From: Eric Dumazet <eric.dumazet@gmail.com>
To: netdev <netdev@vger.kernel.org>
Subject: [RFC] truesize lies
Date: Sun, 09 Oct 2011 21:02:28 +0200 [thread overview]
Message-ID: <1318186948.5276.49.camel@edumazet-laptop> (raw)
I noticed on my laptop a difference if I use wifi or wired internet
connectivity.
On wifi mode, I can see netstat -s giving sign of bad rcvbuf tuning :
371 packets collapsed in receive queue due to low socket buffer
After some analysis, I found full sized tcp skbs (len=1440) had a 2864
bytes truesize on my wifi adapter. Ouch...
On tg3 adapter, truesize is MTU + NET_SKB_PAD + sizeof(sk_buff).
On some devices (say... NIU, but other drivers have same problem :
bnx2x, ), truesize is sizeof(sk_buff) + frame_length.
So if 'truesize' really means to account exact memory cost of frames
(including the empty space after used part), I would say :
1) NIU is lying (it should account in niu_rx_skb_append() not the used
part of the page, but reserved part : PAGE_SIZE/rbr_blocks_per_page)
2) Autotuning is a bit pessimistic : It works only if truesize is MSS +
sizeof(sk_buff) + 16 + MAX_TCP_HEADER
I guess most driver writers adjust truesize to please TCP stack and get
nice performance numbers.
skb->truesize = frame_len + sizeof(sk_buff);
What should we do exactly ?
next reply other threads:[~2011-10-09 19:02 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-09 19:02 Eric Dumazet [this message]
2011-10-09 19:25 ` [RFC] truesize lies Andi Kleen
2011-10-09 21:27 ` David Miller
2011-10-09 21:55 ` Eric Dumazet
2011-10-09 22:10 ` David Miller
2011-10-10 6:54 ` Eric Dumazet
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