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Date:   Thu, 1 Sep 2022 11:45:58 +0100
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From:   John Garry <john.garry@huawei.com>
Subject: Re: [PATCH v2] iommu/iova: Optimize alloc_iova with rbtree_augmented
To:     Peng Zhang <zhangpeng.00@bytedance.com>,
        Ethan Zhao <haifeng.zhao@linux.intel.com>, <joro@8bytes.org>,
        <will@kernel.org>
CC:     <iommu@lists.linux.dev>, <linux-kernel@vger.kernel.org>,
        <robin.murphy@arm.com>
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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