netdev - Re: question about size of sk_buff and skb_shared

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Message-ID: <1492542021.10587.137.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 18 Apr 2017 12:00:21 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Code Soldier1 <codesoldier1@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: question about size of sk_buff and skb_shared_info

On Tue, 2017-04-18 at 10:34 -0700, Code Soldier1 wrote:
> Hi Folks,
> 
> I am sure there is a reason for the current sizes of these structures,
> However the reason is not obvious to me. So please help me understand.
> 
> Currently the size of sk_buff on an x86_64 system is 232 bytes -- Why
> is that. I expected it to be a multiple of 32/64 as they are the most
> common cache lines. Since the alignment calculation will align the
> structure with the hw cache line, it seems like we might be wasting
> space ?
> 
> skb_shared_info on the other hand is perfectly aligned with a size of 320 bytes.
> 
> Thanks,
> 

The alignment is there.
Look at skb_init() code, using SLAB_HWCACHE_ALIGN