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

From: Badari Pulavarty
Subject: [Qemu-devel] Re: [RFC] vhost-blk implementation
Date: Tue, 23 Mar 2010 10:57:33 -0700
User-agent: Thunderbird (Windows/20100228)

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 ?

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.

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.


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