Re: [PATCH v2] vfio iommu type1: Improve vfio_iommu_type1_pin_pages performance

[Eric Farman <farman@linux.ibm.com>]

On 12/9/20 6:54 AM, Cornelia Huck wrote:
> On Tue, 8 Dec 2020 21:55:53 +0800
> "xuxiaoyang (C)" <xuxiaoyang2@huawei.com> wrote:
> 
>> On 2020/11/21 15:58, xuxiaoyang (C) wrote:
>>> vfio_pin_pages() accepts an array of unrelated iova pfns and processes
>>> each to return the physical pfn.  When dealing with large arrays of
>>> contiguous iovas, vfio_iommu_type1_pin_pages is very inefficient because
>>> it is processed page by page.In this case, we can divide the iova pfn
>>> array into multiple continuous ranges and optimize them.  For example,
>>> when the iova pfn array is {1,5,6,7,9}, it will be divided into three
>>> groups {1}, {5,6,7}, {9} for processing.  When processing {5,6,7}, the
>>> number of calls to pin_user_pages_remote is reduced from 3 times to once.
>>> For single page or large array of discontinuous iovas, we still use
>>> vfio_pin_page_external to deal with it to reduce the performance loss
>>> caused by refactoring.
>>>
>>> Signed-off-by: Xiaoyang Xu <xuxiaoyang2@huawei.com>
> 
> (...)
> 
>>
>> hi Cornelia Huck, Eric Farman, Zhenyu Wang, Zhi Wang
>>
>> vfio_pin_pages() accepts an array of unrelated iova pfns and processes
>> each to return the physical pfn.  When dealing with large arrays of
>> contiguous iovas, vfio_iommu_type1_pin_pages is very inefficient because
>> it is processed page by page.  In this case, we can divide the iova pfn
>> array into multiple continuous ranges and optimize them.  I have a set
>> of performance test data for reference.
>>
>> The patch was not applied
>>                      1 page           512 pages
>> no huge pages：     1638ns           223651ns
>> THP：               1668ns           222330ns
>> HugeTLB：           1526ns           208151ns
>>
>> The patch was applied
>>                      1 page           512 pages
>> no huge pages       1735ns           167286ns
>> THP：               1934ns           126900ns
>> HugeTLB：           1713ns           102188ns
>>
>> As Alex Williamson said, this patch lacks proof that it works in the
>> real world. I think you will have some valuable opinions.
> 
> Looking at this from the vfio-ccw angle, I'm not sure how much this
> would buy us, as we deal with IDAWs, which are designed so that they
> can be non-contiguous. I guess this depends a lot on what the guest
> does.

This would be my concern too, but I don't have data off the top of my 
head to say one way or another...

> 
> Eric, any opinion? Do you maybe also happen to have a test setup that
> mimics workloads actually seen in the real world?
> 

...I do have some test setups, which I will try to get some data from in 
a couple days. At the moment I've broken most of those setups trying to 
implement some other stuff, and can't revert back at the moment. Will 
get back to this.

Eric