[Qemu-devel] [Proposal] I/O throttling through new -object interface

[Qemu-devel] [Proposal] I/O throttling through new -object interface

[Manos Pitsidianakis]

In continuation of the discussion at [0], here follows a proposal for a 
new ThrottleGroup object defined through QOM. 

Throttle groups are currently defined when instantiating a drive in the 
command line,, eg.  
-drive file=foo.qcow2,throttling.iops-total=100,throttling.group=bar
and also on QMP with the 'block_set_io_throttle' command, eg:

   { "execute": "block_set_io_throttle",
     "arguments": {
        "device": "virtio0",
        "iops": 100,
        "iops_rd": 0,
        "iops_wr": 0,
        "bps": 0,
        "bps_rd": 0,
        "bps_wr": 0
     }
   }

As part of my GSoC project, IO throttling is moved to a block filter 
driver, which means multiple filter nodes sharing ThrottleGroup 
instances, and also, chaining filter nodes. The old interface will be 
retained as much as possible. By defining ThrottleGroups with QOM the 
following interface can be used:
    -drive file=foo.qcow2,throttling.group=bar \
    -object throttle-group,iops-total=100,id=bar
which will be equivalent to the current command above. The 
'block_set_io_throttle' command can be extended with a "group" argument, 
that when defined instead of "device", will find and edit the object in 
question. Users can hotplug ThrottleGroups with object-add in QMP/HMP, 
then attach drives to a throttle group (ie adding a filter node in the 
drive path that points to that throttle group) with an 'attach-throttle' 
and detach with 'detach-throttle', each taking the drive and throttle 
group name as arguments. An alternative (but uglier) approach would be 
to use 'block_set_io_throttle' for this as well. Something that's also 
sound is a 'block_set_throttle_groups' that takes a list of groups and a 
drive as arguments. I think the last could be best if the relevant 
function does not become complex.
All this means that existing configuration will not be redefined 
everytime the object is referenced unless each limit is specified 
explicitly, like it was mentioned in the previous discussion.

Regarding 'query-block'. This case is complicated by the fact that it 
would be possible for a drive to have many nodes, thus the values 
printed by 'query-block' might be inaccurate if they only report one of 
the node configurations.  The current approach is to report only the 
first filter node in the BDS tree. A user with advanced throttling 
configurations might have no need for this command.

I'm not certain about this, since this could be handled internally by an 
application, but it might be useful to have queries to list all the 
throttle groups that affect a drive, and to list all the drives in a 
throttle group. Maybe defining the drives as a ThrottleGroup QOM 
property will take care of the latter and also attaching/detaching 
drives to a group through the property getter/setter. Thoughts?

[0] https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg06245.html

Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Alberto Garcia]

Hey Manos, thanks for your proposal and for the good work :)

On Thu 08 Jun 2017 08:21:13 PM CEST, Manos Pitsidianakis wrote:

> As part of my GSoC project, IO throttling is moved to a block filter 
> driver, which means multiple filter nodes sharing ThrottleGroup 
> instances, and also, chaining filter nodes. The old interface will be 
> retained as much as possible. By defining ThrottleGroups with QOM the 
> following interface can be used:
>     -drive file=foo.qcow2,throttling.group=bar \
>     -object throttle-group,iops-total=100,id=bar

It should be --object, and I guess the name would be throttling-group?
But otherwise the interface looks good to me.

> 'block_set_io_throttle' command can be extended with a "group"
> argument, that when defined instead of "device", will find and edit
> the object in question.

Note that block_set_io_throttle() already has a 'group' argument. Its
current semantics are:

   - bps or iops != 0   -> set the I/O limits of a throttling group. The
                           selected device is moved to that group if it
                           wasn't there yet.

   - bps and iops == 0  -> remove a device from a throttling group
                           without touching that group's I/O limits.

where 'device' is mandatory and 'group' defaults to the value of
'device' if unset.

If I get it right, what you propose is that 'device' is optional and in
that case 'group' is mandatory and the command updates the throttling
group parameters.

