Re: [PATCH] the dm-loop target

[Christoph Hellwig <hch@infradead.org>]

On Tue, Mar 18, 2025 at 05:34:28PM +0800, Ming Lei wrote:
> On Tue, Mar 18, 2025 at 12:57:17AM -0700, Christoph Hellwig wrote:
> > On Tue, Mar 18, 2025 at 03:27:48PM +1100, Dave Chinner wrote:
> > > Yes, NOWAIT may then add an incremental performance improvement on
> > > top for optimal layout cases, but I'm still not yet convinced that
> > > it is a generally applicable loop device optimisation that everyone
> > > wants to always enable due to the potential for 100% NOWAIT
> > > submission failure on any given loop device.....
> 
> NOWAIT failure can be avoided actually:
> 
> https://lore.kernel.org/linux-block/20250314021148.3081954-6-ming.lei@redhat.com/

That's a very complex set of heuristics which doesn't match up
with other uses of it.

> 
> > 
> > Yes, I think this is a really good first step:
> > 
> > 1) switch loop to use a per-command work_item unconditionally, which also
> >    has the nice effect that it cleans up the horrible mess of the
> >    per-blkcg workers.  (note that this is what the nvmet file backend has
> 
> It could be worse to take per-command work, because IO handling crosses
> all system wq worker contexts.

So do other workloads with pretty good success.

> 
> >    always done with good result)
> 
> per-command work does burn lots of CPU unnecessarily, it isn't good for
> use case of container

That does not match my observations in say nvmet.  But if you have
numbers please share them.