Re: [PATCH 0/8] Optional ZONE_DMA V1

[Jes Sorensen <jes@sgi.com>]

Christoph Lameter wrote:
> On Wed, 13 Sep 2006, Jack Steiner wrote:
> 
>> 	- will any IA64 platform require that ZONE_DMA be enabled (I think
>> 	  the answer is "yes")
> 
> I think that still remains to be discussed. Today there are various means
> to get around address restrictions.

I don't know about USB on ia64, but USB is an issue and we do support
it even on Altix, as crazy as it may seem (I use USB with my SGI Prism
foot-warmer in the office). Also take into account that some ia64 boxes
do not come with IOMMU's, DIG - be afraid, be very afraid. On those
machines you ideally want to have DMA32 zone for this stuff to support
32 bit PCI devices, even if the swiotlb can be used (bounce buffers for
all transactions is just a sick idea), and we get back to the issue of
using generic kernels.

>> 	- if ZONE_DMA is enabled, ALTIX will still use only 1 zone. In your
>> 	  statement above, you say that disabling ZONE_DMA save 1M cpu
>> 	  structures. If ZONE_DMA is enabled, will these 1M structure be allocated
>> 	  on SN even though they are not needed?
> 
> That has always been the case. In SLES9 and SLES10 we are using 1/4th of the pagesets.
> 
> Having no ZONE_DMA provides additional benefits besides saving memory. The 
> VM balancing of allocations within a node becomes not necessary and 
> various VM optimization can be performed. Loops are unrolled. Less zones 
> have to be processed for draining / vm counter updates etc etc.

I agree it sounds appealing, but if reality is that all distro kernels
will switch ZONE_DMA on, then having the option to switch it off is
going have little or zero impact on the end users.

In other words, will this really matter in end user situations?

Cheers,
Jes

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