Re: [dm-devel] [PATCH 1/1] dm mpath: add IO affinity path selector

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Oct 27 2020 at  8:55am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Thu, Oct 22 2020 at  8:27pm -0400,
> Mike Christie <michael.christie@oracle.com> wrote:
> 
> > This patch adds a path selector that selects paths based on a CPU to
> > path mapping the user passes in and what CPU we are executing on. The
> > primary user for this PS is where the app is optimized to use specific
> > CPUs so other PSs undo the apps handy work, and the storage and it's
> > transport are not a bottlneck.
> > 
> > For these io-affinity PS setups a path's transport/interconnect
> > perf is not going to flucuate a lot and there is no major differences
> > between paths, so QL/HST smarts do not help and RR always messes up
> > what the app is trying to do.
> > 
> > On a system with 16 cores, where you have a job per CPU:
> > 
> > fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=4k \
> > --ioengine=libaio --iodepth=128 --numjobs=16
> > 
> > and a dm-multipath device setup where each CPU is mapped to one path:
> > 
> > // When in mq mode I had to set dm_mq_nr_hw_queues=$NUM_PATHS.
> 
> OK, the modparam was/is a means to an end but the default of 1 is very
> limiting (especially in that it becomes one-size-fits-all, which isn't
> true, for all dm-multipath devices in the system).
> 
> If you have any ideas for what a sane heuristic would be for
> dm_mq_nr_hw_queues I'm open to suggestions.  But DM target <-> DM core
> <-> early block core interface coordination is "fun". ;)
> 
> > // Bio mode also showed similar results.
> > 0 16777216 multipath 0 0 1 1 io-affinity 0 16 1 8:16 1 8:32 2 8:64 4
> > 8:48 8 8:80 10 8:96 20 8:112 40 8:128 80 8:144 100 8:160 200 8:176
> > 400 8:192 800 8:208 1000 8:224 2000 8:240 4000 65:0 8000
> > 
> > we can see a IOPs increase of 25%.
> 
> Great. What utility/code are you using to extract the path:cpu affinity?
> Is it array specific?  Which hardware pins IO like this?
> 
> Will you, or others, be enhancing multipath-tools to allow passing such
> io-affinity DM multipath tables?
> 
> > The percent increase depends on the device and interconnect. For a
> > slower/medium speed path/device that can do around 180K IOPs a path
> > if you ran that fio command to it directly we saw a 25% increase like
> > above. Slower path'd devices that could do around 90K per path showed
> > maybe around a 2 - 5% increase. If you use something like null_blk or
> > scsi_debug which can multi-million IOPs and hack it up so each device
> > they export shows up as a path then you see 50%+ increases.
> > 
> > Signed-off-by: Mike Christie <michael.christie@oracle.com>
> > ---
> >  drivers/md/Kconfig          |   9 ++
> >  drivers/md/Makefile         |   1 +
> >  drivers/md/dm-io-affinity.c | 272 ++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 282 insertions(+)
> >  create mode 100644 drivers/md/dm-io-affinity.c
> > 
> > diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> > index 30ba357..c82d8b6 100644
> > --- a/drivers/md/Kconfig
> > +++ b/drivers/md/Kconfig
> > @@ -463,6 +463,15 @@ config DM_MULTIPATH_HST
> >  
> >  	  If unsure, say N.
> >  
> > +config DM_MULTIPATH_IOA
> > +	tristate "I/O Path Selector based on CPU submission"
> > +	depends on DM_MULTIPATH
> > +	help
> > +	  This path selector selects the path based on the CPU the IO is
> > +	  executed on and the CPU to path mapping setup at path addition time.
> > +
> > +	  If unsure, say N.
> > +
> >  config DM_DELAY
> >  	tristate "I/O delaying target"
> >  	depends on BLK_DEV_DM
> > diff --git a/drivers/md/Makefile b/drivers/md/Makefile
> > index 6d3e234..4f95f33 100644
> > --- a/drivers/md/Makefile
> > +++ b/drivers/md/Makefile
> > @@ -59,6 +59,7 @@ obj-$(CONFIG_DM_MULTIPATH)	+= dm-multipath.o dm-round-robin.o
> >  obj-$(CONFIG_DM_MULTIPATH_QL)	+= dm-queue-length.o
> >  obj-$(CONFIG_DM_MULTIPATH_ST)	+= dm-service-time.o
> >  obj-$(CONFIG_DM_MULTIPATH_HST)	+= dm-historical-service-time.o
> > +obj-$(CONFIG_DM_MULTIPATH_IOA)	+= dm-io-affinity.o
> >  obj-$(CONFIG_DM_SWITCH)		+= dm-switch.o
> >  obj-$(CONFIG_DM_SNAPSHOT)	+= dm-snapshot.o
> >  obj-$(CONFIG_DM_PERSISTENT_DATA)	+= persistent-data/
> 
> Thinking about renaming all PS files to have a dm-ps prefix...
> 
> Fact that we have dm-io.c makes dm-io-affinity.c all the more confusing.
> 
> Can you rename to dm-ps-io-affinity.c and post v2?

You know what, I'll take care of it when I deal with the other path
selectors.

We'll need entries like this in Makefile:

dm-io-affinity-y += dm-ps-io-affinity.o

Otherwise the module won't be name dm-io-affinity and "io-affinity" on
DM table wouldn't autoload the kernel module.

Alternatively, MODALIAS can be used... might prefer that as it'd reduce
Makefile complexity for each path-selector.

Mike

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