Re: RFC: I/O bandwidth controller (was Re: Too many I/O controller patches)

Fernando Luis Vázquez Cao wrote:
This seems to be the easiest part, but the current cgroups
infrastructure has some limitations when it comes to dealing with block
devices: impossibility of creating/removing certain control structures
dynamically and hardcoding of subsystems (i.e. resource controllers).
This makes it difficult to handle block devices that can be hotplugged
and go away at any time (this applies not only to usb storage but also
to some SATA and SCSI devices). To cope with this situation properly we
would need hotplug support in cgroups, but, as suggested before and
discussed in the past (see (0) below), there are some limitations.

Even in the non-hotplug case it would be nice if we could treat each
block I/O device as an independent resource, which means we could do
things like allocating I/O bandwidth on a per-device basis. As long as
performance is not compromised too much, adding some kind of basic
hotplug support to cgroups is probably worth it.

(0) http://lkml.org/lkml/2008/5/21/12
What about using major,minor numbers to identify each device and account
IO statistics? If a device is unplugged we could reset IO statistics
and/or remove IO limitations for that device from userspace (i.e. by a
deamon), but pluggin/unplugging the device would not be blocked/affected
in any case. Or am I oversimplifying the problem?
If a resource we want to control (a block device in this case) is
hot-plugged/unplugged the corresponding cgroup-related structures inside
the kernel need to be allocated/freed dynamically, respectively. The
problem is that this is not always possible. For example, with the
current implementation of cgroups it is not possible to treat each block
device as a different cgroup subsytem/resource controlled, because
subsystems are created at compile time.

The whole subsystem is created at compile time, but controller data
structures are allocated dynamically (i.e. see struct mem_cgroup for
memory controller). So, identifying each device with a name, or a key
like major,minor, instead of a reference/pointer to a struct could help
to handle this in userspace. I mean, if a device is unplugged a
userspace daemon can just handle the event and delete the controller
data structures allocated for this device, asynchronously, via
userspace->kernel interface. And without holding a reference to that
particular block device in the kernel. Anyway, implementing a generic
interface that would allow to define hooks for hot-pluggable devices (or
similar events) in cgroups would be interesting.

3. & 4. & 5. - I/O bandwidth shaping & General design aspects

The implementation of an I/O scheduling algorithm is to a certain extent
influenced by what we are trying to achieve in terms of I/O bandwidth
shaping, but, as discussed below, the required accuracy can determine
the layer where the I/O controller has to reside. Off the top of my
head, there are three basic operations we may want perform:
- I/O nice prioritization: ionice-like approach.
- Proportional bandwidth scheduling: each process/group of processes
has a weight that determines the share of bandwidth they receive.
- I/O limiting: set an upper limit to the bandwidth a group of tasks
can use.
Use a deadline-based IO scheduling could be an interesting path to be
explored as well, IMHO, to try to guarantee per-cgroup minimum bandwidth
requirements.
Please note that the only thing we can do is to guarantee minimum
bandwidth requirement when there is contention for an IO resource, which
is precisely what a proportional bandwidth scheduler does. An I missing
something?

Correct. Proportional bandwidth automatically allows to guarantee min
requirements (instead of IO limiting approach, that needs additional
mechanisms to achive this).

In any case there's no guarantee for a cgroup/application to sustain
i.e. 10MB/s on a certain device, but this is a hard problem anyway, and
the best we can do is to try to satisfy "soft" constraints.

-Andrea
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