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RE: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support

To: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Date: Sat, 28 Nov 2009 13:15:04 +0000
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Thread-topic: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support
> The domain lock is in grant_op hypercall. If the multiple tasklets are 
> fighting
> with each other for this big domain lock, it would become a bottleneck and
> hurt the performance.
> Our test system has 16 LP in total, so we have 16 vcpus in dom0 by
> default.
> 10 of them are used to handle the network load. For our test case, dom0's
> totalvcpu utilization is  ~461.64%,  so each vcpu ocupies ~46%.

Having 10 VCPUs for dom0 doesn't seem like a good idea -- it really oughtn't to 
need that many CPUs to handle IO load. Have you got any results with e.g. 2 or 
4 VCPUs?

When we switch over to using netchannel2 by default this issue should largely 
go away anyhow as the copy is not done by dom0. Have you done any tests with 

> Actually the multiple tasklet in netback could already improve the the QoS of 
> the
> system, therefore I think it can also help to get better responseness for
> that vcpu.
> I think I can try to write another patch which replace the tasklet by kthread,
> because I think is a different job with the multi-tasklet netback support.
> (kthread is used to guarantee the responseness of userspace, however 
> multi-tasklet
> netback is used to remove the dom0's cpu utilization bottleneck). However I 
> am not
> sure whether the improvement in QoS by this change is needed In MP system?

Have you looked at the patch that xenserver uses to replace the tasklets by 


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