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

[Huan Yang]

在 2024/7/30 18:42, Christian König 写道:
> Am 30.07.24 um 11:05 schrieb Huan Yang:
> > 在 2024/7/30 16:56, Daniel Vetter 写道:
> > > [????????? daniel.vetter@xxxxxxxx ????????? 
> > > 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
> >
> > > needlessly serialization operations.
> > >
> > > The other issue is that the mmap has some overhead, but might not be 
> > > too
> > > bad.
> > Yes, the time spent on page fault in mmap should be negligible 
> > compared to the time spent on file read.
>
> You should try to avoid mmap as much as possible. Especially the TLB 
> invalidation overhead is really huge on platforms with a large number 
> of CPUs.
But, if not mmap, how to read file from fd to fd? sendfile may 
help(shmemfs may support it, I do not test it), but use pipe buffer is 
not good through test.
> Regards,
> Christian.
>
> > > -Sima
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch/