Re: [PATCH] Documentation: update numastat explanation

[Michal Hocko <mhocko@kernel.org>]

On Thu 07-05-20 14:02:17, Vlastimil Babka wrote:
> During recent patch discussion [1] it became apparent that the "other_node"
> definition in the numastat documentation has always been different from actual
> implementation. It was also noted that the stats can be innacurate on systems
> with memoryless nodes.
> 
> This patch corrects the other_node definition (with minor tweaks to two more
> definitions), adds a note about memoryless nodes and also two introductory
> paragraphs to the numastat documentation.
> 
> [1] https://lore.kernel.org/linux-mm/20200504070304.127361-1-sandipan@linux.ibm.com/T/#u
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  Documentation/admin-guide/numastat.rst | 31 +++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/numastat.rst b/Documentation/admin-guide/numastat.rst
> index aaf1667489f8..08ec2c2bdce3 100644
> --- a/Documentation/admin-guide/numastat.rst
> +++ b/Documentation/admin-guide/numastat.rst
> @@ -6,6 +6,21 @@ Numa policy hit/miss statistics
>  
>  All units are pages. Hugepages have separate counters.
>  
> +The numa_hit, numa_miss and numa_foreign counters reflect how well processes
> +are able to allocate memory from nodes they prefer. If they succeed, numa_hit
> +is incremented on the preferred node, otherwise numa_foreign is incremented on
> +the preferred node and numa_miss on the node where allocation succeeded.
> +
> +Usually preferred node is the one local to the CPU where the process executes,
> +but restrictions such as mempolicies can change that, so there are also two
> +counters based on CPU local node. local_node is similar to numa_hit and is
> +incremented on allocation from a node by CPU on the same node. other_node is
> +similar to numa_miss and is incremented on the node where allocation succeeds
> +from a CPU from a different node. Note there is no counter analogical to
> +numa_foreign.
> +
> +In more detail:
> +
>  =============== ============================================================
>  numa_hit	A process wanted to allocate memory from this node,
>  		and succeeded.
> @@ -14,11 +29,13 @@ numa_miss	A process wanted to allocate memory from another node,
>  		but ended up with memory from this node.
>  
>  numa_foreign	A process wanted to allocate on this node,
> -		but ended up with memory from another one.
> +		but ended up with memory from another node.
>  
> -local_node	A process ran on this node and got memory from it.
> +local_node	A process ran on this node's CPU,
> +		and got memory from this node.
>  
> -other_node	A process ran on this node and got memory from another node.
> +other_node	A process ran on a different node's CPU
> +		and got memory from this node.
>  
>  interleave_hit 	Interleaving wanted to allocate from this node
>  		and succeeded.
> @@ -28,3 +45,11 @@ For easier reading you can use the numastat utility from the numactl package
>  (http://oss.sgi.com/projects/libnuma/). Note that it only works
>  well right now on machines with a small number of CPUs.
>  
> +Note that on systems with memoryless nodes (where a node has CPUs but no
> +memory) the numa_hit, numa_miss and numa_foreign statistics can be skewed
> +heavily. In the current kernel implementation, if a process prefers a
> +memoryless node (i.e.  because it is running on one of its local CPU), the
> +implementation actually treats one of the nearest nodes with memory as the
> +preferred node. As a result, such allocation will not increase the numa_foreign
> +counter on the memoryless node, and will skew the numa_hit, numa_miss and
> +numa_foreign statistics of the nearest node.
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs