Re: iwlagn: order 2 page allocation failures

[Mel Gorman <mel@csn.ul.ie>]

On Mon, Sep 14, 2009 at 11:01:10AM +0800, Zhu Yi wrote:
> On Fri, 2009-09-11 at 16:47 +0800, Mel Gorman wrote:
> > On Thu, Sep 10, 2009 at 02:14:50PM -0700, reinette chatre wrote:
> > > On Thu, 2009-09-10 at 02:02 -0700, Mel Gorman wrote:
> > > 
> > > > 
> > > > As a total aside, there is still the problem that the driver is depending on
> > > > order-2 allocations. On systems without swap, the allocation problem could be
> > > > more severe as there are fewer pages the system can use to regain contiguity.
> > > 
> > > I looked more at the implementation and hardware interface but I do not
> > > see a way around this. We have to provide 8k buffer to device, and we
> > > have to make sure it is aligned. 
> > > 
> > 
> > That would imply an order-1 allocation instead of an order-2 though so
> > it would appear than we are being worse than we have to. It would appear
> > to be because of this +256 bytes that goes onto every buffer.
> > 
> > > Do you have any suggestions?
> > > 
> > 
> > Nothing concrete. Finding an alternative to having the socket buffer
> > 8192+256 to make it an order-1 allocation would be an improvement but I
> > don't know how that should be tackled. Lacking the hardware, I can't
> > experiment myself :(
> 
> Essentially, the hardware only requires an order-1 allocation aligned on
> 256 bytes boundary. But as it is used as an SKB, a trailing struct
> skb_shared_info is added. This forces us to both increase the order and
> do alignment ourselves. I believe some improvement could be done here.
> But it should not be an easy one.
> 

Probably not. I can only assume that moving the location of
skb_shared_info such that it is sometimes located after the buffer and
sometimes allocated via a separate kmalloc() would be a significant
undertaking.

> BTW, does SLAB/SLUB guarantee size of multiple PAGE_SIZE __kmalloc()
> allocation align on PAGE_SIZE (or 256 bytes) boundary?
> 

Page-aligned.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab