Re: [LSF/MM/BPF TOPIC] Multipath bio vs. request

[Mike Snitzer <snitzer@kernel.org>]

Hi John,

On Sat, Mar 22, 2025 at 02:38:29PM -0400, John Meneghini wrote:
> I will be presenting on this topic at LSF/MM/BPF this year, in the IO track.
> 
> Here's an introduction for my talk.
> 
> DMMP currently supports two different kernel IO interfaces: the BIO interface[1] (struct bio) and the Request interface[2] (struct request).
> By default DMMP uses the Request interface and over the years much work has been done test and improve the performance of the DMMP Request
> interface. DMMP can also be manually configured to use the BIO interface. The DMMP BIO interface is supported but little work has been done
> to test and improve its performance. DMMP is currently the only upstream component which continues to use the Request interface for submitting IO.

As I clarified at lunch today, your "DMMP is currently the only
upstream component which continues to use the Request interface for
submitting IO." makes no sense to me.  The request-based DM multipath
target is a blk-mq driver.  It just acts like most blk-mq drivers.

What is different is DM core's request-based code will clone each
request that gets submitted to the request-based DMMP device.  And
then when the request is submitted to an underlying path it gets
directly inserted in the unlering blk-mq request-queue for that path.

So in those aspects request-based DM core and DM multipath are unique
and they do require block interfaces that only benefit DMMP -- but
that has _always_ been the case (nothing else ever needed to clone
requests before submitting them).

> At the ALPSS 2024 conference last October we discussed the possibility of deprecating and eventually removing support the Request interface
> as kernel API. Such a change could impact DMMP so I was asked if Red Hat would be willing to support the effort by measuring the performance
> of DMMP's BIO interface[3] and comparing it to its Request based performance. Having such a comparative performance analysis would be very helpful
> in determining what further changes might be needed to move DMMP away from using the Request interface. This would help with the overall effort
> to improve BIO interface performance and eventually remove support for Request based IO as a kernel API.
> 
> In this presentation I will share the preliminary results of Red Hat's DMMP BIO vs Request performance tests[4] and discuss what the next possible
> steps could be for moving forward.
> 
> The tests and performance graphs in this presentation were developed and run by Samuel Petrovic <spetrovi@redhat.com>.
> Credit goes to Samuel for creating these performance tests and many thanks to Benjamin Marzinski <bmarzins@redhat.com>,
> Mikulas Patocka <mpatocka@redhat.com> and others on the Red Hat DMMP and Performance teams who contributed to this work.
> 
> [1] https://lwn.net/Articles/736534/
> [2] https://lwn.net/Articles/738449/
> [3] https://lore.kernel.org/linux-scsi/643e61a8-b0cb-4c9d-831a-879aa86d888e@redhat.com
> [4] https://people.redhat.com/jmeneghi/LSFMM_2025/DMMP_BIOvsRequest/

Other useful context is the 2007 paper that provides an overview of
why dm-multipath was switched from bio-based to request-based:
https://www.kernel.org/doc/ols/2007/ols2007v2-pages-235-244.pdf