Re: [RFC] a new way to achieve asynchronous IO

[Hao Xu <hao.xu@linux.dev>]

On 6/20/22 20:01, Hao Xu wrote:
> Hi,
> I've some thought on the way of doing async IO. The current model is:
> (given we are using SQPOLL mode)
> 
> the sqthread does:
> (a) Issue a request with nowait/nonblock flag.
> (b) If it would block, reutrn -EAGAIN
> (c) The io_uring layer captures this -EAGAIN and wake up/create
> a io-worker to execute the request synchronously.
> (d) Try to issue other requests in the above steps again.
> 
> This implementation has two downsides:
> (1) we have to find all the block point in the IO stack manually and
> change them into "nowait/nonblock friendly".
> (2) when we raise another io-worker to do the request, we submit the
> request from the very beginning. This isn't a little bit inefficient.

                                           ^is

> 
> 
> While I think we can actually do it in a reverse way:
> (given we are using SQPOLL mode)
> 
> the sqthread1 does:
> (a) Issue a request in the synchronous way
> (b) If it is blocked/scheduled soon, raise another sqthread2
> (c) sqthread2 tries to issue other requests in the same way.
> 
> This solves problem (1), and may solve (2).
> For (1), we just do the sqthread waken-up at the beginning of schedule()
> just like what the io-worker and system-worker do. No need to find all
> the block point.
> For (2), we continue the blocked request from where it is blocked when
> resource is satisfied.
> 
> What we need to take care is making sure there is only one task
> submitting the requests.
> 
> To achieve this, we can maintain a pool of sqthread just like the iowq.
> 
> I've done a very simple/ugly POC to demonstrate this:
> 
> https://github.com/HowHsu/linux/commit/183be142493b5a816b58bd95ae4f0926227b587b 
> 
> 
> I also wrote a simple test to test it, which submits two sqes, one
> read(pipe), one nop request. The first one will be block since no data
> in the pipe. Then a new sqthread was created/waken up to submit the
> second one and then some data is written to the pipe(by a unrelated
> user thread), soon the first sqthread is waken up and continues the
> request.
> 
> If the idea sounds no fatal issue I'll change the POC to real patches.
> Any comments are welcome!
>