Re: libata .sg_tablesize: why always dividing by 2 ?

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2008-02-26 at 13:56 -0800, James Bottomley wrote:
> > Our iommu can use smaller than PAGE_SIZE allocations when PAGE_SIZE is
> > 64k (it can use 4k), and that helps a lot with networking to avoid
> > filling up pSeries small iommu's too fast. Unfortunately, that meant we
> > used to not even get PAGE_SIZE alignment for some cases. The workaround
> > fixes that but does not provide natural size alignment.
> > 
> > If we were to provide size based alignment of sg requests, fragmentation
> > would become so bad we would basically end up failing a large amount of
> > iommu allocations on servers.
> 
> Ben, he just quoted the wrong patch ... that was the segment length
> limit patch.  The boundary alignment patch is this one:
> 
> fb3475e9b6bfa666107512fbd6006c26014f04b8
> 
> And it does alter the ppc iommu to respect the dma_boundary, so all of
> the mechanics for removing the ppc special casing in libata/ide is
> upstream.

And the fact that this will totally blow fragmentation through the roof
and cause half of the dma-map requests to fail on machines with small
iommu has been totally ignored I suppose ?

Ben.