netdev - Re: surprising memory request

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Date:	Mon, 21 Jan 2013 13:21:49 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Dirk Hohndel <dirk@...ndel.org>, netdev@...r.kernel.org,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: surprising memory request

On 01/19/2013 01:54 AM, Eric Dumazet wrote:
> On Fri, 2013-01-18 at 09:46 -0800, Eric Dumazet wrote:
>
>> Thats because Jason thought that tun device had to have an insane number
>> of queues to get good performance.
>>
>> #define MAX_TAP_QUEUES 1024
>>
>> Thats crazy if your machine has say 8 cpus.
>>
>> And Jason didnt care to adapt the memory allocations done in
>> alloc_netdev_mqs(), in order to switch to vmalloc() when kmalloc()
>> fails.
> I suggest using the more reasonable :
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index c81680d..ec18fbf 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -113,7 +113,7 @@ struct tap_filter {
>   * the order of 100-200 CPUs so this leaves us some breathing space if we want
>   * to match a queue per guest CPU.
>   */
> -#define MAX_TAP_QUEUES 1024
> +#define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
>  
>  #define TUN_FLOW_EXPIRE (3 * HZ)
>  

But it's default value 8 is a little too small, we can easily have a kvm
guest with more than 8 vcpus and a host multiqueue card with more than 8
queues. Maybe we can use num_possible_cpus() or just an arbitrary number
such as 256 which seems large enough.


>
>
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