Re: [PATCH 00/18] multiple preferred nodes

[Ben Widawsky <ben.widawsky@intel.com>]

On 20-06-22 13:54:30, Andi Kleen wrote:
> On Fri, Jun 19, 2020 at 09:24:07AM -0700, Ben Widawsky wrote:
> > This patch series introduces the concept of the MPOL_PREFERRED_MANY mempolicy.
> 
> So the reason for having a new policy is that you're worried some legacy
> application passes multiple nodes to MPOL_PREFERRED, where all but the
> first would be currently ignored. Is that right?
> 
> Is there any indication that this is actually the case?
> 
> If not I would prefer to just extend the semantics of the existing MPOL_PREFERRED.
> 
> Even if there was such an legacy application any legacy behavior changes
> are likely not fatal, because preferred is only a hint anyways. Anybody
> who really requires the right nodes would use _BIND.
> 
> -Andi
> 

It does break ABI in a sense as the existing MPOL_PREFERRED does in fact specify
it should fail if more than one node is specified. A decent compromise might be
to add a new flag to be used with MPOL_PREFERRED,  hinting you want like a v2
version of the interface.

Honestly, I don't care either way. I inherited it from Dave Hansen, and I can
only guess at his motivation.

Perhaps more valuable to consider was #3 alternative in the snipped part of the
cover letter:

> 3. Create flags or new modes that helps with some ordering. This offers both a
>    friendlier API as well as a solution for more customized usage. It's unknown
>    if it's worth the complexity to support this. Here is sample code for how
>    this might work:
> 
> // Default
> set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_SOCKET, NULL, 0);
> // which is the same as
> set_mempolicy(MPOL_DEFAULT, NULL, 0);
>
> // The Hare
> set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE, NULL, 0);
>
> // The Tortoise
> set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE_REV, NULL, 0);
>
> // Prefer the fast memory of the first two sockets
> set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE, -1, 2);
>
> // Prefer specific nodes for some something wacky
> set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE_CUSTOM, 0x17c, 1024);