netdev - Re: [PATCH net-next 2/2] mpls: reduce memory usage of routes

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Date:	Tue, 27 Oct 2015 18:39:20 -0700
From:	roopa <roopa@...ulusnetworks.com>
To:	Robert Shearman <rshearma@...cade.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH net-next 2/2] mpls: reduce memory usage of routes

On 10/26/15, 5:37 PM, Robert Shearman wrote:
> Nexthops for MPLS routes have a via address field sized for the
> largest via address that is expected, which is 32 bytes. This means
> that in the most common case of having ipv4 via addresses, 28 bytes of
> memory more than required are used per nexthop. In the other common
> case of an ipv6 nexthop then 16 bytes more than required are
> used. With large numbers of MPLS routes this extra memory usage could
> start to become significant.
>
> To avoid allocating memory for a maximum length via address when not
> all of it is required and to allow for ease of iterating over
> nexthops, then the via addresses are changed to be stored in the same
> memory block as the route and nexthops, but in an array after the end
> of the array of nexthops. New accessors are provided to retrieve a
> pointer to the via address.
>
> To allow for O(1) access without having to store a pointer or offset
> per nh, the via address for each nexthop is sized according to the
> maximum via address for any nexthop in the route, which is stored in a
> new route field, rt_max_alen, but this is in an existing hole in
> struct mpls_route so it doesn't increase the size of the
> structure. Each via address is ensured to be aligned to VIA_ALEN_ALIGN
> to account for architectures that don't allow unaligned accesses.
>
> Signed-off-by: Robert Shearman <rshearma@...cade.com>
> ---
nice way to handle it!. I was going to submit a patch to make MAX_VIA_ALEN = 16bytes as suggested by Eric.
In which case, your patch will only help the case where all nexthops are ipv4.
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