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Message-ID: <a2ae895d871551a9b3ded9ce3874fe28.squirrel@imap.coplanar.net>
Date: Tue, 26 Oct 2010 16:21:16 -0400
From: "Jeremy Jackson" <jerj@...lanar.net>
To: "Eric Dumazet" <eric.dumazet@...il.com>
Cc: "Jeremy Jackson" <jerj@...lanar.net>, netdev@...r.kernel.org
Subject: Re: Unwanted aliasing of UDP checksum failed error counter
> Le mardi 26 octobre 2010 à 15:53 -0400, Jeremy Jackson a écrit :
>> Trying to find source of packet loss on an 8node compute cluster, we
>> find:
>> (not in this example, but on the real cluster)
>>
>> in /proc/sys/net/snmp
>> Udp: InDatagrams NoPorts InErrors OutDatagrams RcvbufErrors SndbufErrors
>> Udp: 976460 1750 0 986795 0 0
>>
>> InErrors *and* RcvbufErrors both go up with full socket buffer, this has
>> made troubleshooting our application more difficult. We were chasing
>> UDP
>> checksum problems, until we checked linux source code, and found
>> aliasing.
>>
>> Is this done for assembly code efficiency? Any reason ENOMEM (ie socket
>> buffer full) can't avoid aliasing to UDP checksum failed errors?
>>
>> in linux-source-2.6.32/net/ipv4/udp.c:__udp_queue_rcv_skb()
>> ....
>> /* Note that an ENOMEM error is charged twice */
>> if (rc == -ENOMEM) {
>> UDP_INC_STATS_BH(sock_net(sk),
>> UDP_MIB_RCVBUFERRORS,
>> is_udplite);
>> atomic_inc(&sk->sk_drops);
>> }
>> goto drop;
>> ...
>> drop:
>> UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
>>
>
> In MIBS, there is no counter for UDP checksum errors
>
> So we use the standard UDP_MIB_INERRORS
Yes, this part I understand, but what I don't understand is why ENOMEM
errors *and* checksum errors both use the same counter, while ENOMEM has
it's own already.
> udpInErrors OBJECT-TYPE
> SYNTAX Counter32
> MAX-ACCESS read-only
> STATUS current
> DESCRIPTION
> "The number of received UDP datagrams that could not be
> delivered for reasons other than the lack of an
> application at the destination port.
>
>
> We could add a LINUX specific MIB entry, eventually...
>
>
>
>
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