Thank you to you both.

I modified the allocator to allocate one large block using hwloc_alloc and then 
use one thread per numa domain to  touch each page according to the tiling 
pattern - unfortunately, I hadn't appreciated that now
always returns the full machine numa mask - and not the numa domain that the 
page was touched by (I guess it only gives the expected answer when 
set_area_membind is used first)

I had hoped to use a dynamic query of the pages (using the first one of a given 
tile) to schedule each task that operates on a given tile to run on the numa 
node that touched it.

I can work around this by using a matrix offset calculation to get the numa 
node, but if there's a way of querying the page directly - then please let me 


From: hwloc-users [] on behalf of Samuel 
Thibault []
Sent: 12 November 2017 10:48
To: Hardware locality user list
Subject: Re: [hwloc-users] question about hwloc_set_area_membind_nodeset

Brice Goglin, on dim. 12 nov. 2017 05:19:37 +0100, wrote:
> That's likely what's happening. Each set_area() may be creating a new "virtual
> memory area". The kernel tries to merge them with neighbors if they go to the
> same NUMA node. Otherwise it creates a new VMA.

Mmmm, that sucks. Ideally we'd have a way to ask the kernel not to
strictly bind the memory, but just to allocate on a given memory
node, and just hope that the allocation will not go away (e.g. due to
swapping), which thus doesn't need a VMA to record the information. As
you describe below, first-touch achieves that but it's not necessarily
so convenient.

> I can't find the exact limit but it's something like 64k so I guess
> you're exhausting that.

It's sysctl vm.max_map_count

>     Question 2 : Is there a better way of achieving the result I'm looking for
>     (such as a call to membind with a stride of some kind to say put N pages 
> in
>     a row on each domain in alternation).
> Unfortunately, the interleave policy doesn't have a stride argument. It's one
> page on node 0, one page on node 1, etc.
> The only idea I have is to use the first-touch policy: Make sure your buffer
> isn't is physical memory yet, and have a thread on node 0 read the "0" pages,
> and another thread on node 1 read the "1" page.

Or "next-touch" if that was to ever get merged into mainline Linux :)

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