netdev - RE: [RFC PATCH 0/3] Faster than SLAB caching of SKBs with qmempool (backed by alf

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Date:	Wed, 10 Dec 2014 14:22:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Jesper Dangaard Brouer' <brouer@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux.com>
CC:	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Hannes Frederic Sowa" <hannes@...essinduktion.org>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: RE: [RFC PATCH 0/3] Faster than SLAB caching of SKBs with qmempool
	(backed by alf_queue)

From: Jesper Dangaard Brouer
> The network stack have some use-cases that puts some extreme demands
> on the memory allocator.  One use-case, 10Gbit/s wirespeed at smallest
> packet size[1], requires handling a packet every 67.2 ns (nanosec).
> 
> Micro benchmarking[2] the SLUB allocator (with skb size 256bytes
> elements), show "fast-path" instant reuse only costs 19 ns, but a
> closer to network usage pattern show the cost rise to 45 ns.
> 
> This patchset introduce a quick mempool (qmempool), which when used
> in-front of the SKB (sk_buff) kmem_cache, saves 12 ns on "fast-path"
> drop in iptables "raw" table, but more importantly saves 40 ns with
> IP-forwarding, which were hitting the slower SLUB use-case.
> 
> 
> One of the building blocks for achieving this speedup is a cmpxchg
> based Lock-Free queue that supports bulking, named alf_queue for
> Array-based Lock-Free queue.  By bulking elements (pointers) from the
> queue, the cost of the cmpxchg (approx 8 ns) is amortized over several
> elements.

It seems to me that these improvements could be added to the
underlying allocator itself.
Nesting allocators doesn't really seem right to me.

	David