Re: [PATCH] kvm:virtio-net: Run TX from the I/O thread

[Alex Williamson <alex.williamson@hp.com>]

On Thu, 2009-01-22 at 12:48 +0000, Mark McLoughlin wrote:
> On Thu, 2009-01-22 at 14:12 +0200, Avi Kivity wrote:
> 
> >  My worry with this change is that increases cpu utilization even more 
> > than it increases bandwidth, so that our bits/cycle measure decreases.  
> 
> Yep, agreed it's important to watch out for this.
> 
> > The descriptors (and perhaps data) are likely on the same cache as the 
> > vcpu, and moving the transmit to the iothread will cause them to move to 
> > the iothread's cache.
> 
> We flush from the I/O thread right now.
> 
> We only ever flush from the vcpu thread when the ring fills up, which
> rarely happens from what I've observed.

Sorry to have come in late to the discussion, but it seems like maybe it
needed another kick after a couple months anyway.  As noted, we are
mostly (almost exclusively?) doing TX from the timer anyway, so this
change or Mark's previous patch series don't really change current cache
effects.  I am curious what happens to latency with Mark's series since
that isn't really addressed by the charts, hopefully good things without
the tx_timer.

A thread per device or perhaps even a thread per RX/TX stream seems like
a logical goal, but these current patches do provide a worthwhile
incremental improvement.  Perhaps we could affinitize the guest to do
I/O on a specific vcpu via _PXM methods in ACPI so we can provide hints
to the scheduler to keep a vcpu thread and it's associated I/O threads
nearby.  Thanks,

Alex

-- 
Alex Williamson                             HP Open Source & Linux Org.