Rob has answered your second question quite well.
As to the first one -- whether to use QUICKDSP for Linux guests -- you first
need to know what QUICKDSP does. It overrides the CP Scheduler's decisions
on whether to admit a virtual machine to the Dispatch List (contending for
resources) or to hold it in the Eligible List (wants to contend for
resources but prohibited due (usually) to lack of storage).
When a QUICKDSP user wants to consume resources, it is automatically
admitted to the Disp List. Judicious use of this setting means that
critical virtual machines always get a shot at resources when they need
them. Overuse of QUICKDSP may cause severe overcommitment of central
storage leading to a situation called page thrash, where little or no
productive work gets done because the system spends all of its resources
moving user pages in and out of storage.
One final note: Just because a virtual machine has QUICKDSP set doesn't
mean it will receive sufficient resources. That's controlled by the SHARE
settings assigned to the various virtual machines in the Dispatch List. And
a discussion of SHARE values is a whole different animal.
Marty
____________________
Martin Zimelis
Principal
maz/Consultancy
> -----Original Message-----
> From: The IBM z/VM Operating System
> [mailto:[email protected]] On Behalf Of Magat, Martin
> Sent: Thursday, October 29, 2009 8:39 PM
> To: [email protected]
> Subject: Re: QUICKDSP and DSPBUF
>
> Hi
>
> I have two queries and kindly hoping I can get some advise:
>
> 1) Is it recommended to define QUICKDSP for high-priority
> zLinux server
> in a resource constraint environment?
> 2) If the DSPBUF values are, DSPBUF : Q1=100 Q2=75 Q3=60, what will be
> the impact to the system (because the default 32767 is I presumed the
> recommended one)?
>
> Many Thanks.
>
> Martin
