netdev - Re: [PATCH net-next V2 05/11] net/mlx5e: Support RX multi-packet WQE (Striding RQ)

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Date:	Tue, 19 Apr 2016 18:39:26 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Saeed Mahameed <saeedm@....mellanox.co.il>,
	Saeed Mahameed <saeedm@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Tal Alon <talal@...lanox.com>,
	Tariq Toukan <tariqt@...lanox.com>,
	Eran Ben Elisha <eranbe@...lanox.com>,
	Achiad Shochat <achiad@...lanox.com>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH net-next V2 05/11] net/mlx5e: Support RX multi-packet WQE
 (Striding RQ)

On Tue, Apr 19, 2016 at 06:25:32PM +0200, Jesper Dangaard Brouer wrote:
> On Mon, 18 Apr 2016 07:17:13 -0700
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Mon, 2016-04-18 at 16:05 +0300, Saeed Mahameed wrote:
> > > On Mon, Apr 18, 2016 at 3:48 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:  
> > > > On Sun, 2016-04-17 at 17:29 -0700, Eric Dumazet wrote:
> > > >  
> > > >>
> > > >> If really you need to allocate physically contiguous memory, have you
> > > >> considered converting the order-5 pages into 32 order-0 ones ?  
> > > >
> > > > Search for split_page() call sites for examples.
> > > >
> > > >  
> > > 
> > > Thanks Eric, we are already evaluating split_page as we speak.
> > > 
> > > We did look but could not find any specific alloc_pages API that

alloc_pages_exact()

> > > allocates many physically contiguous pages with order0 ! so we assume
> > > it is ok to use split_page.  
> > 
> > Note: I have no idea of split_page() performance :
> 
> Maybe Mel knows?

Irrelevant in comparison to the cost of allocating an order-5 pages if
one is not already available.

> And maybe Mel have an opinion about if this is a good
> or bad approach, e.g. will this approach stress the page allocator in a
> bad way?
> 

It'll contend on the zone lock minimally but again, irrelevant in
comparison to having to reclaim/compact an order-5 page if one is not
already free.

It'll appear to work well in benchmarks and then fall apart when the
system is running for long enough.