[PATCH 3/3] block: Polling completion performance optimization

[Sagi Grimberg]

On 12/29/2017 11:50 AM, Christoph Hellwig wrote:
> On Thu, Dec 21, 2017 at 02:34:00PM -0700, Keith Busch wrote:
>> It would be nice, but the driver doesn't know a request's completion
>> is going to be a polled.
> 
> We can trivially set a REQ_POLL bit.  In fact my initial patch kit had
> those on the insitence of Jens, but then I removed it because we had
> no users for it.
> 
>> Even if it did, we don't have a spec defined
>> way to tell the controller not to send an interrupt with this command's
>> compeletion, which would be negated anyway if any interrupt driven IO
>> is mixed in the same queue.
> 
> Time to add such a flag to the spec then..

That would be very useful, ideally we can also hook it into libaio
to submit without triggering an interrupt and have io_getevents to poll
the underlying bdev (blk_poll) similar to how the net stack implements
low latency sockets [1].

Having the ability to suppress interrupts and poll for completions would
be very useful for file servers or targets living in userspace.

https://lwn.net/Articles/551284/