Re: Bonding, GRO and tcp_reordering

[Simon Horman <horms@verge.net.au>]

On Tue, Nov 30, 2010 at 03:42:56PM +0000, Ben Hutchings wrote:
> On Tue, 2010-11-30 at 22:55 +0900, Simon Horman wrote:
> > Hi,
> > 
> > I just wanted to share what is a rather pleasing,
> > though to me somewhat surprising result.
> >
> > I am testing bonding using balance-rr mode with three physical links to try
> > to get > gigabit speed for a single stream. Why?  Because I'd like to run
> > various tests at > gigabit speed and I don't have any 10G hardware at my
> > disposal.
> > 
> > The result I have is that with a 1500 byte MTU, tcp_reordering=3 and both
> > LSO and GSO disabled on both the sender and receiver I see:
> > 
> > # netperf -c -4 -t TCP_STREAM -H 172.17.60.216 -- -m 1472
> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 172.17.60.216
> > (172.17.60.216) port 0 AF_INET
> > Recv   Send    Send                          Utilization       Service Demand
> > Socket Socket  Message  Elapsed              Send     Recv     Send    Recv
> > Size   Size    Size     Time     Throughput  local    remote   local   remote
> > bytes  bytes   bytes    secs.    10^6bits/s  % S      % U      us/KB   us/KB
> > 
> >   87380  16384   1472    10.01      1646.13   40.01    -1.00    3.982  -1.000
> > 
> > But with GRO enabled on the receiver I see.
> > 
> > # netperf -c -4 -t TCP_STREAM -H 172.17.60.216 -- -m 1472
> > TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 172.17.60.216
> > (172.17.60.216) port 0 AF_INET
> > Recv   Send    Send                          Utilization       Service Demand
> > Socket Socket  Message  Elapsed              Send     Recv     Send    Recv
> > Size   Size    Size     Time     Throughput  local    remote   local   remote
> > bytes  bytes   bytes    secs.    10^6bits/s  % S      % U      us/KB   us/KB
> > 
> >  87380  16384   1472    10.01      2613.83   19.32    -1.00    1.211   -1.000
> > 
> > Which is much better than any result I get tweaking tcp_reordering when
> > GRO is disabled on the receiver.
> 
> Did you also enable TSO/GSO on the sender?

It didn't seem to make any difference either way.
I'll re-test just in case I missed something.

> 
> What TSO/GSO will do is to change the round-robin scheduling from one
> packet per interface to one super-packet per interface.  GRO then
> coalesces the physical packets back into a super-packet.  The intervals
> between receiving super-packets then tend to exceed the difference in
> delay between interfaces, hiding the reordering.
> 
> If you only enabled GRO then I don't understand this.
> 
> > Tweaking tcp_reordering when GRO is enabled on the receiver seems to have
> > negligible effect.  Which is interesting, because my brief reading on the
> > subject indicated that tcp_reordering was the key tuning parameter for
> > bonding with balance-rr.
> > 
> > The only other parameter that seemed to have significant effect was to
> > increase the mtu.  In the case of MTU=9000, GRO seemed to have a negative
> > impact on throughput, though a significant positive effect on CPU
> > utilisation.
> [...]
> 
> Increasing MTU also increases the interval between packets on a TCP flow
> using maximum segment size so that it is more likely to exceed the
> difference in delay.

I hadn't considered that, thanks.