netdev - Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

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Date:   Mon, 13 Feb 2017 16:46:15 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Martin KaFai Lau <kafai@...com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

On Mon, 2017-02-13 at 16:34 -0800, Alexander Duyck wrote:
> On Mon, Feb 13, 2017 at 4:22 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> > On Mon, Feb 13, 2017 at 3:47 PM, Alexander Duyck
> > <alexander.duyck@...il.com> wrote:
> >
> >> Actually it depends on the use case.  In the case of pktgen packets
> >> they are usually dropped pretty early in the receive path.  Think
> >> something more along the lines of a TCP syn flood versus something
> >> that would be loading up a socket.
> >
> > So I gave another spin an it, reducing the MTU on the sender to 500
> > instead of 1500 to triple the load (in term of packets per second)
> > since the sender seems to hit some kind of limit around 30Gbit.
> >
> > Number of packets we process on one RX queue , and one TCP flow.
> >
> > Current patch series : 6.3 Mpps
> >
> > Which is not too bad ;)
> >
> > This number does not change putting your __page_frag_cache_drain() trick.
> 
> Well the __page_frag_cache_drain by itself isn't going to add much of
> anything.  You really aren't going to be using it except for in the
> slow path.  I was talking about the bulk page count update by itself.
> All __page_frag_cache_drain gets you is it cleans up the
> page_frag_sub(n-1); put_page(1); code calls.
> 
> The approach I took for the Intel drivers isn't too different than
> what we did for the skbuff page frag cache.  Basically I update the
> page count once every 65535 frames setting it to 64K, and holding no
> more than 65535 references in the pagecnt_bias.  Also in my code I
> don't update the count until after the first recycle call since the
> highest likelihood of us discarding the frame is after the first
> allocation anyway so we wait until the first recycle to start
> repopulating the count.

Alex, be assured that I implemented the full thing, of course.

( The pagecnt_bias field, .. refilled every 16K rounds )

And I repeat : I got _no_ convincing results to justify this patch being
sent in this series.

This series is not about adding 1% or 2% performance changes, but adding
robustness to RX allocations.