Re: 32-bit dma allocations on 64-bit platforms

Andrea Arcangeli
2004-06-24T15:42:09Z

On Thu, Jun 24, 2004 at 09:45:51AM -0500, Terence Ripperda wrote:
> On Thu, Jun 24, 2004 at 04:13:47AM -0700, tiwai@suse.de wrote:
> > > pci_alloc_consistent is limited to 16MB, but so far nobody has really
> > > complained about that. If that should be a real issue we can make
> > > it allocate from the swiotlb pool, which is usually 64MB (and can
> > > be made bigger at boot time)
> >
> > Can't it be called with GFP_KERNEL at first, then with GFP_DMA if the
> > allocated pages are out of dma mask, just like in pci-gart.c?
> > (with ifdef x86-64)
>
> pci_alloc_consistent (at least on x86-64) does do this. one of the problems
> I've seen in experimentation is that GFP_KERNEL tends to allocate from the
> top of memory down. this means that all of the GFP_KERNEL allocations are >
> 32-bits, which forces us down to GFP_DMA and the < 16M allocations.
>
> I've mainly tested this after a cold boot, so after running for a while,
> GFP_KERNEL may hit good allocations a lot more.

it's trivial to change the order in the freelist to allocate from lower
address first, but the point is still that over time that will be
random.

the 16M must be reserved enterely to the __GFP_DMA on any machine with
>=1G of ram, and the lowmem_reserve_ratio algorithm accomplish this and
it scales down the reserve ratio according to the balance between lowmem
and dma zone.

I believe if something you should try with GFP_KERNEL if GFP_DMA fails,
not the other way around. btw, 2.6 is even more efficient in shrinking
and paging out the dma zone than it could be in 2.4.