VM in Xen 4.1; poor packet forwarding performance on xn0

[seanrees at gmail.com]

Hi Roger,

Thanks for the patch -- sadly, it didn't work. No change.

I did have to modify a bit for releng/10.0; for some reason patch refused
to apply it cleanly. It looked fairly straightforward but I attached inline
the patch I ultimately applied below just in case I got it wrong.

Are there any other potential differences between Xen 3.4 and 4.1? (my
provider migrated my problem VPS to a 3.4 host and the problem evaporated;
I am trying this on a new 4.1 VPS that I was able to reproduce the problem
on).

Sean


Index: hvm.c
===================================================================
--- hvm.c (revision 264963)
+++ hvm.c (working copy)
@@ -626,6 +626,7 @@

  xhp.domid = DOMID_SELF;
  xhp.index = HVM_PARAM_CALLBACK_IRQ;
+#if 0
  if (xen_feature(XENFEAT_hvm_callback_vector) != 0) {
  int error;

@@ -638,6 +639,7 @@
  printf("Xen HVM callback vector registration failed (%d). "
     "Falling back to emulated device interrupt\n", error);
  }
+#endif
  xen_vector_callback_enabled = 0;
  if (dev == NULL) {
  /*
@@ -783,7 +785,7 @@
  info.mfn = vtophys(vcpu_info) >> PAGE_SHIFT;
  info.offset = vtophys(vcpu_info) - trunc_page(vtophys(vcpu_info));

- rc = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, cpu, &info);
+ rc = 1;
  if (rc != 0)
  DPCPU_SET(vcpu_info, &HYPERVISOR_shared_info->vcpu_info[cpu]);
  else


On Tue, Apr 22, 2014 at 3:10 PM, Roger Pau Monné <roger.pau at citrix.com>wrote:

> On 18/04/14 16:57, seanrees at gmail.com wrote:
> > Hi there freebsd-xen,
> >
> > I tried first on freebsd-questions@ without success, so I thought to
> retry
> > here.
> >
> > I run OpenVPN on a FreeBSD 10.0-Rp1 VM in Xen 4.1 (HVM). I am
> experiencing
> > slow network performance on xn0 that seems to have developed after I
> > upgraded to FreeBSD 10 (no other changes). I can only achieve about
> 0.5mbps
> > through this interface when forwarding packets; packets in a single
> > direction are fine (e.g; downloading to the VPS or pushing from the VPS)
> > and clock in at many (>10 usually) mbps.
> >
> > Interestingly, my identical VM (configuration managed centrally) running
> on
> > Xen 3.4 (HVM) does *not* have this issue.
>
> Hello,
>
> The difference between Xen 3.4 and Xen 4.1 is that FreeBSD will make use
> of the vector callback, the PV timer and PV IPIs when running on Xen >
> 4.0 (which should provide better performance). I'm attaching a patch
> that will make FreeBSD behave the same way when running on either Xen
> 3.4 or Xen 4.1 (by disabling all this new additions), could you please
> give it a try?
>
> >
> > I did a little debugging and here's what I've noticed:
> >    - Not related to OpenVPN, repro'd using ssh -d.
> >    - Slow VM has a very low rate of context switches (~250) while
> > forwarding, fast VM has a lot more (~2000) sampled over 5 seconds using
> > systat -v.
> >   - I can't repro a context switch limit (tried a limited fork() bomb).
> >   - Tried with *and* without LRO and TSO on xn0 (and all combinations of
> > LRO and TSO on/off)
>
> I've got the feeling that the issue you are seeing is not related to the
> Xen version itself, but the Linux Dom0 kernel version (which I suppose
> is different in the Xen 3.4 and Xen 4.1 hosts). Could you ask your
> provider which Linux Dom0 kernel are they using on the different hosts?
>
> Roger.
>