How to calculate fragmentation?

c memory-management embedded fragmentation
Bigbohne picture Bigbohne · Jan 3, 2011 · Viewed 12.6k times · Source

Imagine you have some memory containing a bunch of bytes:

++++ ++-- ---+ +++-
-++- ++++ ++++ ----
---- ++++ +

Let us say + means allocated and - means free.

I'm searching for the formula of how to calculate the percentage of fragmentation.

Background

I'm implementing a tiny dynamic memory management for an embedded device with static memory. My goal is to have something I can use for storing small amounts of data. Mostly incoming packets over a wireless connection, at about 128 Bytes each.

Answer

psmears picture psmears · Jan 3, 2011

As R. says, it depends exactly what you mean by "percentage of fragmentation" - but one simple formula you could use would be:

(free - freemax)
----------------   x 100%    (or 100% for free=0)
    free

where

free     = total number of bytes free
freemax  = size of largest free block

That way, if all memory is in one big block, the fragmentation is 0%, and if memory is all carved up into hundreds of tiny blocks, it will be close to 100%.

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