[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [BUG] unfairness in Xen's credit scheduler
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; ------- ... } Particularly, [prv->credits_per_tslice] is very important to maintain the *fairness* of credit allocation in csched_acct(). This bug has been there since Xen-4.2.0. Thanks, CHENG Luwei -- PhD student Department of Computer Science The University of Hong Kong Homepage: http://www.cs.hku.hk/~lwcheng _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |