Re: [libvirt] [PATCH v2] Add support for virtio-net.tx_queue_size

On 07/18/2017 08:23 AM, Peter Krempa wrote: > On Mon, Jul 17, 2017 at 15:39:56 +0200, Michal Privoznik wrote: >> https://bugzilla.redhat.com/show_bug.cgi?id=1462653 > > This bugzilla is not public. Okay, I'll drop it from the commit message.

>> Just like I've added support for setting rx_queue_size (in >> c56cdf259 and friends), qemu just gained support for setting tx >> ring size.

>> diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in >> index c12efcf78..58662cf48 100644 >> --- a/docs/formatdomain.html.in >> +++ b/docs/formatdomain.html.in > > [...] > >> @@ -5201,6 +5201,20 @@ qemu-kvm -net nic,model=? /dev/null >> <b>In general you should leave this option alone, unless you >> are very certain you know what you are doing.</b> >> </dd> >> + <dt><code>tx_queue_size</code></dt> >> + <dd> >> + The optional <code>tx_queue_size</code> attribute controls >> + the size of virtio ring for each queue as described above. >> + The default value is hypervisor dependent and may change >> + across its releases. Moreover, some hypervisors may pose >> + some restrictions on actual value. For instance, latest >> + QEMU (as of 2017-07-13) requires value to be a power of two

>> + from [256, 1024] range. >> + <span class="since">Since 3.6.0 (QEMU and KVM only)</span><br/><br/> > > This is ridiculous. Since we can't figure out how to set this, how are > users supposed to figure this out? Well, you've cut off the line that reads; <b>In general you should leave this option alone, unless you are very certain you know what you are doing.</b> So only users that know how virtio works under the hood are expected to also know what rx/tx queue size is and how to set it. But frankly, I

think users setting this are always gonna go with the highest value avaliable (1024). Such detailed description is a copy of rx_virtio_queue size description which is result of review. > > Is it really needed? How should it be configured? Can't we or qemu pick > a sane value? > No. Some users need bigger virtio rings otherwise they see a packet drop. So this is a fine tuning that heavily depends on the use case. Thus libvirt should not try to come up with some value.