Note that you can do that already if you select a device that is already
in the group. These new semantics would allow modifying a group that is
not being used by any block device.

I'm not a big fan of overloading block_set_io_throttle() even more, but
this proposal does actually sound fine.

> Users can hotplug ThrottleGroups with object-add in QMP/HMP, then
> attach drives to a throttle group (ie adding a filter node in the
> drive path that points to that throttle group) with an
> 'attach-throttle' and detach with 'detach-throttle', each taking the
> drive and throttle group name as arguments. An alternative (but
> uglier) approach would be to use 'block_set_io_throttle' for this as
> well.

Yeah I agree that the block_set_io_throttle() approach would be uglier.

> Something that's also sound is a 'block_set_throttle_groups' that
> takes a list of groups and a drive as arguments.

Would that simply be a foreach(g, groups) { attach(device, g); } ?

> Regarding 'query-block'. This case is complicated by the fact that it
> would be possible for a drive to have many nodes, thus the values
> printed by 'query-block' might be inaccurate if they only report one
> of the node configurations.  The current approach is to report only
> the first filter node in the BDS tree. A user with advanced throttling
> configurations might have no need for this command.

I guess the user can use 'query-block' and walk the backing chain
looking for throttling filter nodes?

Berto

Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Manos Pitsidianakis]

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On Fri, Jun 09, 2017 at 03:42:41PM +0200, Alberto Garcia wrote:
>Hey Manos, thanks for your proposal and for the good work :)
>
>On Thu 08 Jun 2017 08:21:13 PM CEST, Manos Pitsidianakis wrote:
>
>> As part of my GSoC project, IO throttling is moved to a block filter
>> driver, which means multiple filter nodes sharing ThrottleGroup
>> instances, and also, chaining filter nodes. The old interface will be
>> retained as much as possible. By defining ThrottleGroups with QOM the
>> following interface can be used:
>>     -drive file=foo.qcow2,throttling.group=bar \
>>     -object throttle-group,iops-total=100,id=bar
>
>It should be --object

Both seem to have the result in my qemu build.

>, and I guess the name would be throttling-group?

Hm, I thought throttle-group would be nicer because it's shorter, but I 
can change it, of course. I am not certain if there's a semantic 
difference (because I am not a native speaker).

>
>> 'block_set_io_throttle' command can be extended with a "group"
>> argument, that when defined instead of "device", will find and edit
>> the object in question.
>
>Note that block_set_io_throttle() already has a 'group' argument. Its
>current semantics are:
>
>   - bps or iops != 0   -> set the I/O limits of a throttling group. The
>                           selected device is moved to that group if it
>                           wasn't there yet.
>
>   - bps and iops == 0  -> remove a device from a throttling group
>                           without touching that group's I/O limits.
>
>where 'device' is mandatory and 'group' defaults to the value of
>'device' if unset.
>
>If I get it right, what you propose is that 'device' is optional and in
>that case 'group' is mandatory and the command updates the throttling
>group parameters.
>
>Note that you can do that already if you select a device that is already
>in the group. These new semantics would allow modifying a group that is
>not being used by any block device.

That was my thinking as well.
>
>I'm not a big fan of overloading block_set_io_throttle() even more, but
>this proposal does actually sound fine.
>
>> Users can hotplug ThrottleGroups with object-add in QMP/HMP, then
>> attach drives to a throttle group (ie adding a filter node in the
>> drive path that points to that throttle group) with an
>> 'attach-throttle' and detach with 'detach-throttle', each taking the
>> drive and throttle group name as arguments. An alternative (but
>> uglier) approach would be to use 'block_set_io_throttle' for this as
>> well.
>
>Yeah I agree that the block_set_io_throttle() approach would be uglier.
>
>> Something that's also sound is a 'block_set_throttle_groups' that
>> takes a list of groups and a drive as arguments.
>
>Would that simply be a foreach(g, groups) { attach(device, g); } ?

