maximally misaligned

[Jason Evans]

On Dec 26, 2012, at 3:50 PM, Bradley C. Kuszmaul <kuszmaul at gmail.com> wrote:
> It would be useful if there were a way to configure jemalloc to give
> the worst acceptable alignment for every returned object.
> 
> I'm currently debugging a program that uses O_DIRECT for file I/O.
> This option requires that all pointers passed to read() and write() be
> 512-byte aligned.  There is an occasional function that uses malloc()
> to create a buffer instead of something like memalign().  I'd like it
> if the malloc() could be tweaked so that the returned pointer from
> malloc(4096) was aligned to be off by, say 16 bytes.  Otherwise there
> fact that malloc(4096) seems to return a 512-byte aligned pointer
> masks a bug in the program.
> 
> I've implemented this misaligned mallocator on top of malloc() for my purposes.
> 
> Is this something that might belong in the jemalloc library instead of
> in my code?  Maybe it's already there and I don't know about it.
> Maybe this is a dumb idea and it really belongs in the application, as
> I've done it.

I have tentative plans to make alignment of allocations more random, in order to reduce CPU cache conflicts.  However, I don't think intentional minimal alignment for allocations will ever fit well in jemalloc, because jemalloc tries hard to pack objects in ways that conflict with the misalignment feature you built.

Jason