Message-ID: <51C28FA7.2070901@redhat.com>
Date: Thu, 20 Jun 2013 13:14:15 +0800
From: Jason Wang <jasowang@redhat.com>
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To: Eric Dumazet <eric.dumazet@gmail.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>, davem@davemloft.net,
        edumazet@google.com, hkchu@google.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [net-next rfc 1/3] net: avoid high order memory allocation for
 queues by using flex array
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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.
>
>
>

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