Something like 

foreach(g, groups) { attach(device, g); }
foreach(g, device.groups - groups) { detach(device, g); } #where '-' is 
the set minus operation)

This would mean you would have to redefine previous groups as well 
everytime you use 'block_set_throttle_groups' otherwise they would get 
removed from the device. At least that's a way to avoid using more 
commands for attaching/detaching.
>
>> Regarding 'query-block'. This case is complicated by the fact that it
>> would be possible for a drive to have many nodes, thus the values
>> printed by 'query-block' might be inaccurate if they only report one
>> of the node configurations.  The current approach is to report only
>> the first filter node in the BDS tree. A user with advanced throttling
>> configurations might have no need for this command.
>
>I guess the user can use 'query-block' and walk the backing chain
>looking for throttling filter nodes?

I think it would be trivial to do this. Isn't the BDS tree always 
"short"? Otherwise the filter nodes should be cached in BlockBackend for 
performance, if it can be argued that it's needed.
>
>Berto
>

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Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Kevin Wolf]

Am 08.06.2017 um 20:21 hat Manos Pitsidianakis geschrieben:
> In continuation of the discussion at [0], here follows a proposal
> for a new ThrottleGroup object defined through QOM.
> 
> Throttle groups are currently defined when instantiating a drive in
> the command line,, eg.  -drive
> file=foo.qcow2,throttling.iops-total=100,throttling.group=bar
> and also on QMP with the 'block_set_io_throttle' command, eg:
> 
>   { "execute": "block_set_io_throttle",
>     "arguments": {
>        "device": "virtio0",
>        "iops": 100,
>        "iops_rd": 0,
>        "iops_wr": 0,
>        "bps": 0,
>        "bps_rd": 0,
>        "bps_wr": 0
>     }
>   }
> 
> As part of my GSoC project, IO throttling is moved to a block filter
> driver, which means multiple filter nodes sharing ThrottleGroup
> instances, and also, chaining filter nodes. The old interface will
> be retained as much as possible. By defining ThrottleGroups with QOM
> the following interface can be used:
>    -drive file=foo.qcow2,throttling.group=bar \
>    -object throttle-group,iops-total=100,id=bar
> which will be equivalent to the current command above.

Looks okay. This is a "mixed" configuration, where modern -object
throtte-group is used, but still legacy -drive. We'll keep supporting
this at least for compatibility, but possibly also as a convenience
feature. Whether we consider it just a legacy feature or a convenience
feature for manual users determines whether new functionality needs to
be supported with it.

libvirt is expected to go all the way and use -blockdev driver=throttle
explicitly together with -object.

> The 'block_set_io_throttle' command can be extended with a "group"
> argument, that when defined instead of "device", will find and edit
> the object in question. Users can hotplug ThrottleGroups with
> object-add in QMP/HMP, then attach drives to a throttle group (ie
> adding a filter node in the drive path that points to that throttle
> group) with an 'attach-throttle' and detach with 'detach-throttle',
> each taking the drive and throttle group name as arguments.

Let's not do more automagic graph manipulations than are strictly needed
to maintain compatibility with the existing interfaces. We will have to
automatically insert or remove new nodes if 'block_set_io_throttle' is
used, but let's not extend it. It only makes things complicated.

If people are using new features, they will be using new interfaces and
we can make them use more natural syntax. For example, we can expect
that they create throttle nodes manually (management tools always, human
users at least for more complicated cases). What we may want is to
update the settings of a throttle node at runtime (though inserting a
new filter and then removing the old one could do the trick as well).

If we want to change the settings of a throttle node, then I think the
appropriate low-level interface is bdrv_reopen(). We need to figure out
whether we can expose a full bdrv_reopen() on QMP or whether we need
more specific commands. I'd prefer finally doing the "real thing" rather
than adding more and more special-case commands to update nodes.

> An alternative (but uglier) approach would be to use
> 'block_set_io_throttle' for this as well. Something that's also
> sound is a 'block_set_throttle_groups' that takes a list of groups
> and a drive as arguments. I think the last could be best if the
> relevant function does not become complex.
> All this means that existing configuration will not be redefined
> everytime the object is referenced unless each limit is specified
> explicitly, like it was mentioned in the previous discussion.
> 
> Regarding 'query-block'. This case is complicated by the fact that
> it would be possible for a drive to have many nodes, thus the values
> printed by 'query-block' might be inaccurate if they only report one
> of the node configurations.  The current approach is to report only
> the first filter node in the BDS tree. A user with advanced
> throttling configurations might have no need for this command.

