On Mon, Mar 24, 2025 at 04:20:12PM -0400, Mike Snitzer wrote: > 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.
Sorry for typoe: s/unlering/underlying/ > 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 >
