[dm-devel] dm kcopyd: Increase sub-job size to 512KiB

[Mike Snitzer]

On Tue, Jul 16 2019 at 10:11am -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Tue, Jul 16 2019 at  9:59am -0400,
> Nikos Tsironis <ntsironis at arrikto.com> wrote:
> 
> > On 7/15/19 9:22 PM, Mike Snitzer wrote:
> > > On Fri, Jul 12 2019 at  9:45am -0400,
> > > Nikos Tsironis <ntsironis at arrikto.com> wrote:
> > > 
> > >> Hi Mike,
> > >>
> > >> A kind reminder about this patch. Do you require any changes or will you
> > >> merge it as is?
> > > 
> > > I think we need changes to expose knob(s) to tune this value on a global
> > > _and_ device level via sysfs.  E.g.:
> > > 
> > > 1) dm_mod module param for global
> > > 2) but also allow a per-device override, like:
> > >    echo 512 > /sys/block/dm-X/dm/kcopyd_subjob_size
> > > 
> > 
> > Hi Mike,
> > 
> > Thanks for your feedback. I agree, this sounds like the best thing to do.
> > 
> > > 1 is super easy and is a start.  Layering in 2 is a bit more involved.
> > 
> > Maybe I could help with (2). We could discuss about it and how you think
> > it's best to do it and then I could proceed with an implementation.
> > 
> > Please let me know what you think.
> > 
> > > 
> > > In hindsight (given how risk-averse I am on changing the default) I
> > > should've kept the default 128 but allowed override with modparam
> > > dm_mod.kcopyd_subjob_size=1024
> > > 
> > > Would this be an OK first step?
> > 
> > Yes, this would be great.
> > 
> > > 
> > > If so, we're still in the 5.3 merge window, I'll see what I can do.
> > 
> > Shall I proceed with a patch adding the dm_mod.kcopyd_subjob_size
> > modparam?
> 
> Sure.  And it could be that we won't need 2.
> 
> Ideally the default would work for every setup.  Less knobs the better.
> But as a stop-gap I think we need to expose a knob that allows override.
> 
> Thinking further, I don't think changing the default to 512K is too
> risky (famous last words).  So please just update your original patch to
> include the modparam so that users can get the old 64K back if needed.
> 
> BTW, the param name should probably be "kcopyd_subjob_size_kb" to
> reflect the value is KB.

One other thing: not sure what the max should be on this
modparam.. maybe 1024K?

Mike