Date:	Fri, 28 Dec 2012 10:01:21 +0000
From:	Mark Zealey <netdev@...kandruth.co.uk>
To:	netdev@...r.kernel.org
Subject: UDP multi-core performance on a single socket and SO_REUSEPORT

I appreciate that this question has come up a number of times over the 
years, most recently as far as I can see in this thread: 
http://markmail.org/message/hcc7zn5ln5wktypv . I'm going to explain my 
problem and present some performance numbers to back this up.

The problem: I'm doing some research on scaling a dns server (powerdns) 
to work well on multi-core boxes (in this case testing with 2*E5-2650 
processors ie linux sees 32 cores).

My powerdns configuration uses a shared socket with one thread for each 
core in the box listening on that socket using poll()/recvmsg(). I've 
modified powerdns so in my tests it is doing the absolute minimum of 
work to answer packets (all queries are for the same record, it keeps 
the response in memory and just changes a few fields before calling 
sendmsg()). I'm binding to a single 10.xxx address and using this for 
all local and remote tests.

The numbers below are generated using 16 parallel queryperf's on 
localhost (it doesn't really matter if it is from remote hosts or the 
localhost; the numbers don't change much).

Using stock centos 6.3 kernel I see powerdns performing at around 
120kqps (uses at most about 12 cpus)
Using 3.7.1 kernel (from elrepo) I see this increase to 200-240kqps 
maxing out all cpu's in the box (soft interrupt cpu time is about 8* 
higher than on centos 6.3 kernel at 40% and system cpu time is at 50% - 
powerdns only uses 10% of the cpu time)
Using stock centos 6.3 kernel with the google SO_REUSEPORT patch from 
2010 (modified slightly so it applies) I see 500-600kqps from remote; or 
1mqps when doing localhost queries. powerdns doesn't go past using 8 
cpus - it appears that the limit it is hitting then is to do with some 
lock in sendmsg().

I've not been able to get the 2010 SO_REUSEPORT patch working on the 
3.7.1 kernel I suspect it would make for even better performance as 
sendmsg() should have been significantly improved.

Now, I don't believe that SO_REUSEPORT is needed in the kernel in this 
case, however the numbers above clearly show that the current UDP 
implementation for recvmsg() on a single socket across multiple cores on 
kernel 3.7.1 is still locking badly. A perf report on 3.7.1 (using 16 
local queryperf's) shows:

     68.34%  pdns_server  [kernel.kallsyms]    [k] _raw_spin_lock_bh
             |
             --- 0x7fa472023a2d
                 system_call_fastpath
                 sys_recvmsg
                 __sys_recvmsg
                 sock_recvmsg
                 inet_recvmsg
                 udp_recvmsg
                 skb_free_datagram_locked
                |
                |--100.00%-- lock_sock_fast
                |          _raw_spin_lock_bh
                 --0.00%-- [...]

      3.10%  pdns_server  [kernel.kallsyms]    [k] _raw_spin_lock_irqsave
             |
             --- 0x7fa472023a2d
                 system_call_fastpath
                 sys_recvmsg
                 __sys_recvmsg
                 sock_recvmsg
                 inet_recvmsg
                 udp_recvmsg
                |
                |--99.69%-- __skb_recv_datagram
                |          |
                |          |--77.68%-- _raw_spin_lock_irqsave
                |          |
                |          |--14.56%-- prepare_to_wait_exclusive
                |          |          _raw_spin_lock_irqsave
                |          |
                |           --7.76%-- finish_wait
                |                     _raw_spin_lock_irqsave
                 --0.31%-- [...]
                ...

Any advice or patches welcome... :-)

Mark

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