netdev - Re: [PATCH] net/sock: move memory_allocated over to percpu

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Date:   Fri, 7 Sep 2018 11:32:57 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Olof Johansson <olof@...om.net>,
        David Miller <davem@...emloft.net>,
        Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Vladislav Yasevich <vyasevich@...il.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        linux-decnet-user@...ts.sourceforge.net,
        kernel-team <kernel-team@...com>
Subject: Re: [PATCH] net/sock: move memory_allocated over to percpu_counter
 variables

On Thu, Sep 06, 2018 at 12:33:58PM -0700, Eric Dumazet wrote:
> On Thu, Sep 6, 2018 at 12:21 PM Olof Johansson <olof@...om.net> wrote:
> >
> > Today these are all global shared variables per protocol, and in
> > particular tcp_memory_allocated can get hot on a system with
> > large number of CPUs and a substantial number of connections.
> >
> > Moving it over to a per-cpu variable makes it significantly cheaper,
> > and the added overhead when summing up the percpu copies is still smaller
> > than the cost of having a hot cacheline bouncing around.
> 
> I am curious. We never noticed contention on this variable, at least for TCP.

Yes these variables are heavily amortised so I'm surprised that
they would cause much contention.

> Please share some numbers with us.

Indeed.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt