netdev - Re: /proc/net/tcp6 inconsistent line length

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Date:	Tue, 17 Jan 2012 16:13:17 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	abirvalg@...abit.com
Cc:	netdev@...r.kernel.org
Subject: Re: /proc/net/tcp6 inconsistent line length

Le mardi 17 janvier 2012 à 16:14 +0000, abirvalg@...abit.com a écrit :

> Thank you, David. I first fread() the whole file to membuf and then process it thusly trying to find a line that matches my source port (porthex) :
> 
> while(1){
>     memcpy(buffer,&tcp6_membuf[184+170*i],4);
>     if ( !memcmp ( porthex, buffer, 4 ) ){ //match!
>         goto endloop;
>     }
>      i++;
>      continue;


Wow... no comment on this code.

> 
> N.B. 184 is the offset of source port on line 2 from the beginning of file (line 1 is used for column headings)
> 
> my /proc/net/tcp6 file contains 300 - 500 lines and I parse it at least from 4 to 10 times per sec. So the maximum is 5000 iterations per sec. It would be very costly CPU wise to do strchr(buf, '\n')
>  
> I'm not skillful enough to understand the code in kernel sources.
> My concern boils down to this:
> 
> Was /proc/net/tcp* meant to have an equal length of each line?
> If yes, then I discovered a bug.
> If no, then would it be possible to patch the kernel to enable an equal length on each line? 
> 

netlink can give you the needed information much faster.

Take a look at what is done by :

"ss -emoi src :22"
State      Recv-Q Send-Q                   Local Address:Port
Peer Address:Port 
ESTAB      0      0                       192.168.20.108:22
10.37.168.20:39444  timer:(keepalive,115min,0) ino:6852
sk:ffff88011ca81900
	 mem:(r0,w0,f8192,t0) ts sack cubic wscale:4,4 rto:208 rtt:8/11 ato:51
cwnd:10 ssthresh:24 send 14.5Mbps rcv_rtt:15 rcv_space:14480


sendmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000},
msg_iov(3)=[{"`\0\0\0\22\0\1\3@\342\1\0\0\0\0\0\2\0\0\17\0\0\0\0\0\0\0\0
\0\0\0\0"..., 76}, {"\24\0\1\0", 4}, {"\7\20\24\0\0\0\0\0\26\0\0\0\0\0\0
\0", 16}], msg_controllen=0, msg_flags=0}, 0) = 96
recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000},
msg_iov(1)=[{"\344\0\0\0\22\0\2\0@\342\1\0l\f\0\0\2\1\1\0\0\26\232\24
\300\250\24l320 "..., 8192}], msg_controllen=0, msg_flags=0}, 0) = 228




One sendmsg() for the request, one recvmsg() for the answer (with _all_
details included), even if your /proc/net/tcp*** has 1.000.000 lines.

Really say no to /proc files.



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