netdev - Re: [PATCH net] net/mlx5e: Do not recycle pages from emergency reserve

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Date:   Mon, 23 Jan 2017 10:14:55 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     brouer@...hat.com, Tom Herbert <tom@...bertland.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Davem <davem@...emloft.net>,
        Saeed Mahameed <saeedm@....mellanox.co.il>
Subject: Re: [PATCH net] net/mlx5e: Do not recycle pages from emergency
 reserve

On Sat, 21 Jan 2017 11:26:49 -0800
Eric Dumazet <eric.dumazet@...il.com> wrote:

> > My previous measurements show approx 20℅ speedup on a UDP test with
> > delivery to remote CPU.
> >   
> I find this a bit strange. When you have time (ie not while driving your
> car or during week end) please give more details, for example on message
> size.

I tested this with both 64 bytes and 1500 bytes.  After I moved to 50G
and 100G testing then I don't need to use 64 bytes packets to provoke
the bottlenecks in the stack ;-)

> Was it before skb_condense() was added ?

It tested this just before skb_condense() was added.  BUT
skb_condense() does not get activated when using mlx5, because uses
build_skb() ie. not using frags.  

For people that don't realize this:
 Eric's optimization in skb_condense() is about trading remote CPU
 atomic refcnt (put_page) for copy + local CPU refcnt dec.

My measurements show cycles cost local=31 vs. remote=208, thus a
estimated saving around 177 cycles.  Which is spend on calling a fairly
complex function __pskb_pull_tail(), and only works for more complex
SKBs with frags.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer