Re: Xen dom0 network I/O scalability

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, May 09, 2011 at 09:13:09PM -0500, Kaushik Kumar Ram wrote:
> On Apr 27, 2011, at 12:50 PM, Konrad Rzeszutek Wilk wrote:
> >> So the current implementation of netback does not scale beyond a single CPU core, thanks to the use of tasklets, making it a bottleneck (please correct me if I am wrong). I remember coming across some patches which attempts to use softirqs instead of tasklets to solve this issue. But the latest version of linux-2.6-xen.hg repo does not include them. Are they included in some other version of dom0 Linux? Or will they be included in future? 
> > 
> > You should be using the 2.6.39 kernel or the 2.6.32 to take advantage of those patches.
> 
> Thanks Konrad. I got hold of a pvops dom0 kernel from Jeremy's git repo (xen/stable-2.6.32.x). As you pointed out it did include those patches. I spent some time studying the new netback design and ran some experiments. I have a few questions regarding them. 
> 
> I am using the latest version of the hypervisor from the xen-unstable.hg repo. I ran the experiments on a dual socket AMD quad-core opteron machine (with 8 CPU cores). My experiments simply involved running 'netperf' between 1 or 2 pairs of VMs on the same machine. I allocated 4 vcpus for dom0 and one each for the VMs. None of the vcpus were pinned.
> 
> - So the new design allows you to choose between tasklets and kthreads within netback, with tasklets being the default option. Is there any specific reason for this?

Not sure where the thread is for this - but when the patches for that were posted it showed
a big improvement in performance over 10GB. But it did require spreading the netback across
the CPUs.
> 
> - The inter-VM performance (throughput) is worse using both tasklets and kthreads as compared to the old version of netback (as in linux-2.6-xen.hg repo). I observed about 50% drop in throughput in my experiments. Has anyone else observed this? Is the new version yet to be optimized?

That is not surprising. The "new" version of netback copies pages. It does not "swizzel" or "map" then between
domains (so zero copying).
> 
> - Two tasklets (rx and tx) are created per vcpu within netback. But in my experiments I noticed that only one vcpu was being used during the experiments (even with 4 VMs).  I also observed that all the event channel notifications within netback are always sent to vcpu 0. So my conjecture is that since the tasklets are always scheduled by vcpu 0, all of them are run only on vcpu 0. Is this a BUG?

Yes. We need to fix 'irqbalance' to work properly. There is something not working right.
> 
> - Unlike with tasklets, I observed the CPU utilization go up when I used kthreads and increased the number of VMs. But the performance never scaled up. On profiling the code (using xenoprof) I observed significant synchronization overhead due to lock contention. The main culprit seems to be the per-domain lock acquired inside the hypervisor (specifically within do_grant_table_op). Further, packets are copied (inside gnttab_copy) while this lock is held. Seems like a bad idea?

Ian was thinking (and he proposed a talk at Linux Plumbers Conference) to reintroduce the zero copying
functionality back. But it is not an easy problem b/c the way the pages go through the Linux kernel
plumbing.

> 
> - A smaller source of overhead is when the '_lock' is acquired within netback in netif_idx_release(). Shouldn't this lock be per struct xen-netbk instead of being global (declared as static within the function)? Is this a BUG?

Ian, what is your thought?
> 
> If some (or all) of these points have already been discussed before, I apologize in advance! 
> 
> I appreciate any feedback or pointers.
> 
> Thanks.
> 
> --Kaushik