Re: [Xen-devel] [BUG] unfairness in Xen's credit scheduler

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Sep 2, 2013 at 10:54 AM,  <lwcheng@xxxxxxxxx> wrote:
> Hi all,
>
> Since Xen 4.2.0, users can change time slice of the scheduler at
> runtime via xl command line, a very nice feature. However, it is
> not *correctly* implemented.
>
> Problem description
> --------------------
> say you set the 'cap' of one VM to 50 (a half core),
> -when setting the time slice to be *greater* than 30ms, the VM gets
> much *less* CPU cycles than its allocation
> -when setting the time slice to be *smaller* than 30ms, the VM gets
> much *more* CPU cycles than its allocation
>
>
> Problem happens in sched_credit.c/csched_sys_cntl():
> --------------------
> after changing prv->tslice_ms, other parameters *should* also be
> changed accordingly:
>
> csched_sys_cntl() {
>     ...
>     prv->tslice_ms = params->tslice_ms;
>     prv->ratelimit_us = params->ratelimit_us;
> -------
>     /* my patch: these parameters should also be changed */
>     prv->ticks_per_tslice = CSCHED_TICKS_PER_TSLICE;
>     if ( prv->tslice_ms < prv->ticks_per_tslice )
>         prv->ticks_per_tslice = 1;
>     prv->tick_period_us = prv->tslice_ms * 1000 / prv->ticks_per_tslice;
>     prv->credits_per_tslice = CSCHED_CREDITS_PER_MSEC * prv->tslice_ms;

Oh, right -- sorry, that was my patch, and I don't know what I was thinking.

If you have time, could you make a patch which creates a function to
do this, and have both SCHEDOP_put_into and csched_init() call it?
That way we avoid duplicating the code.

Some hints for making a good patch and sending it to the list can be found here:

http://wiki.xen.org/wiki/Submitting_Xen_Patches

Thanks!
 -George

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