Subject: hard scheduler, was Re: [vserver] Disk limits problem?
From: ADNET Ghislain <gadnet@aqueos.com>
Date: Fri, 11 Sep 2009 18:45:26 +0200
Fri, 11 Sep 2009 18:45:26 +0200

> i ask because i'm using 2.6.27 with cgroup cpu scheduling [1] and have
> found it comparable to the description of hard limit + idle time [2] (as i
> never got it to work).
>
> [1] http://linux-vserver.org/util-vserver:Cgroups
> [2] http://linux-vserver.org/CPU_Scheduler
>   
the hard scheduler let you put hard limits on a guest. with cgroups it 
seems to me you can only balance the load. IE if one month 9 out of 10 
guest does nothing the 10th will have 100% of the cpu and as the 9 other 
guest  start to do things his cpu share will lower to 1/10. If all the 
guest are for you then all is good but..

  This could bring issue when you ressell part of this cpu time as 
customers will have widly different experiences betwee the 10% an 100% 
of the cpu without knowing why. With hard limit you could give them a 
hard limit a 15% cpu so that they gain a little but cannot use 100% even 
if the whole server is idle. Limiting the "but yesterday it worked fine 
and i do not changed anything and we have the same amount of visitors 
!!!" effet to the hotline.

  the other possibility that is not cpu nor planet friendly is to run 
one guest with cpuburn on it so that you consume allway the idle time of 
the cgroups forcing the limit as a side effect. Of course hard scheduler 
will be the prefered way ;)

  Most of the time vserver stuff is like ethernet: beautifully simple 
and efficient. cgroups looks like token ring you know, not those strnage 
just send a packet and pray for no collision thing. At the end 
simplicity win the day but the kernel is such a moving target i think we 
will allways have trouble getting the funding to allow herbert and  
Daniel to spend more time on it. I think until a good sized sponsor 
comes up it will be hard to have the full feature set following the 
incredible change rate the kernel has in this particular area.

-- 
Cordialement,
Ghislain


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