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Subject: Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count
To:     Christoph Hellwig <hch@infradead.org>
CC:     Dave Chinner <david@fromorbit.com>,
        Matthew Wilcox <willy@infradead.org>,
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References: <20181012060014.10242-1-jhubbard@nvidia.com>
 <20181012060014.10242-5-jhubbard@nvidia.com> <20181013035516.GA18822@dastard>
 <7c2e3b54-0b1d-6726-a508-804ef8620cfd@nvidia.com>
 <20181013164740.GA6593@infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
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On 10/13/18 9:47 AM, Christoph Hellwig wrote:
> On Sat, Oct 13, 2018 at 12:34:12AM -0700, John Hubbard wrote:
>> In patch 6/6, pin_page_for_dma(), which is called at the end of get_user_pages(),
>> unceremoniously rips the pages out of the LRU, as a prerequisite to using
>> either of the page->dma_pinned_* fields. 
>>
>> The idea is that LRU is not especially useful for this situation anyway,
>> so we'll just make it one or the other: either a page is dma-pinned, and
>> just hanging out doing RDMA most likely (and LRU is less meaningful during that
>> time), or it's possibly on an LRU list.
> 
> Have you done any benchmarking what this does to direct I/O performance,
> especially for small I/O directly to a (fast) block device?

Not yet. I can go do that now. If you have any particular test suites, benchmarks,
or just programs to recommend, please let me know. So far, I see

   tools/testing/selftests/vm/gup_benchmark.c


-- 
thanks,
John Hubbard
NVIDIA