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.
>
> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com>
> ---
>
> diff to v1:
> - Resend after 2e7d4916967 which introduced yet another qemu capability and
> thus made my patch not apply cleanly.
>
> docs/formatdomain.html.in | 16 +++++++++++++++-
> docs/schemas/domaincommon.rng | 5 +++++
> src/conf/domain_conf.c | 16 ++++++++++++++++
> src/conf/domain_conf.h | 1 +
> src/qemu/qemu_capabilities.c | 4 ++++
> src/qemu/qemu_capabilities.h | 3 +++
> src/qemu/qemu_command.c | 8 ++++++++
> src/qemu/qemu_domain.c | 16 +++++++++++-----
> ...e.args => qemuxml2argv-net-virtio-rxtxqueuesize.args} | 4 ++--
> ...ize.xml => qemuxml2argv-net-virtio-rxtxqueuesize.xml} | 2 +-
> tests/qemuxml2argvtest.c | 5 +++--
> ...e.xml => qemuxml2xmlout-net-virtio-rxtxqueuesize.xml} | 2 +-
> tests/qemuxml2xmltest.c | 2 +-
> 13 files changed, 71 insertions(+), 13 deletions(-)
> rename tests/qemuxml2argvdata/{qemuxml2argv-net-virtio-rxqueuesize.args =>
qemuxml2argv-net-virtio-rxtxqueuesize.args} (85%)
> rename tests/qemuxml2argvdata/{qemuxml2argv-net-virtio-rxqueuesize.xml =>
qemuxml2argv-net-virtio-rxtxqueuesize.xml} (93%)
> rename tests/qemuxml2xmloutdata/{qemuxml2xmlout-net-virtio-rxqueuesize.xml =>
qemuxml2xmlout-net-virtio-rxtxqueuesize.xml} (96%)
>
> 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.
Michal