On 17 Apr 2019, at 18:47, Ben Pfaff wrote:
On Wed, Apr 17, 2019 at 10:09:53AM +0200, Eelco Chaudron wrote:
On 16 Apr 2019, at 21:55, Ben Pfaff wrote:
AF_XDP is a faster way to access the existing kernel devices. If we
take that point of view, then it would be ideal if AF_XDP were
automatically used when it was available, instead of adding a new
network device type. Is there a reason that this point of view is
wrong? That is, when AF_XDP is available, is there a reason not to
use
it?
This needs support by all the ingress and egress ports in the system,
and
currently, there is no API to check this.
Do you mean for performance or for some other reason? I would suspect
that, if AF_XDP was not available, then everything would still work OK
via AF_PACKET, just slower.
Yes, it will become slower and people do not understand why. For
example, it's easy to combine kernel, XDP and DPDK ports. But receiving
one and tx on another becomes slow. It could even impact the DPDK/XDP
performance as now syscall’s need to be executed stalling the PMD loop
causing packets to be dropped.
Maybe we should add something about this in the documentation, for
example, that the kernel receive loop is done in the main thread,
XDP/DPDK in dedicated PMD threads, etc. etc.
There are also features like traffic shaping that will not work.
Maybe it
will be worth adding the table for AF_XDP in
http://docs.openvswitch.org/en/latest/faq/releases/
AF_XDP is comparable to DPDK/userspace, not to the Linux kernel
datapath.
The table currently conflates the userspace datapath with the DPDK
network device. I believe that the only entry there that depends on
the
DPDK network device is the one for policing. It could be replaced by
a
[*] with a note like this:
YES - for DPDK network devices.
NO - for system or AF_XDP network devices.
This would work, just want to make sure it’s tested rather than assume
it will work as there might be corner cases.
You said that your goal for the next version is to improve
performance
and add optimizations. Do you think that is important before we
merge
the series? We can continue to improve performance after it is
merged.
The previous patch was rather unstable and I could not get it running
with
the PVP test without crashing. I think this patchset should get some
proper
testing and reviews by others. Especially for all the features being
marked
as supported in the above-mentioned table.
If it's unstable, we should fix that before adding it in.
However, the bar is lower for new features that don't break existing
features, especially optional ones and ones that can be easily be
removed if they don't work out in the end. DPDK support was
considered
"experimental" for a long time, it's possible that AF_XDP would be in
the same boat for a while.
Thats fine, as long as there are some serious reviews of it. I’ll work
on it once I return from PTO, but I guess more would be welcome.
If we set performance aside, do you have a reason to want to wait to
merge this? (I wasn't able to easily apply this series to current
master, so it'll need at least a rebase before we apply it. And I
have
only skimmed it, not fully reviewed it.)
Other than the items above, do we really need another datapath?
It's less than a new datapath. It's a new network device
implementation.
Sorry yes, from OVS terminology it is…
With this, we use two or more cores for processing packets. If we
poll
two physical ports it could be 300%, which is a typical use case with
bonding. What about multiple queue support, does it work? Both in
kernel and DPDK mode we use multiple queues to distribute the load,
with this scenario does it double the number of CPUs used? Can we use
the poll() mode as explained here,
https://linuxplumbersconf.org/event/2/contributions/99/, and how will
it work with multiple queues/pmd threads? What about any latency
tests, is it worse or better than kernel/dpdk? Also with the AF_XDP
datapath, there is no to leverage hardware offload, like DPDK and
TC. And then there is the part that it only works on the most recent
kernels.
These are good questions. William will have some of the answers.
To me looking at this I would say it’s far from being ready to be
merged
into OVS. However, if others decide to go ahead I think it should be
disabled, not compiled in by default.
Yes, that seems reasonable to me.
_______________________________________________
dev mailing list
d...@openvswitch.org
https://mail.openvswitch.org/mailman/listinfo/ovs-dev
