Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag

Daniel Vetter
2024-08-05T17:53:34Z

On Thu, Aug 01, 2024 at 10:53:45AM +0800, Huan Yang wrote:
>
> 在 2024/8/1 4:46, Daniel Vetter 写道:
> > On Tue, Jul 30, 2024 at 08:04:04PM +0800, Huan Yang wrote:
> > > 在 2024/7/30 17:05, Huan Yang 写道:
> > > > 在 2024/7/30 16:56, Daniel Vetter 写道:
> > > > > [????????? daniel.vetter@ffwll.ch ?????????
> > > > > https://aka.ms/LearnAboutSenderIdentification?????????????]
> > > > >
> > > > > On Tue, Jul 30, 2024 at 03:57:44PM +0800, Huan Yang wrote:
> > > > > > UDMA-BUF step:
> > > > > >    1. memfd_create
> > > > > >    2. open file(buffer/direct)
> > > > > >    3. udmabuf create
> > > > > >    4. mmap memfd
> > > > > >    5. read file into memfd vaddr
> > > > > Yeah this is really slow and the worst way to do it. You absolutely want
> > > > > to start _all_ the io before you start creating the dma-buf, ideally
> > > > > with
> > > > > everything running in parallel. But just starting the direct I/O with
> > > > > async and then creating the umdabuf should be a lot faster and avoid
> > > > That's greate,  Let me rephrase that, and please correct me if I'm wrong.
> > > >
> > > > UDMA-BUF step:
> > > >   1. memfd_create
> > > >   2. mmap memfd
> > > >   3. open file(buffer/direct)
> > > >   4. start thread to async read
> > > >   3. udmabuf create
> > > >
> > > > With this, can improve
> > > I just test with it. Step is:
> > >
> > > UDMA-BUF step:
> > >   1. memfd_create
> > >   2. mmap memfd
> > >   3. open file(buffer/direct)
> > >   4. start thread to async read
> > >   5. udmabuf create
> > >
> > >   6 . join wait
> > >
> > > 3G file read all step cost 1,527,103,431ns, it's greate.
> > Ok that's almost the throughput of your patch set, which I think is close
> > enough. The remaining difference is probably just the mmap overhead, not
> > sure whether/how we can do direct i/o to an fd directly ... in principle
> > it's possible for any file that uses the standard pagecache.
>
> Yes, for mmap, IMO, now that we get all folios and pin it. That's mean all
> pfn it's got when udmabuf created.
>
> So, I think mmap with page fault is helpless for save memory but increase
> the mmap access cost.(maybe can save a little page table's memory)
>
> I want to offer a patchset to remove it and more suitable for folios
> operate(And remove unpin list). And contains some fix patch.
>
> I'll send it when I test it's good.
>
>
> About fd operation for direct I/O, maybe use sendfile or copy_file_range?
>
> sendfile base pipe buffer, it's low performance when I test is.
>
> copy_file_range can't work due to it's not the same file system.
>
> So, I can't find other way to do it. Can someone give some suggestions?

Yeah direct I/O to pagecache without an mmap might be too niche to be
supported. Maybe io_uring has something, but I guess as unlikely as
anything else.
-Sima
--
Daniel Vetter
Software Engineer, Intel Corporation