Re: [PATCH v2] iommu/iova: Optimize alloc_iova with rbtree_augmented

[John Garry]

On 26/08/2022 11:28, Peng Zhang wrote:
> > Though only 3-4 drivers use alloc_iova() directly, in my understanding
> >
> > your test has simulated the worst case, rcache doesn't work at all,
> >
> > "alloc_iova" +“remove_iova” number looks great for worst case.
>
> There is another case, when the size to allocate greater to 2^5, even if 
> alloc_iova_fast() is used, alloc_iova() will always be called because 
> the maximum iova size that rcache supports to allocate is 32.
> IOVA_RANGE_CACHE_MAX_SIZE specifies the maximum size.


If you really have a performance issue with alloc_iova_fast() -> 
alloc_iova() then I suggest that you consider trying to use 
dma_opt_mapping_size() to teach the DMA engine driver to not create 
requests whose overall size exceeds to the rcache limit.

Thanks,
John