[Qemu-devel] [PATCH 0/1] migration: calculate expected_downtime considering redirtied ram

[Qemu-devel] [PATCH 0/1] migration: calculate expected_downtime considering redirtied ram

[bala24]

From: Balamuruhan S <bala24@linux.vnet.ibm.com>

Based on the discussion with Dave and David Gibson earlier with respect
to expected_downtime calculation, 

https://lists.gnu.org/archive/html/qemu-devel/2018-04/msg02418.html

got suggestions that the calculation is of not accurate and we need to
consider the ram that gets redirtied during the time when we would have
actually transferred ram in the current iteration.

so I have came up with a calculation by considering the ram that could
get redirtied during the current iteration at the time we would have
transferred the remaining ram in current iteration. By this way,
the total ram to be transferred will be remaining ram + redirtied ram
and dividing with bandwidth would yield us better expected_downtime
value.

Please help to review and suggest about this approach.

Balamuruhan S (1):
  migration: calculate expected_downtime considering redirtied ram

 migration/migration.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

-- 
2.14.5

[Qemu-devel] [PATCH 1/1] migration: calculate expected_downtime considering redirtied ram

[bala24]

From: Balamuruhan S <bala24@linux.vnet.ibm.com>

currently we calculate expected_downtime by time taken to transfer
remaining ram, but during the time we had transferred remaining ram
few pages of ram might be redirtied and we need to retransfer it,
so it is better to consider them for calculating expected_downtime
for getting more accurate values.

Total ram to be transferred = remaining ram + (redirtied ram at the
                                               time when the remaining
                                               ram gets transferred)

redirtied ram = dirty_pages_rate * time taken to transfer remaining ram

redirtied ram = dirty_pages_rate * (remaining ram / bandwidth)

expected_downtime = (remaining ram + redirtied ram) / bandwidth

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
---
 migration/migration.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index ffc4d9e556..dc38e9a380 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2903,7 +2903,13 @@ static void migration_update_counters(MigrationState *s,
      * recalculate. 10000 is a small enough number for our purposes
      */
     if (ram_counters.dirty_pages_rate && transferred > 10000) {
-        s->expected_downtime = ram_counters.remaining / bandwidth;
+        /* Time required to transfer remaining ram */
+        remaining_ram_transfer_time = ram_counters.remaining / bandwidth
+
+        /* redirty of ram at the time remaining ram gets transferred*/
+        newly_dirtied_ram = ram_counters.dirty_pages_rate * remaining_ram_transfer_time
+
+        s->expected_downtime = (ram_counters.remaining + newly_dirtied_ram) / bandwidth;
     }
 
     qemu_file_reset_rate_limit(s->to_dst_file);
-- 
2.14.5

Re: [Qemu-devel] [PATCH 1/1] migration: calculate expected_downtime considering redirtied ram

[Juan Quintela]

bala24@linux.vnet.ibm.com wrote:
> From: Balamuruhan S <bala24@linux.vnet.ibm.com>
>
> currently we calculate expected_downtime by time taken to transfer
> remaining ram, but during the time we had transferred remaining ram
> few pages of ram might be redirtied and we need to retransfer it,
> so it is better to consider them for calculating expected_downtime
> for getting more accurate values.
>
> Total ram to be transferred = remaining ram + (redirtied ram at the
>                                                time when the remaining
>                                                ram gets transferred)
>
> redirtied ram = dirty_pages_rate * time taken to transfer remaining ram
>
> redirtied ram = dirty_pages_rate * (remaining ram / bandwidth)
>
> expected_downtime = (remaining ram + redirtied ram) / bandwidth
>
> Suggested-by: David Gibson <david@gibson.dropbear.id.au>
> Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> ---
>  migration/migration.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index ffc4d9e556..dc38e9a380 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2903,7 +2903,13 @@ static void migration_update_counters(MigrationState *s,
>       * recalculate. 10000 is a small enough number for our purposes
>       */
>      if (ram_counters.dirty_pages_rate && transferred > 10000) {
> -        s->expected_downtime = ram_counters.remaining / bandwidth;
> +        /* Time required to transfer remaining ram */
> +        remaining_ram_transfer_time = ram_counters.remaining / bandwidth

missing semicolon

> +
> +        /* redirty of ram at the time remaining ram gets transferred*/
> +        newly_dirtied_ram = ram_counters.dirty_pages_rate * remaining_ram_transfer_time

the same.

