Re: [PATCH 2/4] iommu: add a bitmap based dma address allocator

["Woodhouse, David" <david.woodhouse-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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On Wed, 2015-11-25 at 15:46 +0100, jroedel-l3A5Bk7waGM@public.gmane.org wrote:
> On Tue, Nov 24, 2015 at 02:05:12PM -0800, Shaohua Li wrote:
> > The lib/iommu-common.c uses a bitmap and a lock. This implementation
> > actually uses a percpu_ida which completely avoids locking. It would be
> > possible to make lib/iommu-common.c use percpu_ida too if somebody wants
> > to do it, but I think this shouldn't be a blocker for these patches
> > giving it has huge performance gain.
> 
> It doesn't "completely avoids locking", the percpu_ida code uses a lock
> internally too. Also, what is the memory and device address space
> overhead per cpu?

A percpu lock doesn't bounce cachelines between CPUs very much, so from
that point of view it might as well not exist :)

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David Woodhouse                            Open Source Technology Centre
David.Woodhouse-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org                              Intel Corporation



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