Hi,

On 2020-12-11 01:44:38 +0000, Pavel Begunkov wrote:

In general the current hybrid polling doesn't work well with high QD,
that's because statistics it based on are not very resilient to all sorts
of problems. And it might be a problem I described long ago

https://www.spinics.net/lists/linux-block/msg61479.html
https://lkml.org/lkml/2019/4/30/120

Interesting.

Are you interested in it just out of curiosity, or you have a good
use case? Modern SSDs are so fast that even with QD1 the sleep overhead
on sleeping getting considerable, all the more so for higher QD.

It's a bit more than just idle curiosity, but not a strong need (yet). I
was experimenting with using it for postgres WAL writes. The CPU cost of
"classic" polling is high enough to make it not super attractive in a
lot of cases.  Often enough the QD is just 1 for data integrity writes
on fast drives, but there's also cases (bulk load particularly, or high
concurrency OLTP) where having multiple IOs in flight is important.

Because if there is no one who really cares, then instead of adding
elaborated correction schemes, I'd rather put max(time, 10ms) and
that's it.

I wonder if it's doable to just switch from hybrid polling to classic
polling if there's more than one request in flight?

Greetings,

Andres Freund