netdev - Re: [PATCH net-next] net: sched: use no more than one page in struct fw

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Message-ID: <1395065631.9668.44.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 17 Mar 2014 07:13:51 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	David Miller <davem@...emloft.net>,
	John Fastabend <john.fastabend@...il.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: sched: use no more than one page in
 struct fw_head

On Mon, 2014-03-17 at 13:51 +0000, Thomas Graf wrote:
> On 03/16/14 at 09:06am, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > In commit b4e9b520ca5d ("[NET_SCHED]: Add mask support to fwmark
> > classifier") Patrick added an u32 field in fw_head, making it slightly
> > bigger than one page.
> > 
> > Change the layout of this structure and let compiler emit a reciprocal
> > divide for fw_hash(), as this makes the core more readable and
> > more efficient those days.
> 
> I think you  need to educate me a bit on this. objdump
> spits out the following:
> 
> static u32 fw_hash(u32 handle)
> {
>         return handle % HTSIZE;
>   1d:   bf ff 01 00 00          mov    edi,0x1ff
>   22:   89 f0                   mov    eax,esi
>   24:   31 d2                   xor    edx,edx
>   26:   f7 f7                   div    edi
> 
> Doesn't look like a reciprocal div to me. Where did I
> screw up or why doesn't gcc optimize it properly?
> --

Thats because on your cpu, gcc knows the divide is cheaper than anything
else (a multiply followed by a shift)

What are your exact CFLAGS ?



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