Il 18/12/2012 23:18, Rolf Eike Beer ha scritto:

Paolo Bonzini wrote:

quoted

Hi all,

this series adds multiqueue support to the virtio-scsi driver, based
on Jason Wang's work on virtio-net.  It uses a simple queue steering
algorithm that expects one queue per CPU.  LUNs in the same target always
use the same queue (so that commands are not reordered); queue switching
occurs when the request being queued is the only one for the target.
Also based on Jason's patches, the virtqueue affinity is set so that
each CPU is associated to one virtqueue.

I tested the patches with fio, using up to 32 virtio-scsi disks backed
by tmpfs on the host.  These numbers are with 1 LUN per target.

FIO configuration
-----------------
[global]
rw=read
bsrange=4k-64k
ioengine=libaio
direct=1
iodepth=4
loops=20

overall bandwidth (MB/s)
------------------------

# of targets    single-queue    multi-queue, 4 VCPUs    multi-queue, 8 VCPUs
1                  540               626                     599
2                  795               965                     925
4                  997              1376                    1500
8                 1136              2130                    2060
16                1440              2269                    2474
24                1408              2179                    2436
32                1515              1978                    2319

(These numbers for single-queue are with 4 VCPUs, but the impact of adding
more VCPUs is very limited).

avg bandwidth per LUN (MB/s)
----------------------------

# of targets    single-queue    multi-queue, 4 VCPUs    multi-queue, 8 VCPUs
1                  540               626                     599
2                  397               482                     462
4                  249               344                     375
8                  142               266                     257
16                  90               141                     154
24                  58                90                     101
32                  47                61                      72

Is there an explanation why 8x8 is slower then 4x8 in both cases?

Regarding the "in both cases" part, it's because the second table has
the same data as the first, but divided by the first column.

In general, the "strangenesses" you find are probably within statistical
noise or due to other effects such as host CPU utilization or contention
on the big QEMU lock.

Paolo


 8x1 and 8x2

being slower than 4x1 and 4x2 is more or less expected, but 8x8 loses against 
4x8 while 8x4 wins against 4x4 and 8x16 against 4x16.

Eike