[PATCH 0/2] Block: Give option to force io polling

[Derrick, Jonathan]

Hi Stephen,

I've got a patch I'm going to send out shortly which does it for a block-device file (as Jeff suggested). It's still Mjölnir but seems to be more in the right place.

-----Original Message-----
From: Stephen Bates [mailto:stephen.bates at microsemi.com] 
Sent: Thursday, May 12, 2016 11:28 AM
To: hch at infradead.org
Cc: axboe at fb.com; linux-block at vger.kernel.org; Busch, Keith <keith.busch at intel.com>; linux-nvme at lists.infradead.org; Derrick, Jonathan <jonathan.derrick at intel.com>
Subject: RE: [PATCH 0/2] Block: Give option to force io polling

> 
> 
> On Mon, May 09, 2016 at 02:53:30PM +0000, Stephen Bates wrote:
> > Christoph, this is a DRAM based NVMe device, the code for polling in
> NVMe was merged in 4.5 right? We are using the inbox NVMe driver. Here 
> is some performance data:
> >
> > QD=1, single thread, random 4KB reads
> >
> > Polling Off: 12us Avg / 40us 99.99% ; Polling On: 9.5us Avg / 25us 
> > 99.99%
> >
> > Both the average and 99.99% reduction are of interest.
> 
> How does CPU usage look for common workloads with polling force enabled?

Christoph, CPU load added. 

Polling Off: 12us Avg / 40us 99.99% ; CPU 0.39 H/W Threads Polling On: 9.5us Avg / 25us 99.99% ; CPU 0.98 H/W Threads

> If it's really an overall win we should just add a quirk to the NVMe 
> driver to always force polling for this device based on the PCI ID.

While I like the "big hammer" approach I think we have to have some control over when it is swung ;-). Always turning on polling for a specific device seems a bit too Mjölnir [1] of a solution. The bonus of polling is only when QD and/or thread-count is low and not everyone will use the device that way.  I also suspect other low latency NVMe devices are coming and having a quirk for each and every one of them does not make sense. We could use a module param or sysfs entry in the NVMe driver itself if we want to avoid having this control in the block layer?

Stephen

[1] Mjölnir = Thor's Hammer ;-).