[linux-mm-cc] Queries about Compcache allocator

[John McCabe-Dansted]

On Thu, Oct 22, 2009 at 7:18 PM, Nitin Gupta <ngupta at vflare.org> wrote:
> I have never gone though jemalloc details but if it satisfies above requirements
> and also gives low fragmentation under these constraints then it can surely be
> used for compcache.

AFAICT, Out-of-the-box jemalloc would be useless. From the excerpt
below, jemalloc is optimized for allocations less than 512 bytes,
while compcache will often want to allocate ~2k. Under compcache,
jemalloc would essentially act as a power of 2 slab allocator. It
wasn't obvious from skimming the paper whether it would be practical
to modify jemalloc to have "quantum-spaced" allocations up to 4k, and
I didn't check the source code.

http://people.freebsd.org/~jasone/jemalloc/bsdcan2006/jemalloc.pdf
page 5
--

Figure 4 shows the size classes for all allocation sizes.
    Category   Subcategory       Size
    Small
                    Tiny
                                             2  B
                                             4  B
                                             8  B
                  Quantum-spaced
                                             16  B
                                             32  B
                                             48  B
                                               ...
                                             480   B
                                             496   B
                                             512   B
                  Sub-page
                                             1  kB
                                             2  kB
    Large
                                             4  kB
                                             8  kB
                                            16  kB
                                                ...
                                        256   kB
                                        512   kB
                                           1  MB
    Huge
                                           2  MB
                                          4  MB
                                          6  MB
                                             ...
Figure 4: Default size classes, assuming runtime defaults, 4 kB pages
and a 16 byte quantum.
  It would be simpler to have no subcategories for small allocations
by doing away with the quantum-
spaced size classes. However, most applications primarily allocate
objects that are smaller than 512
bytes, and quantum spacing of size classes substantially reduces
average internal fragmentation.

--
John C. McCabe-Dansted