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[Qemu-devel] Re: [RFC] vhost-blk implementation

From: Michael S. Tsirkin
Subject: [Qemu-devel] Re: [RFC] vhost-blk implementation
Date: Tue, 23 Mar 2010 20:06:44 +0200
User-agent: Mutt/1.5.19 (2009-01-05)

On Tue, Mar 23, 2010 at 10:57:33AM -0700, Badari Pulavarty wrote:
> Michael S. Tsirkin wrote:
>> On Mon, Mar 22, 2010 at 05:34:04PM -0700, Badari Pulavarty wrote:
>>> Write Results:
>>> ==============
>>> I see degraded IO performance when doing sequential IO write
>>> tests with vhost-blk compared to virtio-blk.
>>> # time dd of=/dev/vda if=/dev/zero bs=2M oflag=direct
>>> I get ~110MB/sec with virtio-blk, but I get only ~60MB/sec with
>>> vhost-blk. Wondering why ?
>> Try to look and number of interrupts and/or number of exits.
> I checked interrupts and IO exits - there is no major noticeable  
> difference between
> vhost-blk and virtio-blk scenerios.
>> It could also be that you are overrunning some queue.
>> I don't see any exit mitigation strategy in your patch:
>> when there are already lots of requests in a queue, it's usually
>> a good idea to disable notifications and poll the
>> queue as requests complete. That could help performance.
> Do you mean poll eventfd for new requests instead of waiting for new  
> notifications ?
> Where do you do that in vhost-net code ?

vhost_disable_notify does this.

> Unlike network socket, since we are dealing with a file, there is no  
> ->poll support for it.
> So I can't poll for the data. And also, Issue I am having is on the  
> write() side.

Not sure I understand.

> I looked at it some more - I see 512K write requests on the
> virtio-queue  in both vhost-blk and virtio-blk cases. Both qemu or
> vhost is doing synchronous  writes to page cache (there is no write
> batching in qemu that is affecting this  case).  I still puzzled on
> why virtio-blk outperforms vhost-blk.
> Thanks,
> Badari

If you say the number of requests is the same, we are left with:
- requests are smaller for some reason?
- something is causing retries?


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