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

Huan Yang
2024-07-30T11:33:25Z

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@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
>>
>>> 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/
>>>
>