On 18 Oct 2018, at 18:13, Sriharsha Basavapatna via dev wrote:
With the current OVS offload design, when an offload-device fails to
add a
flow rule and returns an error, OVS adds the rule to the kernel
datapath.
The flow gets processed by the kernel datapath for the entire life of
that
flow. This is fine when an error is returned by the device due to lack
of
support for certain keys or actions.
But when an error is returned due to temporary conditions such as lack
of
resources to add a flow rule, the flow continues to be processed by
kernel
even when resources become available later. That is, those flows never
get
offloaded again. This problem becomes more pronounced when a flow that
has
been initially offloaded may have a smaller packet rate than a later
flow
that could not be offloaded due to lack of resources. This leads to
inefficient use of HW resources and wastage of host CPU cycles.
This patch-set addresses this issue by providing a way to detect
temporary
offload resource constraints (Out-Of-Resource or OOR condition) and to
selectively and dynamically offload flows with a higher
packets-per-second
(pps) rate. This dynamic rebalancing is done periodically on netdevs
that
are in OOR state until resources become available to offload all
pending
flows.
The patch-set involves the following changes at a high level:
1. Detection of Out-Of-Resources (OOR) condition on an offload-capable
netdev.
2. Gathering flow offload selection criteria for all flows on an OOR
netdev;
i.e, packets-per-second (pps) rate of flows for offloaded and
non-offloaded (pending) flows.
3. Dynamically replacing offloaded flows with a lower pps-rate, with
non-offloaded flows with a higher pps-rate, on an OOR netdev. A new
OpenvSwitch configuration option - "offload-rebalance" to enable
this policy.
Cost/benefits data points:
1. Rough cost of the new rebalancing, in terms of CPU time:
Ran a test that replaced 256 low pps-rate flows(pings) with 256
high
pps-rate flows(iperf), in a system with 4 cpus (Intel Xeon E5 @
2.40GHz;
2 cores with hw threads enabled, rest disabled). The data showed
that cpu
utilization increased by about ~20%. This increase occurs during
the
specific second in which rebalancing is done. And subsequently
(from the
next second), cpu utilization decreases significantly due to
offloading
of higher pps-rate flows. So effectively there's a bump in cpu
utilization
at the time of rebalancing, that is more than compensated by
reduced cpu
utilization once the right flows get offloaded.
2. Rough benefits to the user in terms of offload performance:
The benefits to the user is reduced cpu utilization in the host,
since
higher pps-rate flows get offloaded, replacing lower pps-rate
flows.
Replacing a single offloaded flood ping flow with an iperf flow
(multiple
connections), shows that the cpu %used that was originally 100% on
a
single cpu (rebalancing disabled) goes down to 35% (rebalancing
enabled).
That is, cpu utilization decreased 65% after rebalancing.
3. Circumstances under which the benefits would show up:
The rebalancing benefits would show up once offload resources are
exhausted and new flows with higher pps-rate are initiated, that
would
otherwise be handled by kernel datapath costing host cpu cycles.
This can be observed using 'ovs appctl dpctl/ dump-flows' command.
Prior
to rebalancing, any high pps-rate flows that couldn't be offloaded
due to
resource crunch would show up in the output of 'dump-flows
type=ovs' and
after rebalancing such flows would appear in the output of
'dump-flows type=offloaded'.
Before I review the individual patches, hope to do this tomorrow, I have
some general concerns/comments.
Once committed will this feature be marked double experimental? Just
want to clarify if it goes in as part of HW offload the experimental tag
for this is not removed once HW offload becomes mainstream.
Currently, in OOR state both phases (insert, and rebalance) are ran.
Rather than have offload-rebalance true, or false. Maybe we can have,
disable, retry, retry-rebalance. I think only retrying might be
beneficial as well, not changing existing flows.
My main objection against offloading flows at a later stage is packet
re-ordering. As soon as you move from kernel datapath to hardware
offload packet might be sent out of order. This is mainly a problem for
TCP streams, which do not have the problem if the stream is directly
offloaded at the start.
To make this even worse is that the current implementation has no
protection for flows fighting to be offloaded, i.e. ping/ponging from HW
to SW, hence causing a lot of out of order packets.
Based on the above I was wondering if any tests where done to measure
the out of order packets/jitter on rebalancing?
Last implementation item I have is the packet throughput through the
kernel datapath is rather low, <200Kpps. This low throughput might make
it hard to determine which not offloaded flow might be the best suited
for HW offload. The kernel might drop packets for a flow with a way
higher potential than the packets that do make it through the kernel. I
do not have a solution for this, but I guess it worth keeping in mind
when this gets enabled.
As a general question, where other solutions being considered to cope
with inadequate resources? Maybe a solution that will give the operator
more control over what would be offloaded? Like giving all configured
flows a relative priority. Either at individual flow creation or some
general overlay, i.e. all TCP flows for example.h
Cheers,
Eelco
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