Memory allocation in kernel -- what to use in which situation? What is the best for page-sized allocations?

Kostik Belousov kostikbel at
Sun Oct 2 13:06:53 UTC 2011

On Sun, Oct 02, 2011 at 04:21:09PM +0400, Lev Serebryakov wrote:
> Hello, Freebsd-hackers.
>   Here are several memory-allocation mechanisms in the kernel. The two
> I'm aware of is MALLOC_DEFINE()/malloc()/free() and uma_* (zone(9)).
>   As far as I understand, malloc() is general-purpose, but it has
> fixed "transaction cost" (in term of memory consumption) for each
> block allocated, and is not very suitable for allocation of many small
> blocks, as lots of memory will be wasted for bookkeeping.
>   zone(9) allocator, on other hand, have very low cost of each
> allocated block, but could allocate only pre-configured fixed-size
> blocks, and ideal for allocation tons of small objects (and provide
> API for reusing them, too!).
>   Am I right?
>    But what if I need to allocate a lot (say, 16K-32K) of page-sized
> blocks? Not in one chunk, for sure, but in lifetime of my kernel
> module. Which allocator should I use? It seems, the best one will be
> very low-level only-page-sized allocator. Is here any in kernel?

Very short answer is that you could use kmem_alloc() or kmem_malloc().
kmem_alloc() sleeps, while kmem_malloc() may be directed to not sleep.
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