Re: Bandwidth Allocations under CFQ I/O Scheduler

[Ric Wheeler]

Jens Axboe wrote:
> On Tue, Oct 17 2006, Arjan van de Ven wrote:
>
> > On Mon, 2006-10-16 at 16:46 -0400, Phetteplace, Thad (GE Healthcare,
> > consultant) wrote:
> >
> > > The I/O priority levels available under the CFQ scheduler are
> > > nice (no pun in intended), but I remember some talk back when
> > > they first went in that future versions might include bandwidth
> > > allocations in addition to the 'niceness' style.  Is anyone out
> > > there working on that?  If not, I'm willing to hack up a proof
> > > of concept... I just wan't to make sure I'm not reinventing
> > > the wheel.
> >
> >
> > Hi,
> >
> > it's a nice idea in theory. However... since IO bandwidth for seeks is
> > about 1% to 3% of that of sequential IO (on disks at least), which
> > bandwidth do you want to allocate? "worst case" you need to use the
> > all-seeks bandwidth, but that's so far away from "best case" that it may
> > well not be relevant in practice. Yet there are real world cases where
> > for a period of time you approach worst case behavior ;(
>
>
> Bandwidth reservation would have to be confined to special cases, you
> obviously cannot do it "in general" for the reasons Arjan lists above.
> So you absolutely have to limit any meta data io that would cause seeks,
> and the file in question would have to be laid out in a closely
> sequential fashion. As long as the access pattern generated by the app
> asking for reservation is largely sequential, the kernel can do whatever
> it needs to help you maintain the required bandwidth.
>
> On a per-file basis the bandwidth reservation should be doable, to the
> extent that generic hardware allows.

I agree - bandwidth allocation is really tricky to do in a useful way.

On one hand, you could "time slice" the disk with some large quanta as 
we would do with a CPU to get some reasonably useful allocation for 
competing, streaming workloads.

On the other hand, this kind of thing would kill latency if/when you hit 
any synchronous writes (or cold reads).

One other possible use for allocation is throttling a background 
workload (say, an interative checker for a file system or some such 
thing) where the workload can run effectively forever, but should be 
contained to not interfere with foreground workloads. A similar time 
slice might be used to throttle this load done unless there is no 
competing work to be done.




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