netdev - Re: [net-next rfc 1/3] net: avoid high order memory allocation for queues by using flex array

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Message-ID: <1371635763.3252.289.camel@edumazet-glaptop>
Date:	Wed, 19 Jun 2013 02:56:03 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, davem@...emloft.net,
	edumazet@...gle.com, hkchu@...gle.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [net-next rfc 1/3] net: avoid high order memory allocation for
 queues by using flex array

On Wed, 2013-06-19 at 12:11 +0300, Michael S. Tsirkin wrote:

> Well KVM supports up to 160 VCPUs on x86.
> 
> Creating a queue per CPU is very reasonable, and
> assuming cache line size of 64 bytes, netdev_queue seems to be 320
> bytes, that's 320*160 = 51200. So 12.5 pages, order-4 allocation.
> I agree most people don't have such systems yet, but
> they do exist.

Even so, it will just work, like a fork() is likely to work, even if a
process needs order-1 allocation for kernel stack.

Some drivers still use order-10 allocations with kmalloc(), and nobody
complained yet.

We had complains with mlx4 driver lately only bcause kmalloc() now gives
a warning if allocations above MAX_ORDER are attempted.

Having a single pointer means that we can :

- Attempts a regular kmalloc() call, it will work most of the time.
- fallback to vmalloc() _if_ kmalloc() failed.

Frankly, if you want one tx queue per cpu, I would rather use
NETIF_F_LLTX, like some other virtual devices.

This way, you can have real per cpu memory, with proper NUMA affinity.

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