Declaration of the new variables is also missing.

> +        s->expected_downtime = (ram_counters.remaining + newly_dirtied_ram) / bandwidth;
>      }
>  
>      qemu_file_reset_rate_limit(s->to_dst_file);

About the numbers, I am not against it.  It is an heuristic.  Without
numbers (and it is very load dependent) it is not clear that this one is
going to be much worse/better than previous one (this should be a bit
better, though).

Thanks, Juan.

Re: [Qemu-devel] [PATCH 1/1] migration: calculate expected_downtime considering redirtied ram

[Peter Xu]

On Wed, Jan 23, 2019 at 05:35:03PM +0100, Juan Quintela wrote:
> bala24@linux.vnet.ibm.com wrote:
> > From: Balamuruhan S <bala24@linux.vnet.ibm.com>
> >
> > currently we calculate expected_downtime by time taken to transfer
> > remaining ram, but during the time we had transferred remaining ram
> > few pages of ram might be redirtied and we need to retransfer it,
> > so it is better to consider them for calculating expected_downtime
> > for getting more accurate values.
> >
> > Total ram to be transferred = remaining ram + (redirtied ram at the
> >                                                time when the remaining
> >                                                ram gets transferred)
> >
> > redirtied ram = dirty_pages_rate * time taken to transfer remaining ram
> >
> > redirtied ram = dirty_pages_rate * (remaining ram / bandwidth)
> >
> > expected_downtime = (remaining ram + redirtied ram) / bandwidth
> >
> > Suggested-by: David Gibson <david@gibson.dropbear.id.au>
> > Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> > ---
> >  migration/migration.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/migration/migration.c b/migration/migration.c
> > index ffc4d9e556..dc38e9a380 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -2903,7 +2903,13 @@ static void migration_update_counters(MigrationState *s,
> >       * recalculate. 10000 is a small enough number for our purposes
> >       */
> >      if (ram_counters.dirty_pages_rate && transferred > 10000) {
> > -        s->expected_downtime = ram_counters.remaining / bandwidth;
> > +        /* Time required to transfer remaining ram */
> > +        remaining_ram_transfer_time = ram_counters.remaining / bandwidth
> 
> missing semicolon
> 
> > +
> > +        /* redirty of ram at the time remaining ram gets transferred*/
> > +        newly_dirtied_ram = ram_counters.dirty_pages_rate * remaining_ram_transfer_time
> 
> the same.
> 
> Declaration of the new variables is also missing.
> 
> > +        s->expected_downtime = (ram_counters.remaining + newly_dirtied_ram) / bandwidth;
> >      }
> >  
> >      qemu_file_reset_rate_limit(s->to_dst_file);
> 
> About the numbers, I am not against it.  It is an heuristic.  Without
> numbers (and it is very load dependent) it is not clear that this one is
> going to be much worse/better than previous one (this should be a bit
> better, though).

Actually I have had a question on how expected_downtime is defined and
how it will be used by users.

My understanding is that the expected_downtime is defined as: how long
time the guest will be down if we stop the VM right now and migrate
all the rest of pages.

This definition makes sense in that it helps the customer to
dynamically decide whether it's a good point to go into the last phase
of migration.  Currently we should be able to achieve that by setting
a very high target downtime.

And if that definition is the thing we want, the current calculation
seems exactly the number we want, since if we stop the VM right now
then there won't be any more data to be dirtied as well.

Regards,

-- 
Peter Xu

Re: [Qemu-devel] [PATCH 1/1] migration: calculate expected_downtime considering redirtied ram

[Balamuruhan S]

