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 <spetr...@redhat.com>. > Credit goes to Samuel for creating these performance tests and many thanks to > Benjamin Marzinski <bmarz...@redhat.com>, > Mikulas Patocka <mpato...@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-879aa86d8...@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
