Re: [Xen-users] VCPU amount

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

2009/3/25 Vladislav Karpenko <vladislav@xxxxxxxxxxxxxx>:
>>> is it
>>> good to make more vcpu than physical processors
>>
>> No.
>
>
> Thank u, but how many vcpus must be totaly asigned to domus if i have 8
> cores?

Depends on what you need, actually.
In my case I always dedicate one core (core 0) to dom0, plus limit its
initial memory usage. This is done using grub.conf like this (see
dom0_mem and dom0_vcpus_pin)

#=========================
title Red Hat Enterprise Linux Server (2.6.18-128.1.1.el5xen)
        root (hd0,0)
        kernel /xen.gz-3.3.1 com1=115200,8n1 console=vga,com1 dom0_mem=512M
dom0_vcpus_pin
        module /vmlinuz-2.6.18-128.1.1.el5xen ro root=/dev/rootVG/rootLV rhgb
quiet console=vga console=ttyS0
        module /initrd-2.6.18-128.1.1.el5xen.img
#=========================

and these settings on /etc/xen/xend-config.sxp

#=========================
(dom0-min-mem 256)
(dom0-cpus 1)
#=========================

combined, both settings ensure that dom0 only uses 256-512MB memory,
plus it only uses cpu0. On all domU config I use something like this
(relevant cpu settings for windows HVM)

vcpus=1
cpus="1-7"
cpu_cap=100
cpu_weight=128

it means I only use 1 vcpu, and it can be in cpu 1-7 (thus leaving
cpu0 for dom0 exclusive usage). This ensures dom0 can handle disk and
network requests for domUs at all times.

If you want to give some domU priority over another, adjust cpu_weight
as necessary.

Note that on Windows HVM you probably need GPLPV to have a usable
performance. However when using GPLPV,  I/O throughput with vcpus >1
is lower when compared to vcpus=1 (although it's still MUCH higher
compared to emulated QEMU drivers). So you see, adding vcpu does not
always mean higher performance.

You need to experiment and see which settings suit you best. Probably
vcpus=1 for db server and vcpus=6 for terminal server (that is
assuming your windows version can handle 6 cpus).

Regards,

Fajar

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