Status of Credit2 and Credit1 [was: Re: Xen 4.6 Development Update (four months reminder)]

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Wed, 2015-05-13 at 06:01 +0100, wei.liu2@citrix.com wrote:

> = Prognosis =
> 
> The states are: none -> fair -> ok -> good -> done
> 
> none - nothing yet
> fair - still working on it, patches are prototypes or RFC
> ok   - patches posted, acting on review
> good - some last minute pieces
> done - all done, might have bugs

> == Hypervisor == 

> *  Credit2 production ready (none)
>    cpu reservation
>   -  George Dunlap
>
No work on this yet, AFAIK. I may be able to look into it in a couple of
weeks, but I still am not sure.

> === Hypervisor X86 === 

> *  HT enabled with credit has 7.9 per perf drop. (none)
>    kernbench demonstrated it
>    http://www.gossamer-threads.com/lists/xen/devel/339409
>    This has existed since credit1 introduction.
>   -  Dario Faggioli
>
Let me clarify this a bit.

First of all, let me point out that the issue only shows up in
under-loaded systems (i.e., with less active vcPUs then the host has
pCPUs). In any case, this is something inherent in Credit1 architecture,
and I don't have a sensible idea on how to deal with it without causing
other problems.

I am working on some improvement to Credit1 (e.g., introducing load
tracking and basing some decisions on it), which hopefully will help
better performance even in this case, but I'm not actively working on
fixing this (if it can be fixed).

Regards,
Dario

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