least one process has a handle to it
  • Bright pixels are pixels referenced by a process and dark pixels are not referenced by any process.
  • This is a Hilbert curve, it preserves locality; meaning that close addresses are close on the picture.

Initial state

After compacting memory

After dropping caches

After compacting memory again

Same as a gif

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Sources & References

  1. github.com/tatref- linux-mem
  2. docs.kernel.org - Documentation for /proc/sys/vm/
  3. Wikipedia - Hilbert curve
  4. grimoire.carcano.ch - Memory Management – The Buddy Allocator

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