netdev - Re: net: more accurate skb truesize

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Date:	Tue, 10 Apr 2012 11:52:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	monstr@...str.eu
Cc:	netdev@...r.kernel.org,
	John Williams <john.williams@...alogix.com>,
	David Miller <davem@...emloft.net>
Subject: Re: net: more accurate skb truesize - regression on Microblaze

On Tue, 2012-04-10 at 11:38 +0200, Eric Dumazet wrote:
> On Tue, 2012-04-10 at 11:29 +0200, Michal Simek wrote:
> > On 04/10/2012 11:24 AM, Eric Dumazet wrote:
> > > On Tue, 2012-04-10 at 11:11 +0200, Michal Simek wrote:
> > >
> > >> ~ # cat /proc/sys/net/ipv4/tcp_rmem
> > >> 4096    87380   130048
> > >
> > > Are they default values, or tuned by admin ?
> > >
> > > 130048 bytes isnt enough to let TCP open its rcv window.
> > 
> > yep. Default value after powerup. What's wrong with that?
> > 
> > Michal
> > 
> 
> I guess your tcp performance is driven by these numbers mostly.
> 
> receive window wont grow above 64K in these case. a tcpdump could
> confirm the issue.
> 
> Do you have 130000 pages of memory ?
> 
> Seems the net/ipv4/tcp.c code is wrong (or the comment is wrong)
> 
> Since its not 1/128 but 1/1024 ....
> 
> 
>         /* Set per-socket limits to no more than 1/128 the pressure threshold */
>         limit = nr_free_buffer_pages() << (PAGE_SHIFT - 10);
>         limit = max(limit, 128UL);
>         max_share = min(4UL*1024*1024, limit);
> 
>         sysctl_tcp_wmem[0] = SK_MEM_QUANTUM;
>         sysctl_tcp_wmem[1] = 16*1024;
>         sysctl_tcp_wmem[2] = max(64*1024, max_share);
> 
>         sysctl_tcp_rmem[0] = SK_MEM_QUANTUM;
>         sysctl_tcp_rmem[1] = 87380;
>         sysctl_tcp_rmem[2] = max(87380, max_share);
> 

OK there is a bug introduced in commit
c43b874d5d714f271b80d4c3f49e05d0cbf51ed2
(tcp: properly initialize tcp memory limits)

I'll send a patch to change : 

limit = nr_free_buffer_pages() << (PAGE_SHIFT - 10);

back to

limit = nr_free_buffer_pages() << (PAGE_SHIFT - 7);


You could try this :)



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