[Xen-devel] Re: RFC: I/O bandwidth controller

[Hirokazu Takahashi <taka@xxxxxxxxxxxxx>]

Hi, Naveen,

> > If we are pursuing a I/O prioritization model à la CFQ the temptation is
> > to implement it at the elevator layer or extend any of the existing I/O
> > schedulers.
> >
> > There have been several proposals that extend either the CFQ scheduler
> > (see (1), (2) below) or the AS scheduler (see (3) below). The problem
> > with these controllers is that they are scheduler dependent, which means
> > that they become unusable when we change the scheduler or when we want
> > to control stacking devices which define their own make_request_fn
> > function (md and dm come to mind). It could be argued that the physical
> > devices controlled by a dm or md driver are likely to be fed by
> > traditional I/O schedulers such as CFQ, but these I/O schedulers would
> > be running independently from each other, each one controlling its own
> > device ignoring the fact that they part of a stacking device. This lack
> > of information at the elevator layer makes it pretty difficult to obtain
> > accurate results when using stacking devices. It seems that unless we
> > can make the elevator layer aware of the topology of stacking devices
> > (possibly by extending the elevator API?) evelator-based approaches do
> > not constitute a generic solution. Here onwards, for discussion
> > purposes, I will refer to this type of I/O bandwidth controllers as
> > elevator-based I/O controllers.
> 
> It can be argued that any scheduling decision wrt to i/o belongs to
> elevators. Till now they have been used to improve performance. But
> with new requirements to isolate i/o based on process or cgroup, we
> need to change the elevators.
> 
> If we add another layer of i/o scheduling (block layer I/O controller)
> above elevators
> 1) It builds another layer of i/o scheduling (bandwidth or priority)
> 2) This new layer can have decisions for i/o scheduling which conflict
> with underlying elevator. e.g. If we decide to do b/w scheduling in
> this new layer, there is no way a priority based elevator could work
> underneath it.

I seems like the same goes for the current Linux kernel implementation
that if processes issued a lot of I/O requests and the io-request queue
of a disk is overflowed, all the I/O requests after will be blocked
and the priorities of them are meaningless.
In other word, it won't work if it receives lots of requests more than
the ability/bandwidth of a disk.

It doesn't seem so weird if it won't work if a cgroup issues lots of
I/O requests more than the bandwidth which is assigned to the cgroup.

> If a custom make_request_fn is defined (which means the said device is
> not using existing elevator), it could build it's own scheduling
> rather than asking kernel to add another layer at the time of i/o
> submission. Since it has complete control of i/o.

Thanks,
Hirokazu Takahashi


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