[Xen-users] slow dom0 write operations with 2.6.32 pv-ops and Xen4.0.0-rc4

[benco <benco@xxxxxxx>]

Hi,

i did a simple write test on my system and found out that there is a big
performance difference regarding to write operations between non-xen and xen
kernels.

HW is dual 4-core Xeon5570 with 32GB RAM and FC attached storage with SATA
drives (RAID5).

SW is linux 2.6.32 pv-ops with Xen4.0.0-rc4 on Debian Lenny. There were no
VMs running. I did simple 'dd' write test with 3GB file on LVM partition.


dom0 with 2GB RAM and 2CPU
==========================

linux2.6.32 pv-ops + Xen4.0.0-rc4

title           Xen 4.0.0-rc4 / Debian GNU/Linux, kernel 2.6.32
root            (hd0,0)
kernel          /xen-4.0.0-rc4.gz dom0_mem=2048M dom0_max_vcpus=2
module          /vmlinuz-2.6.32 root=/dev/sda6 ro console=tty0
module          /initrd.img-2.6.32

#xm vcpu-pin Domain-0 0 0
#xm vcpu-pin Domain-0 1 1
#time dd if=/dev/zero of=zerofile.tst bs=1k count=3145728
3145728+0 records in
3145728+0 records out
3221225472 bytes (3.2 GB) copied, 48.4788 s, 66.4 MB/s

real    0m48.531s
user    0m1.940s
sys     0m12.745s



dom0 with no memory and cpu limit
=================================

linux2.6.32 pv-ops + Xen4.0.0-rc4 

title           Xen 4.0.0-rc4 / Debian GNU/Linux, kernel 2.6.32
root            (hd0,0)
kernel          /xen-4.0.0-rc4.gz
module          /vmlinuz-2.6.32 root=/dev/sda6 ro console=tty0
module          /initrd.img-2.6.32

#time dd if=/dev/zero of=zerofile.tst bs=1k count=3145728
3145728+0 records in
3145728+0 records out
3221225472 bytes (3.2 GB) copied, 15.7325 s, 205 MB/s

real    0m15.735s
user    0m2.312s 
sys     0m13.273s


bare metal
==========

linux2.6.32 pv-ops

title           Debian GNU/Linux, kernel 2.6.32
root            (hd0,0)
kernel          /vmlinuz-2.6.32 root=/dev/sda6 ro
initrd          /initrd.img-2.6.32

#time dd if=/dev/zero of=zerofile.tst bs=1k count=3145728
3145728+0 records in
3145728+0 records out
3221225472 bytes (3.2 GB) copied, 6.42111 s, 502 MB/s

real    0m6.422s
user    0m0.304s
sys     0m6.024s

Any idea what to check/modify to improve?

thanks,

roman

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