WinVNC & NT: 100% CPU utilization

James [Wez] Weatherall jnw "at"
Wed, 25 Mar 1998 10:25:07 +0000

On Tue, 24 Mar 1998, dave w capella wrote:

> I fired up the NT's TaskMgr to monitor CPU & memory use. To my 
> surprise, I found that with only WinVNC running, serving 2 clients,
> my CPU utilization hit the ceiling (100%). It's only got 32mb, but only
> 28mb were committed (no thrashing). A check of all processes
> revealed that WinVNC was taking 85% - 98% of the CPU's time. The only
> other processes running were the standard NT server services: IIS,FTP,
> standard TCP services.

Connecting multiple clients to WinVNC is known to increase CPU usage. 
However, under Windows NT this should rarely prove to be a problem, since
WinVNC will be scheduled in the same way as any other user application and
so will not lock out other processes.

Note also that WinVNC's CPU usage will rise proportionally to the area of
the screen being updated.  This means that dragging large windows, or
having large windows regularly updating (e.g. the system clock maxmised to
fill the screen), will cause CPU usage to peak.

However, I suspect the real reason for this apparent performance hit is
that you're running the Task Manager on the desktop of an NT box that is
being viewed by two VNC clients.  So a large proportion of the CPU cycles
being used by WinVNC will be coming from the fact that you have the Task
Manager updating it's display every half second or so, to two clients.  I
managed to get roughly the CPU usage you describe (although I have two
CPU's, so I was looking for only ~40% usage on average, as being
comparable).  When I close the task manager, CPU usage drops markedly.

> I've been telling colleagues that this is *not* resource hungry, and
> I could have sworn that my earlier tests bore this out. Any idea why
> this 'virgin' machine would exhibit such behavior? Or were my earlier
> results wrong?

Were you performing similar tasks in your original trials to the case
described above?  How many clients did you originally have connected?


James "Wez" Weatherall
Olivetti & Oracle Research Laboratory, Cambridge, UK.
Tel : Work - 343000