Re: [Xen-devel] [RFC] [Design] Better XL support of RTDS scheduler for Xen 4.6

[Meng Xu <xumengpanda@xxxxxxxxx>]

[Adding Denys and Andri from globallogic  for their advice about the
user interface and why we need such functionality]

2015-02-21 10:51 GMT-05:00 Meng Xu <xumengpanda@xxxxxxxxx>:
> Hi all,
>
> This is for Xen 4.6: Enabling XL to set the parameters of each
> individual VCPU of a domain for RTDS scheduler.
>
> [Goal]
> Right now, xl sched-rtds tool can only set the VCPUs of a domain to
> the same parameter although the scheduler did support VCPUs with
> different parameters. This work is to provide the âxl sched-rtdsâ tool
> the ability to configure the VCPUs of a domain with different
> parameters.
>
> After this has been implemented, an example of the functionality of XL
> will look like this:
>
> #xl sched-rtds
> Cpupool Pool-0: sched=RTDS
> Name                                ID VCPU Period Budget
> Domain-0                             0    0  10000  10000
> Domain-0                             0    1  20000  20000
> Domain-0                             0    2  30000  30000
> Domain-0                             0    3  10000  10000
> litmus1                              1    0  10000   4000
> litmus1                              1    1  10000   4000
> //comment: VCPU is the index of the VCPUs in the domain. The
> parameters of VCPUs in the same domain are different, while they are
> the same in Xen 4.5.
>
> //set the parameters of the vcpu 1 of domain litmus1:
> # xl sched-rtds -d litmus1 -v 1 -p 20000 -b 10000
>
> //domain litmus1's vcpu 1's parameters are changed, display each VCPU's
> parameters separately:
> # xl sched-rtds -d litmus1
> Name                                ID VCPU Period Budget
> litmus1                              1    0  10000   4000
> litmus1                              1    1  20000  10000
>
> [Design sketch]
> We submitted the patch along with the RTDS scheduler before last year.
> The thread can be found at
> http://lists.xen.org/archives/html/xen-devel/2014-08/msg02245.html.
>
> In order to implement the get function, which return the parameters of
> all VCPUs of the same domain, we can bounce an array that has all
> parameters of all VCPUs of the domain from hypervisor to userspace and
> output it. (This part didn't cause much buzz. :-) )
> The other choice is using a hypercall to get the information of each
> VCPU. If there are x VCPUs in a domain, we will issue x hypercalls to
> get all information of this domain. This choice causes more overhead
> since the number of hypercalls is linear to the number of VCPUs in a
> domain. The first choice only needs one hypercall for a domain to get
> all needed information.
>
> In order to implement the set function, which set the parameters of
> one specific VCPU of a domain, we have two different approaches to
> implement it:
> Choice 1): When users set the parameters of k^th VCPUs of a domain,
> which has x VCPUs in total, the toolstack passes "only" the modified
> VCPUsâ information from userspace to hypervisor via hypercall;  So
> this implementation does "not" need an array to bounce between
> userspace and hypervisor. (The implementation in lilbxc was at
> http://lists.xen.org/archives/html/xen-devel/2014-08/msg02248.html)
> Choice 2): When users set the parameters of k^th VCPUs of a domain,
> which has x VCPUs in total, the toolstack will create an array with
> length x and set the information (period, budget) of the k^th element
> in the array. The other elements in the array are marked as 0. After
> the toolstack bounce the array into the hypervisor, it will only set
> the parameters of the k^th VCPU.
>
> My questions are:
> 1) Should we allow users to set the parameters of "several" VCPUs with
> one command:
> For example, should we allow users to set the parameters of VCPU 1 and
> 2 of domain litmus1 with one xl sched-rtds command?
> # xl sched-rtds -d litmus1 -v 1 -p 20000 -b 10000 -v 2 -p 30000 -b 20000
> IMO, we'd better not allow them to do this because 1) setting the
> parameters of VCPUs of a domain should not be very frequent; 2) the
> command will become quite long when users want to set the parameters
> of many VCPUs of a domain.
>
> 2) Which design choice should we choose to implement the set function,
> Choice 1 or Choice 2? or do you have better suggestion?
>
> Thank you very much!
>
> best,
>
> Meng
> -----------
> Meng Xu
> PhD Student in Computer and Information Science
> University of Pennsylvania

Meng

-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania

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