On Thu, Jan 24, 2019 at 03:18:22PM +0800, Peter Xu wrote:
> On Wed, Jan 23, 2019 at 05:35:03PM +0100, Juan Quintela wrote:
> > bala24@linux.vnet.ibm.com wrote:
> > > From: Balamuruhan S <bala24@linux.vnet.ibm.com>
> > >
> > > currently we calculate expected_downtime by time taken to transfer
> > > remaining ram, but during the time we had transferred remaining ram
> > > few pages of ram might be redirtied and we need to retransfer it,
> > > so it is better to consider them for calculating expected_downtime
> > > for getting more accurate values.
> > >
> > > Total ram to be transferred = remaining ram + (redirtied ram at the
> > >                                                time when the remaining
> > >                                                ram gets transferred)
> > >
> > > redirtied ram = dirty_pages_rate * time taken to transfer remaining ram
> > >
> > > redirtied ram = dirty_pages_rate * (remaining ram / bandwidth)
> > >
> > > expected_downtime = (remaining ram + redirtied ram) / bandwidth
> > >
> > > Suggested-by: David Gibson <david@gibson.dropbear.id.au>
> > > Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
> > > ---
> > >  migration/migration.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index ffc4d9e556..dc38e9a380 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -2903,7 +2903,13 @@ static void migration_update_counters(MigrationState *s,
> > >       * recalculate. 10000 is a small enough number for our purposes
> > >       */
> > >      if (ram_counters.dirty_pages_rate && transferred > 10000) {
> > > -        s->expected_downtime = ram_counters.remaining / bandwidth;
> > > +        /* Time required to transfer remaining ram */
> > > +        remaining_ram_transfer_time = ram_counters.remaining / bandwidth
> > 
> > missing semicolon
> > 
> > > +
> > > +        /* redirty of ram at the time remaining ram gets transferred*/
> > > +        newly_dirtied_ram = ram_counters.dirty_pages_rate * remaining_ram_transfer_time
> > 
> > the same.
> > 
> > Declaration of the new variables is also missing.
> > 
> > > +        s->expected_downtime = (ram_counters.remaining + newly_dirtied_ram) / bandwidth;
> > >      }
> > >  
> > >      qemu_file_reset_rate_limit(s->to_dst_file);
> > 
> > About the numbers, I am not against it.  It is an heuristic.  Without
> > numbers (and it is very load dependent) it is not clear that this one is
> > going to be much worse/better than previous one (this should be a bit
> > better, though).
> 
> Actually I have had a question on how expected_downtime is defined and
> how it will be used by users.
> 
> My understanding is that the expected_downtime is defined as: how long
> time the guest will be down if we stop the VM right now and migrate
> all the rest of pages.
> 
> This definition makes sense in that it helps the customer to
> dynamically decide whether it's a good point to go into the last phase
> of migration.  Currently we should be able to achieve that by setting
> a very high target downtime.
> 
> And if that definition is the thing we want, the current calculation
> seems exactly the number we want, since if we stop the VM right now
> then there won't be any more data to be dirtied as well.

Thank you Peter, I thought about your definition and it makes sense
with your definition that existing calculation is appropriate and
correct.

-- Bala
> 
> Regards,
> 
> -- 
> Peter Xu
> 

Re: [Qemu-devel] [PATCH 0/1] migration: calculate expected_downtime considering redirtied ram

[no-reply]

Patchew URL: https://patchew.org/QEMU/20190122150543.16889-1-bala24@linux.vnet.ibm.com/



Hi,

This series failed the docker-mingw@fedora build test. Please find the testing commands and
their output below. If you have Docker installed, you can probably reproduce it
locally.

=== TEST SCRIPT BEGIN ===
#!/bin/bash
time make docker-test-mingw@fedora SHOW_ENV=1 J=14
=== TEST SCRIPT END ===

  CC      net/util.o
  CC      net/hub.o
/tmp/qemu-test/src/migration/migration.c: In function 'migration_update_counters':
/tmp/qemu-test/src/migration/migration.c:2907:9: error: 'remaining_ram_transfer_time' undeclared (first use in this function)
         remaining_ram_transfer_time = ram_counters.remaining / bandwidth
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/qemu-test/src/migration/migration.c:2907:9: note: each undeclared identifier is reported only once for each function it appears in
/tmp/qemu-test/src/migration/migration.c:2907:73: error: expected ';' before 'newly_dirtied_ram'
         remaining_ram_transfer_time = ram_counters.remaining / bandwidth
                                                                         ^
                                                                         ;


The full log is available at
http://patchew.org/logs/20190122150543.16889-1-bala24@linux.vnet.ibm.com/testing.docker-mingw@fedora/?type=message.
---
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