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

From: Eric Dumazet
Date: Wed Jun 19 2013 - 05:56:22 EST

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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