[linux-mm-cc] Queries about Compcache allocator

[Leonidas .]

On Thu, Oct 22, 2009 at 6:35 AM, John McCabe-Dansted <gmatht at gmail.com> wrote:
> 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
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>


Hi,
Sorry for the late response.

Yes, you are right, Jemalloc is optimal for allocation pattern which
involves frequent
small allocations i.e. 512 bytes. So perhaps, this would be ideal for
scenarios which
which require this kind of allocation pattern i.e. may be for embedded
devices, as
per compcache requirement, this might not be ideal in its current form.

-Leo.