Re: memory access op ideas

[Hao Xu <haoxu.linux@gmail.com>]

在 4/22/22 9:38 PM, Jens Axboe 写道:
> On 4/22/22 6:52 AM, Hao Xu wrote:
>> Hi Avi,
>> ? 4/13/22 6:33 PM, Avi Kivity ??:
>>> Unfortunately, only ideas, no patches. But at least the first seems very easy.
>>>
>>>
>>> - IORING_OP_MEMCPY_IMMEDIATE - copy some payload included in the op itself (1-8 bytes) to a user memory location specified by the op.
>>>
>>>
>>> Linked to another op, this can generate an in-memory notification useful for busy-waiters or the UMWAIT instruction
>>>
>>>
>>> This would be useful for Seastar, which looks at a timer-managed memory location to check when to break computation loops.
>>>
>>>
>>> - IORING_OP_MEMCPY - asynchronously copy memory
>>>
>>>
>>> Some CPUs include a DMA engine, and io_uring is a perfect interface to exercise it. It may be difficult to find space for two iovecs though.
>>
>> I have a question about the 'DMA' here, do you mean DMA device for
>> memory copy? My understanding is you want async memcpy so that the
>> cpu can relax when the specific hardware is doing memory copy. the
>> thing is for cases like busy waiting or UMAIT, the length of the memory
>> to be copied is usually small(otherwise we don't use busy waiting or
>> UMAIT, right?). Then making it async by io_uring's iowq may introduce
>> much more overhead(the context switch).
> 
> Nothing fast should use io-wq. But not sure why you think this would
> need it, as long as you can start the operation in a sane fashion and
> get notified when done, why would it need io-wq?
Ah, yes, forgot the some basic logic while sending the email.
>