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

[Hannes Reinecke <hare@suse.de>]

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...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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