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

[mdf at FreeBSD.org]

2011/10/2 Lev Serebryakov <lev at freebsd.org>:
> 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?

No one has quite answered this question, IMO, so here's my 2 cents.

malloc(9) on smaller sizes (<= PAGE_SIZE) uses uma(9) under the
covers.  There are a set of uma zones for 16, 32, 64, 128, ...
PAGE_SIZE bytes and malloc(9) looks up the malloc size in a small
array to determine which uma zone to allocate from.

So malloc(9) on small sizes doesn't have overhead of bookkeeping, but
it does have overhead of rounding to the next highest malloc uma
bucket.  At $WORK we found, for example, that 48 bytes and 96 bytes
were very common sizes and so I added uma zones there (and few other
odd sies determined by using the malloc statistics option).

>   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?

4k allocations, as has been pointed out, get a single kernel page in
both the virtual space and physical space.  They (like all the large
allocations) use a field in the vm_page for the physical page backing
the virtual address to record info about the allocation.

Any allocation PAGE_SIZE and larger will round up to the next multiple
of pages and allocate whole pages.  IMO the problems here are (1) as
was pointed out, TLB shootdown on free(9), and (2) the current
algorithm for finding space in a kmem_map is a linear search and
doesn't track where there are fragmented chunks, so it's not terribly
efficient when finding larger sies, and the PAGE_SIZE allocations will
not fill in fragmented areas.

Cheers,
matthew