On 8/14/13 11:39 AM, Lawrence Stewart wrote:
On 08/14/13 03:29, Julian Elischer wrote:
I have been tracking down a performance embarrassment on AMAZON EC2 and
have found it I think.
Let us please avoid conflating performance with throughput. The
behaviour you go on to describe as a performance embarrassment is
actually a throughput difference, and the FreeBSD behaviour you're
describing is essentially sacrificing throughput and CPU cycles for
lower latency. That may not be a trade-off you like, but it is an
important factor in this discussion.
it was an embarrassment in that in one class of test we performed very
poorly.
It was not a disaster or a show-stopper, but for our product it is a
critical number.
It is a throughput difference, as you say but that is a very important
part of performance...
The latency of linux didn't seem to be any worse
than FreeBSD, just the throughput was a lot higher in the same scenario.
Don't fall into the trap of labelling Linux's propensity for maximising
throughput as superior to an alternative approach which strikes a
different balance. It all depends on the use case.
well the linux balance seems t be "be better all around" at this
moment so that is
embarrassing. :-) I could see no latency reversion.
Our OS cousins over at Linux land have implemented some interesting
behaviour when TSO is in use.
They seem to aggregate ACKS when there is a lot of traffic so that they
can create the
largest possible TSO packet. We on the other hand respond to each and
every returning ACK, as it arrives and thus generally fall into the
behaviour of sending a bunch of small packets, the size of each ack.
There's a thing controlled by ethtool called GRO (generic receive
offload) which appears to be enabled by default on at least Ubuntu and I
guess other Linux's too. It's responsible for aggregating ACKs and data
to batch them up the stack if the driver doesn't provide a hardware
offload implementation. Try rerunning your experiments with the ACK
batching disabled on the Linux host to get an additional comparison point.
I will try that as soon as I get back to the machines in question.
for two examples look at:
http://www.freebsd.org/~julian/LvsF-tcp-start.tiff
and
http://www.freebsd.org/~julian/LvsF-tcp.tiff
in each case, we can see FreeBSD on the left and Linux on the right.
The first case shows the case as the sessions start, and the second case
shows
some distance later (when the sequence numbers wrap around.. no particular
reason to use that, it was just fun to see).
In both cases you can see that each Linux packet (white)(once they have got
going) is responding to multiple bumps in the send window sequence
number (green and yellow lines) (representing the arrival of several ACKs)
while FreeBSD produces a whole bunch of smaller packets, slavishly
following
exactly the size of each incoming ack.. This gives us quite a
performance debt.
Again, please s/performance/what-you-really-mean/ here.
ok, In my tests this makes FreeBSD data transfers much slower, by as
much as 60%.
Notice that this behaviour in Linux seems to be modal.. it seems to
'switch on' a little bit
into the 'starting' trace.
In addition, you can see also that Linux gets going faster even in the
beginning where
TSO isn't in play, by sending a lot more packets up-front. (of course
the wisdom of this
can be argued).
They switched to using an initial window of 10 segments some time ago.
FreeBSD starts with 3 or more recently, 10 if you're running recent
9-STABLE or 10-CURRENT.
I tried setting initial values as shown:
net.inet.tcp.local_slowstart_flightsize: 10
net.inet.tcp.slowstart_flightsize: 10
it didn't seem to make too much difference but I will redo the test.
Has anyone done any work on aggregating ACKs, or delaying responding to
them?
As noted by Navdeep, we already have the code to aggregate ACKs in our
software LRO implementation. The bigger problem is that appropriate byte
counting places a default 2*MSS limit on the amount of ACKed data the
window can grow by i.e. if an ACK for 64k of data comes up the stack,
we'll grow the window by 2 segments worth of data in response. That
needs to be addressed - we could send the ACK count up with the
aggregated single ACK or just ignore abc_l_var when LRO is in use for a
connection.
so, does "Software LRO" mean that LRO on hte NIC should be ON or OFF
to see this?
Cheers,
Lawrence
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