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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Mar 01, 2010 at 12:04:20AM +0100, benco wrote:
> 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?
> 

You're measuring buffered (cached) performance, so when you limit
the amount of cache to 2 GB the performance will be slower.

Try adding "oflag=direct" do your dd commands and test again.

-- Pasi


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