[uClinux-dev] Reclaiming Memory

[Gavin Lambert]

Quoth Harry Gunnarsson:
> Hm, you are saying one thing that gets me thinking. I assumed, 
> perhaps incorrectly, that the 'chunks' of memory in the 
> power-of-two scheme were pre-allocated at kernel start. 
> That is, n * 128k, m * 256k, y * 512k etc And I thought that 
> the big chunks of 2M and 4M were also pre-allocated at startup 
> and never used for small allocations. 
>
> I thought that this was the point of the SLAB allocator, i.e. 
> avoiding fragmentations with fixed size pre-allocated blocks.

Well, I'm not entirely certain how the SLAB allocator operates, since I
haven't really looked at its code at all.  But I do know (from testing) that
larger blocks can be chosen and sliced to create smaller blocks.  I don't
know what its criteria is for choosing blocks; my *assumption* is that it
walks up the chain (ie. if it needs a 256k block, it'll look in the 256k
free list, and only if there aren't any will it go up to the 512k free list,
and so on until it finds a block it can slice up and distribute).  But I
really don't know.  Maybe it tries to do something clever instead.

> So there is no 'guaranteed' way of making sure the system can 
> deliver reliably a 4M chunk of memory over the time of execution, right? 

If you want to absolutely guarantee it, then don't let Linux have the RAM in
the first place :)  (Either by lying to it about how much RAM is there or by
using something like the bootmem interface.)  Usually you don't need to go
that far, though.