Re: [Qemu-devel] High speed polling

[Clay Andreasen <cda@cray.com>]

Thank you for your reply.
I am still a novice with Qemu so pardon me if I don't make any sense.

I tried --enable-io-thread.  I get the error:
cpus.o: In function `qemu_kvm_eat_signal':
cpus.c:(.text+0x111a): undefined reference to `kvm_on_sigbus_vcpu'

so I assume it requires KVM.  I'm not using KVM because I don't have 
full control
over the host I am running on.
I have 8 host processors running 4 Qemu copies (1 vcpu each) plus my 
network simulator.
I have tried polling via a call in vl.c:mainloop and via qemu_mod_timer().
There doesn't appear to be much difference.
The guest is a full-blown x86_64 OS.
I am polling to minimize latency.
I am looking at other ways to tolerate the current latency in case I can't
do much better.

Clay


On 06/15/11 01:22, Stefan Hajnoczi wrote:
> On Tue, Jun 14, 2011 at 11:32 PM, Clay Andreasen<cda@cray.com>  wrote:
>> I have a network device simulation that I am connecting to multiple
>> instances of Qemu (nodes) via a shared memory queue.  It works pretty well
>> as
>> long as all of the nodes are initiating communication but when one node is
>> passive, it must poll to get packets.  So far the fastest I have been able
>> to
>> get it to poll is about every 2M emulated clocks.
>> This is with CONFIG_HIGH_RES_TIMERS and CONFIG_NO_HZ on the host.
>> I also set MIN_TIMER_REARM_NS in qemu-timer.c to 100000.
>> Is there some way to increase the polling rate by about an order of
>> magnitude?
> Without more details it's hard to say what is going on:
>
> Running an x86 guest?  Are you using ./configure --enable-io-thread?
> It sounds like you may not be using KVM?  How many vcpus are running
> on the host in total compared to the number of logical CPUs on the
> host?
>
> You haven't given details on how you are polling in the guest.  Are
> you running a polling loop in ring 0 or is the guest running a
> full-blown OS and polling from userspace?
>
> Why are you polling in the first place - to minimize latency?
>
> Stefan