Re: [PATCHSET] blk-throttle: implement proper hierarchy support

[Vivek Goyal]

On Thu, May 02, 2013 at 11:44:26AM -0700, Tejun Heo wrote:

[..]
> Also, if you're actually thinking about reimplementing blk-throttle,
> please do consider the followings.
> 
> * Currently, blk-throttle doesn't throttle the number of bios being
>   queued.  Note that this breaks the basic back-pressure mechanism
>   where IO pressure is propagated back to the issuer by throttling the
>   issuing task.  blk-throttle breaks that link and converts it to a
>   memory pressure.

Agreed. Implementing something along the lines of per group nr_requests
is needed.

> 
> * It's almost inherently unscalable with highops devices.  Given that
>   IO limiting doesn't require very fine granularity, I think doing
>   this per-cpu shouldn't be too hard.  e.g. build a per-cpu token
>   distributing hierarchy with rebalancing across CPUs happening
>   periodically.

Interesting. I thought for highops devices we will these multi queue
patches from jens and there we can probably implement per queue tokens
and rebalance tokens across queues periodically.

> 
> In short, right now, the goal is getting the hierarchy support
> acceptably working ASAP and yeap we wanna get the nested limits and at
> least certain level of fairness, but let's please implement something
> simple for now and strive for sophistification later because it's
> holding back everyone else.

I am fine with this. Having some hierarchical algorithm and not blocking
full hierarchical support for cgroup is better than not having any
hierachical support and wait for better implementation.

Thanks
Vivek
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