Re: [PATCHv2 0/2] block,nvme: latency-based I/O scheduler

[Keith Busch <kbusch@kernel.org>]

On Fri, Apr 05, 2024 at 08:21:14AM +0200, Hannes Reinecke wrote:
> On 4/4/24 23:14, Keith Busch wrote:
> > On Wed, Apr 03, 2024 at 04:17:54PM +0200, Hannes Reinecke wrote:
> > > Hi all,
> > > 
> > > there had been several attempts to implement a latency-based I/O
> > > scheduler for native nvme multipath, all of which had its issues.
> > > 
> > > So time to start afresh, this time using the QoS framework
> > > already present in the block layer.
> > > It consists of two parts:
> > > - a new 'blk-nlatency' QoS module, which is just a simple per-node
> > >    latency tracker
> > > - a 'latency' nvme I/O policy
> > Whatever happened with the io-depth based path selector? That should
> > naturally align with the lower latency path, and that metric is cheaper
> > to track.
> 
> Turns out that tracking queue depth (on the NVMe level) always requires
> an atomic, and with that a performance impact.
> The qos/blk-stat framework is already present, and as the numbers show
> actually leads to a performance improvement.
> 
> So I'm not quite sure what the argument 'cheaper to track' buys us here...

I was considering the blk_stat framework compared to those atomic
operations. I usually don't enable stats because all the extra
ktime_get_ns() and indirect calls are relatively costly. If you're
enabling stats anyway though, then yeah, I guess I don't really have a
point and your idea here seems pretty reasonable.