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

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Message-ID: <478BB722.1020004@nortel.com>
Date:	Mon, 14 Jan 2008 13:25:22 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Eric Dumazet <dada1@...mosbay.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

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.

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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