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Re: [Qemu-devel] [PATCH] starting qemu vnc session on a pre-allocated po

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] starting qemu vnc session on a pre-allocated port
Date: Mon, 25 Jun 2007 06:55:05 -0500
User-agent: Thunderbird (X11/20070604)

Gilad Ben-Yossef wrote:
Hi Anthony,

Thanks for the feedback.

I'm afraid I'm to blame for the idea to this patch (but Shahar was the one
that actually did the real work, I'm just bothering him).

Anthony Liguori wrote:

The problem with the solution you suggest is that all VNC traffic will be first sent to the unix domain socket, and then copied to the TCP socket. This double work may be acceptable if we're talking about one instance of qemu, but as I said, I run many concurrent sessions which create too much load. In the solution I suggest, this extra copying is not needed.

You're optimizing prematurely. The overhead of the copy is negligible for something like VNC. Under normal circumstances, we're talking about 30-100k/s. During idle usage, the bandwidth drops to almost nothing.

There are also the double context switches, more file descriptors and
extra proccess to handle the copy but you are abosutly right - we have
no indication what so ever that this really has any measurable impact
on perfomance.

I guess it's easier to resort to perfomance as an excuse since it involves
things you can measure (even if they are meaningless) rather then trying
to justify a design decision because it simply looks better. ;-)

I'll try to do just that, anyway:

Using Unix domain sockets would require adding extra code in some other proccess that will handle the socket to socket transfer. About 15 lines of code that must be running for as long as qemu does to handle the communication. That code still needs to be mnaintained, seperate from qemu, by anyone that trying to do something
similar (so we have sync problems etc.)

On the other hand, the change to qemu is ~5 lines (option parsing not included ;-) It's initaliation code only (no suprises mid run) and is maintained as part of qemu
with exact same functionality.

Here are the reasons why the Unix domain socket approach is superior:

Sharing a file descriptor implies a parent/child relationship. It also implies that the daemon will be running for the entire lifetime of the VM. Since VM's are meant to run for very long periods of time, this is quite limiting. By utilizing a domain socket, you gain the ability to record on disk the state of the daemon and then restart. The layer of redirection also allows you to let your uses change the VNC server properties while the VM is running (so you change the listening vnc display from localhost:3 to :22 without restarting the VM).

Plus, live migration has no hope of working if you're passing file descriptors on the command line as they're meaningless once you've migrated.


Anthony Liguor
If you think users other then us will use the patch (and we believe they will),
we think it'll be useful for this to be included in qemu mainline.

Anyway, thanks for reading this long email and for qemu VNC support in general :-)


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