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
