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Re: [Qemu-devel] Queries on dataplane mechanism

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Queries on dataplane mechanism
Date: Fri, 24 Jun 2016 11:15:06 +0100
User-agent: Mutt/1.6.1 (2016-04-27)

On Thu, Jun 23, 2016 at 08:56:34PM +0530, Gaurav Sharma wrote:
> Hi,
> I am trying to explore how the data plane mechanism works in QEMU. I
> understand the behavior of QEMU big lock. Can someone clarify the following
> w.r.t. to data plane :
> 1. Currently only virtio-blk-pci and virtio-scsi-pci have data plane
> enabled ?


> 2. From qemu 2.1.0 data plane is enabled by default.

No "enabled by default" would mean that existing QEMU command-lines
enable dataplane.  This is not the case.  You have to explicitly define
an iothread object and then associate a virtio-blk/virtio-scsi device
with it.

> I specify the
> following options in the command line to enable :
> -enable-kvm -drive if=none,id=drive1,file=file_name -object
> iothread,id=iothread2 -device
> virtio-blk-pci,id=drv0,drive=drive1,iothread=iothread2
> Is the above syntax correct ?


> 3. What is the best possible scenario to test data plane ? Currently, I
> have a test set up wherein i have two different devices [dev1 and dev2]. If
> i process a write to dev1 which i made blocking by putting a sleep
> statement, will i be able to process write on dev2 ? My understanding is
> that as in case of dataplane we have a different event loop, i should be
> able to process write on dev2. Is this correct ?

Dataplane improves scalability for high IOPS workloads when there are
multiple disks.

You do not need to modify any code in order to benchmark dataplane.  Run
fio inside an SMP 4 guest with 4 disks (you can use the host Linux
kernel's null_blk driver) and you should find that QEMU without
dataplane has lower iops.  The difference should become clear around 4
or 8 vcpus/disks.


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