[Qemu-devel] QEMU throughput is down with SMP

["Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>]

Code: Mainline QEMU (git://git.qemu.org/qemu.git)
Machine: LS21 blade.
Disk: Local disk through VirtIO.
Did not select any cache option. Defaulting to writethrough.

Command tested:
3 parallel instances of : dd if=/dev/zero of=/pmnt/my_pw bs=4k count=100000

QEMU with  smp=1
19.3 MB/s + 19.2 MB/s + 18.6 MB/s = 57.1 MB/s

QEMU with smp=4
15.3 MB/s + 14.1 MB/s + 13.6 MB/s = 43.0 MB/s

Is this expected?


Thanks,
JV

=== Details ===
smp = 1

time dd if=/dev/zero of=/pmnt/my_pw bs=4k count=100000 & time dd if=/dev/zero 
of=/pmnt/my_pw1 bs=4k count=100000 & time dd if=/dev/zero of=/pmnt/my_pw2 bs=4k 
count=100000 &


[root@localhost ~]# 100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 21.2747 s, 19.3 MB/s

real	0m21.377s
user	0m0.040s
sys	0m1.655s
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 21.2799 s, 19.2 MB/s

real	0m21.374s
user	0m0.046s
sys	0m1.660s
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 22.0735 s, 18.6 MB/s

real	0m22.153s
user	0m0.043s
sys	0m1.642s


smp = 4
[root@localhost ~]# time dd if=/dev/zero of=/pmnt/my_pw bs=4k count=100000 & 
time dd if=/dev/zero of=/pmnt/my_pw1 bs=4k count=100000 & time dd if=/dev/zero 
of=/pmnt/my_pw2 bs=4k count=100000 &
[root@localhost ~]# 100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 26.8055 s, 15.3 MB/s

real	0m26.869s
user	0m0.079s
sys	0m3.333s
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 28.9583 s, 14.1 MB/s

real	0m29.018s
user	0m0.053s
sys	0m4.313s
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 30.1739 s, 13.6 MB/s

real	0m30.238s
user	0m0.065s
sys	0m4.124s