Re: Bandwidth Allocations under CFQ I/O Scheduler

[Ric Wheeler <ric@emc.com>]

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.