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

From: Jason Wang <[email-masked]>
Date: Thu, 20 Jun 2013 13:14:15 +0800

On 06/19/2013 05:56 PM, Eric Dumazet wrote:
> 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.

A drawback of NETIF_F_LLTX is that we may contend on qdisc lock
especially when we have a huge number of tx queues.
>
> This way, you can have real per cpu memory, with proper NUMA affinity.
>
>
>