linux-kernel - Re: [Q] Default SLAB allocator

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Date:	Fri, 19 Oct 2012 09:01:03 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	JoonSoo Kim <js1304@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Ezequiel Garcia <elezegarcia@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Tim Bird <tim.bird@...sony.com>,
	celinux-dev@...ts.celinuxforum.org
Subject: Re: [Q] Default SLAB allocator

On Fri, 2012-10-19 at 09:03 +0900, JoonSoo Kim wrote:
> Hello, Eric.
> Thank you very much for a kind comment about my question.
> I have one more question related to network subsystem.
> Please let me know what I misunderstand.
> 
> 2012/10/14 Eric Dumazet <eric.dumazet@...il.com>:
> > In latest kernels, skb->head no longer use kmalloc()/kfree(), so SLAB vs
> > SLUB is less a concern for network loads.
> >
> > In 3.7, (commit 69b08f62e17) we use fragments of order-3 pages to
> > populate skb->head.
> 
> You mentioned that in latest kernel skb->head no longer use kmalloc()/kfree().

I hadnt the time to fully explain what was going on, only to give some
general ideas/hints.

Only incoming skbs, delivered by NIC are built this way.

I plan to extend this to some kind of frames, for example TCP ACK.
(They have a short life, so using __netdev_alloc_frag makes sense)

But when an application does a tcp_sendmsg() we use GFP_KERNEL
allocations and thus still use kmalloc().

> But, why result of David's "netperf RR" test on v3.6 is differentiated
> by choosing the allocator?

Because outgoing skb are still using a kmalloc() for their skb->head

RR sends one frame, receives one frame for each transaction.

So with 3.5, each RR transaction using a NIC needs 3 kmalloc() instead
of 4 for previous kernels.

Note that loopback traffic is different, since we do 2 kmalloc() per
transaction, and there is no difference on 3.5 for this kind of network
load.

> As far as I know, __netdev_alloc_frag may be introduced in v3.5, so
> I'm just confused.
> Does this test use __netdev_alloc_skb with "__GFP_WAIT | GFP_DMA"?
> 
> Does normal workload for network use __netdev_alloc_skb with
> "__GFP_WAIT | GFP_DMA"?
> 

Not especially.


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