netdev - Re: questions on NAPI processing latency and dropped network packets

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Date:	Mon, 14 Jan 2008 20:33:24 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	Ray Lee <ray-lk@...rabbit.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: questions on NAPI processing latency and dropped network packets

Chris Friesen a écrit :
> Eric Dumazet wrote:
>> Chris Friesen a écrit :
>
>>> Based on profiling and instrumentation it seems like the cost of 
>>> sctp_endpoint_lookup_assoc() more than triples, which means that the 
>>> amount of time that bottom halves are disabled in that function also 
>>> triples.
>>
>> Any idea of the size of sctp hash size you have ?
>> (your dmesg probably includes a message starting with SCTP: Hash 
>> tables configured...
>> How many concurrent sctp sockets are handled ?
>
> Our lab is currently rebooting, but I'll try and get this once it's 
> back up.
>
>> Maybe sctp_assoc_hashfn() is too weak for your use, and some chains 
>> are *really* long.
>
> Based on the profiling information we're spending time in 
> sctp_endpoint_lookup_assoc() which doesn't actually use hashes, so I 
> can't see how the hash would be related.  I'm pretty new to SCTP 
> though, so I may be missing something.
Well, it does use hashes :)

        hash = sctp_assoc_hashfn(ep->base.bind_addr.port, rport);
        head = &sctp_assoc_hashtable[hash];
        read_lock(&head->lock);
        sctp_for_each_hentry(epb, node, &head->chain) {
            /* maybe your machine is traversing here a *really* long 
chain */
            }

>
> Here's the top results from readprofile, unfortunately these are 
> aggregated across both cpus so they don't really show what's going on. 
> The key thing is that sctp_endpoint_lookup_assoc() is the most 
> expensive kernel routine on this entire system.
>
>   3147 .power4_idle                              22.4786
>   1712 .native_idle                              20.3810
>   1234 .sctp_endpoint_lookup_assoc                2.1725

>   1212 ._spin_unlock_irqrestore                   6.4468
>    778 .do_futex                                  0.3791
>    447 ._spin_unlock_irq                          4.2981
>    313 .fget                                      1.7784
>    277 .fput                                      3.8472
>    275 .kfree                                     0.7473
>    234 .__kmalloc                                 0.5571
>    131 SystemCall_common                          0.3411
>    130 .sctp_assoc_is_match                       0.6373
>    123 .lock_sock                                 0.4155
>    119 .find_vma                                  0.6919
>    116 .kmem_cache_alloc                          0.3580
>    111 .kmem_cache_free                           0.3343
>    106 .skb_release_data                          0.4907
>    102 .__copy_tofrom_user                        0.0724
>    100 .exit_elf_binfmt                           1.9231
>    100 .do_select                                 0.0820
>
>
> Chris
> -- 
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