Re: swiotlb detection should be memory hotplug aware ?

[FUJITA Tomonori]

On Fri, 23 Jul 2010 17:23:33 +0200
Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:

> 
> > I was thinking about this at some point. I think the first step is to
> > make SWIOTLB use the debugfs to actually print out how much of its
> > buffers are used - and see if the 64MB is a good fit.
> 
> swiotlb is near always wrongly sized. For most system it's far too much, 
> but for some
> not enough. I have some systemtap scripts around to instrument it.

True, it's impossible to preallocate the best iotlb size statically.

 
> Also it depends on the IO load, so if you size it reasonable you
> risk overflow on large IO (however these days this very rarely happens 
> because
> all "serious" IO devices don't need swiotlb anymore)

Yeah, nowadays it's pointless to try to get the good performance with
swiotlb.


> The other problem is that using only  two bits for the needed address 
> space is also extremly
> inefficient (4GB and 16MB on x86). Really want masks everywhere and 
> optimize for the
> actual requirements.

swiotlb doesn't allocate GFP_DMA memory. It handles only GFP_DMA32.

swiotlb doesn't work for drivers with some odd dma mask (non 32bit)
but we have been lived with it so I don't think that it's a big issue.


I think, supporting expanding swiotlb dynamically is enough. The
default swiotlb size, 64MB is too large for majority.

I have a half-baked patch for it. I'll send it later.
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