[LEDE-DEV] lantiq: reduced throughput with SMP disabled

[James Finnie]

I've been chasing down some poor performance with my HH5A running LEDE
(I've used r1142 - r1227 for testing).  Most notable when using
multiple interfaces, particularly the combination of 2.4G wireless
with VDSL at high throughput (am lucky enough to get 80 down / 20 up
on my connection), resulting in very "bursty" performance during
internet speed testing (thinkbroadband), and an inability to max out
the connection (which was possible with stock firmware).  Brief
inspection with top looked like there was no idle CPU available during
such testing.  Some more in-depth tests using iperf3 [0] seemed to
indicate that full rate VDSL and wifi was never going to be possible
on this CPU.

Via a PM on said forum a suggestion came that it might be worth trying
to revert the disabling of SMP (which was put in place due to an issue
with dual eth interfaces).  I got a build environment up and running
and locally reverted this change; it seems this solves my issues of
CPU contention, pretty much saturating the VDSL over 2.4G wireless
with CPU to spare, and results in quite a lot higher maximum
throughput measured with iperf3 via both localhost (RX/TX 428/428Mbit
vs 486/574Mbit) and ethernet (53/98Mbit vs 71/201Mbit).

I understand there are reasons to have SMP disabled; but it does seem
that if like me you have fast connections then the penalty for this is
pretty high due to the CPU choking.  It would be nice to get SMP
re-instated if possible.

[0] http://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=149&sid=7b589ebbda7bc5db24667174ca9b401b&start=10#p1189