Re: EXPERIMENTAL: mmap-based memory context / allocator

On 15.2.2015 20:56, Heikki Linnakangas wrote:
> On 02/15/2015 08:57 PM, Tomas Vondra wrote:
>> One of the wilder ideas (I mentined beer was involved!) was a memory
>> allocator based on mmap [2], bypassing the libc malloc implementation
>> altogether. mmap() has some nice features (e.g. no issues with returning
>> memory back to the kernel, which may be problem with sbrk). So I hacked
>> a bit and switched the AllocSet implementation to mmap().
>
> glibc's malloc() also uses mmap() for larger allocations. Precisely
> because those allocations can then be handed back to the OS. I don't
> think we'd want to use mmap() for small allocations either. Let's not
> re-invent malloc()..

malloc() does that only for allocations over MAP_THRESHOLD, which is
128kB by default. Vast majority of blocks we allocate are <= 8kB, so
mmap() almost never happens.

At least that's my understanding, I may be wrong of course.

>
> - Heikki
>

--
Tomas Vondra http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services