netdev - Re: [RFC] socket sk_sndmsg

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Message-ID: <1323188117.2448.24.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Tue, 06 Dec 2011 17:15:17 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] socket sk_sndmsg_page waste

Le mardi 06 décembre 2011 à 07:53 +0100, Eric Dumazet a écrit :
> TCP can steer one page of memory per socket to cook outgoing frames.
> 
> This means a machine handling long living sockets can consume a lot of
> ram.
> 
> 1.000.000 tcp sockets : up to 4GB of allocated memory, if some writes
> had been done on these sockets.
> 
> It would make sense to use a per thread page as a pool, instead of a per
> socket pool, and remove sk_sndmsg_page/off fields.
> 
> Problem with this strategy is impact outside of net tree, and a cost at
> thread creation/destruction.
> 
> [ But this could be used in fs/pipe.c or fs/splice.c code..., so that
> small writes() dont allocate a full page but try to reuse the "per
> task_struct" page ]
> 
> 

Another idea would be to use a percpu variable, to get proper NUMA
affinity as well, and no extra cost at thread create/delete time.

Only 'problem' is we can sleep (pagefault) in
skb_copy_to_page_nocache(), so special care must be taken (disabling
preemption wont prevent another thread on same cpu can use the same
page)



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