I maintain my stance that I express off-list, which is that query-block
should _only_ display limits that belong to the throttle node that was
internally created for satisfying a 'block_set_io_throttle' request on a
BlockBackend.

If the user created a throttle node themselves, they already know about
it and can inspect the throttle node directly.

> I'm not certain about this, since this could be handled internally
> by an application, but it might be useful to have queries to list
> all the throttle groups that affect a drive, and to list all the
> drives in a throttle group. Maybe defining the drives as a
> ThrottleGroup QOM property will take care of the latter and also
> attaching/detaching drives to a group through the property
> getter/setter. Thoughts?

I'm not sure if you aren't taking a somewhat simplified view here. Let's
look at a still relatively simple setup where we have a local qcow2 file
with a backing file that is accessed over the network.

    virtio-blk
        |
        |
        v
   BlockBackend
        |
        |
        v     (bs->backing)
      qcow2  ---------------->  raw
        |                        |
        |(bs->file)              |(bs->file)
        v                        v
    file-posix                  http

You have four arrows below BlockBackend, and a filter node (or multiple)
could be inserted on each of them, resulting in different behaviour. All
of them arguably affect the drive (virtio-blk), but does a list where
all of them are treated the same really make sense?

I can make things a bit more interesting still by adding an NBD server
just for fun. Let's expose the raw backing file there, we always wanted
to have a HTTP-to-NBD bridge.

    virtio-blk               NBD server
        |                        |
        |                        |
        v                        v
   BlockBackend             BlockBackend
        |                        |
        |                        |
        v     (bs->backing)      |
      qcow2  ---------------->  raw
        |                        |
        |(bs->file)              |(bs->file)
        v                        v
    file-posix                  http

Now a throttle node between raw and http isn't clearly related to just
virtio-blk any more, it throttles requests that come from two sources.
Would you still include it in the list?

What if we additionally insert a throttle node below the NBD server's
BlockBackend? It will indirectly affect virtio-blk by giving it a higher
share of the bandwith because the NBD server uses less of it. Is this a
node that your list considers as affecting virtio-blk?


We could probably think of more complicated setups, but I think we
should really leave such decisions to the user. If they inserted the
throttle nodes, they can as well inspect them one by one. The only case
where the user can't do this is if they don't know that the nodes exist,
i.e. when they are using legacy interfaces.


While we're talking about the graph, did you consider changes to the
graph yet that are done by other operations? For example, if you have a
simple setup with throttling today, this looks like:

    BlockBackend (+ throttling) --> raw --> file-posix

If you take a snapshot, you get a new qcow2 node inserted that has the
old top-level node as its backing file:

    BlockBackend (+ throttling) --> qcow2 --> file-posix
                                      |
                                      +--> raw --> file-posix

However, if the throttling is done by a filter node, if we're not
careful, we'll end up with the following (which is not a transparent
implementation of the old semantics):

    BlockBackend --> qcow2 --> file-posix
                       |
                       +--> throttle --> raw --> file-posix

The throttle node affects only the backing file now, but not the new
top-level node. With manually created filter nodes we may need to
provide a way so that the QMP command tells where in the tree the
snapshot is actually taken. With automatically created ones, we need to
make sure that they always stay on top.

Similar problems arise when block jobs manipulate the graph. For
example, think of a commit block job, which will remove the topmost node
from the backing file chain. We don't want it to remove the throttling
filter together with the qcow2 node. (Or do we? It might be something
that needs to be configurable.)

We also have several QMP that currently default to working on the
topmost image of the chain. Some of them may actually want to work on
the topmost node that isn't an automatically inserted filter.


I hope this helps you understand why I keep saying that automatic
insertion/removal of filter nodes is tricky and we should do it as
little as we possibly can.

