Re: [Qemu-devel] [PATCH v12 1/6] migration: introduce postcopy-blocktime capability

[Alexey Perevalov]

On 01/03/2018 12:20 AM, Eric Blake wrote:
> On 10/30/2017 08:16 AM, Alexey Perevalov wrote:
> > Right now it could be used on destination side to
> > enable vCPU blocktime calculation for postcopy live migration.
> > vCPU blocktime - it's time since vCPU thread was put into
> > interruptible sleep, till memory page was copied and thread awake.
> >
> > Reviewed-by: Dr. David Alan Gilbert <address@hidden>
> > Signed-off-by: Alexey Perevalov <address@hidden>
> > ---
> >   migration/migration.c | 9 +++++++++
> >   migration/migration.h | 1 +
> >   qapi/migration.json   | 6 +++++-
> >   3 files changed, 15 insertions(+), 1 deletion(-)
> Is there any reason this has to be a new capability rather than
> unconditionally enabled?  What are the trade-offs for enabling vs.
> disabling the capability that warrant it being a knob?  Can we do a
> better job of documenting in which cases the user would want to change
> the knob from its default value, if we even need it to be a knob?
Hello Eric,
sorry for late response, it's holiday in whole Russia.

The reason we decided to introduce new capability it's
performance penalty, memory & cpu usage, in current version
it's not so high as in initial, it affects hot path of post-copy live 
migration.
Regarding documentation part, I'll answer in
"[PATCH v12 6/6] migration: add postcopy total blocktime into query-migrate"
thread.

> > +++ b/qapi/migration.json
> > @@ -352,12 +352,16 @@
> >   #
> >   # @x-multifd: Use more than one fd for migration (since 2.11)
> >   #
> > +# @postcopy-blocktime: Calculate downtime for postcopy live migration
> > +#                     (since 2.11)
> > +#
> >   # Since: 1.2
> >   ##
> >   { 'enum': 'MigrationCapability',
> >     'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> >              'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
> > -           'block', 'return-path', 'pause-before-switchover', 'x-multifd' ] }
> > +           'block', 'return-path', 'pause-before-switchover', 'x-multifd',
> > +           'postcopy-blocktime' ] }
> >   
> >   ##
> >   # @MigrationCapabilityStatus:

--
Best regards,
Alexey Perevalov