On 2017年05月10日 17:42, Anton Ivanov wrote:
On 10/05/17 09:56, Jason Wang wrote:
On 2017年05月10日 13:28, Anton Ivanov wrote:
On 10/05/17 03:18, Jason Wang wrote:
On 2017年05月09日 23:11, Stefan Hajnoczi wrote:
On Tue, May 09, 2017 at 08:46:46AM +0100, Anton Ivanov wrote:
I have figured it out. Two issues.
1) skb->xmit_more is hardly ever set under virtualization because
the qdisc
is usually bypassed because of TCQ_F_CAN_BYPASS. Once
TCQ_F_CAN_BYPASS is
set a virtual NIC driver is not likely see skb->xmit_more (this
answers my
"how does this work at all" question).
2) If that flag is turned off (I patched sched_generic to turn it
off in
pfifo_fast while testing), DQL keeps xmit_more from being set. If
the driver
is not DQL enabled xmit_more is never ever set. If the driver is DQL
enabled
the queue is adjusted to ensure xmit_more stops happening within
10-15 xmit
cycles.
That is plain *wrong* for virtual NICs - virtio, emulated NICs, etc.
There,
the BIG cost is telling the hypervisor that it needs to "kick" the
packets.
The cost of putting them into the vNIC buffers is negligible. You
want
xmit_more to happen - it makes between 50% and 300% (depending on
vNIC
design) difference. If there is no xmit_more the vNIC will
immediately
"kick" the hypervisor and try to signal that the packet needs to
move
straight away (as for example in virtio_net).
How do you measure the performance? TCP or just measure pps?
In this particular case - tcp from guest. I have a couple of other
benchmarks (forwarding, etc).
One more question, is the number for virtio-net or other emulated vNIC?
Other for now - you are cc-ed to keep you in the loop.
Virtio is next on my list - I am revisiting the l2tpv3.c driver in
QEMU and looking at how to preserve bulking by adding back sendmmsg
(as well as a list of other features/transports).
We had sendmmsg removed for the final inclusion in QEMU 2.1, it
presently uses only recvmmsg so for the time being it does not care.
That will most likely change once it starts using sendmmsg as well.
An issue is that qemu net API does not support bulking, do you plan to
add it?
Thanks