Kevin

Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Alberto Garcia]

On Fri 09 Jun 2017 06:06:09 PM CEST, Manos Pitsidianakis wrote:

>>>     -drive file=foo.qcow2,throttling.group=bar \
>>>     -object throttle-group,iops-total=100,id=bar
>>
>>, and I guess the name would be throttling-group?
>
> Hm, I thought throttle-group would be nicer because it's shorter, but
> I can change it, of course. I am not certain if there's a semantic
> difference (because I am not a native speaker).

I'm not a native speaker either, I was pointing out that we would have
throttling.group for -drive and then the throttle-group object.

Berto

Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Stefan Hajnoczi]

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On Thu, Jun 08, 2017 at 09:21:13PM +0300, Manos Pitsidianakis wrote:
> The old interface will be retained as much as possible.

I think "retained" means "reused" here but in case it doesn't:

The old interface must remain 100% functional.  Existing software like
libvirt must continue to work without any changes.  QEMU releases are
backwards compatible except for a long-term deprecation schedule.

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Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Stefan Hajnoczi]

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On Thu, Jun 08, 2017 at 09:21:13PM +0300, Manos Pitsidianakis wrote:
> Users can hotplug ThrottleGroups with object-add in QMP/HMP, then
> attach drives to a throttle group (ie adding a filter node in the drive path
> that points to that throttle group) with an 'attach-throttle' and detach
> with 'detach-throttle', each taking the drive and throttle group name as
> arguments.

BDS nodes are added/removed with blockdev-add/blockdev-del.  Why are new
attach-throttle/detach-throttle commands necessary?

Stefan

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Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Stefan Hajnoczi]

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On Mon, Jun 12, 2017 at 02:37:34PM +0200, Alberto Garcia wrote:
> On Fri 09 Jun 2017 06:06:09 PM CEST, Manos Pitsidianakis wrote:
> 
> >>>     -drive file=foo.qcow2,throttling.group=bar \
> >>>     -object throttle-group,iops-total=100,id=bar
> >>
> >>, and I guess the name would be throttling-group?
> >
> > Hm, I thought throttle-group would be nicer because it's shorter, but
> > I can change it, of course. I am not certain if there's a semantic
> > difference (because I am not a native speaker).
> 
> I'm not a native speaker either, I was pointing out that we would have
> throttling.group for -drive and then the throttle-group object.

I prefer "throttling-group" but it's not a big deal either way.

Stefan

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Re: [Qemu-devel] [Proposal] I/O throttling through new -object interface

[Manos Pitsidianakis]

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On Mon, Jun 12, 2017 at 01:15:43PM +0200, Kevin Wolf wrote:
>Am 08.06.2017 um 20:21 hat Manos Pitsidianakis geschrieben:
>to maintain compatibility with the existing interfaces. We will have to
>automatically insert or remove new nodes if 'block_set_io_throttle' is
>used, but let's not extend it. It only makes things complicated.
>
>If people are using new features, they will be using new interfaces and
>we can make them use more natural syntax. For example, we can expect
>that they create throttle nodes manually (management tools always, human
>users at least for more complicated cases). What we may want is to
>update the settings of a throttle node at runtime (though inserting a
>new filter and then removing the old one could do the trick as well).
>
>If we want to change the settings of a throttle node, then I think the
>appropriate low-level interface is bdrv_reopen(). We need to figure out
>whether we can expose a full bdrv_reopen() on QMP or whether we need
>more specific commands. I'd prefer finally doing the "real thing" rather
>than adding more and more special-case commands to update nodes.

So basically, restrict our involvement in the graph with 
'block_set_io_throttle', which will only deal with editing the node at 
the top of the tree, just below BB? (Unless the group name instead of 
device name is specified). Can the user sufficiently manage a complex 
configuration on their own with existing interfaces (ie blockdev-add)?  

>I maintain my stance that I express off-list, which is that query-block
>should _only_ display limits that belong to the throttle node that was
>internally created for satisfying a 'block_set_io_throttle' request on a
>BlockBackend.

Assuming the answer to my first question is yes, that will always be the 
throttle node below the queried BB.

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