With Sub-NUMA Cluster mode enabled the scope of monitoring resources is
per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in
their name refer to Sub-NUMA nodes instead of L3 cache ids.
Users should be aware that SNC mode also affects the amount of L3 cache
available for allocation within each SNC node.
Signed-off-by: Tony Luck <[email protected]>
---
Changes since v5:
Added addtional details about challenges tracking tasks when SNC
mode is enabled.
---
diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
index cb05d90111b4..222c507089a5 100644
--- a/Documentation/arch/x86/resctrl.rst
+++ b/Documentation/arch/x86/resctrl.rst
@@ -345,9 +345,9 @@ When control is enabled all CTRL_MON groups will also contain:
When monitoring is enabled all MON groups will also contain:
"mon_data":
- This contains a set of files organized by L3 domain and by
- RDT event. E.g. on a system with two L3 domains there will
- be subdirectories "mon_L3_00" and "mon_L3_01". Each of these
+ This contains a set of files organized by L3 domain or by NUMA
+ node (depending on whether Sub-NUMA Cluster (SNC) mode is disabled
+ or enabled respectively) and by RDT event. Each of these
directories have one file per event (e.g. "llc_occupancy",
"mbm_total_bytes", and "mbm_local_bytes"). In a MON group these
files provide a read out of the current value of the event for
@@ -452,6 +452,23 @@ and 0xA are not. On a system with a 20-bit mask each bit represents 5%
of the capacity of the cache. You could partition the cache into four
equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
+Notes on Sub-NUMA Cluster mode
+==============================
+When SNC mode is enabled Linux may load balance tasks between Sub-NUMA
+nodes much more readily than between regular NUMA nodes since the CPUs
+on Sub-NUMA nodes share the same L3 cache and the system may report
+the NUMA distance between Sub-NUMA nodes with a lower value than used
+for regular NUMA nodes. Users who do not bind tasks to the CPUs of a
+specific Sub-NUMA node must read the "llc_occupancy", "mbm_total_bytes",
+and "mbm_local_bytes" for all Sub-NUMA nodes where the tasks may execute
+to get the full view of traffic for which the tasks were the source.
+
+The cache allocation feature still provides the same number of
+bits in a mask to control allocation into the L3 cache. But each
+of those ways has its capacity reduced because the cache is divided
+between the SNC nodes. The values reported in the resctrl
+"size" files are adjusted accordingly.
+
Memory bandwidth Allocation and monitoring
==========================================
On Tue, Oct 3, 2023 at 11:30 PM Tony Luck <[email protected]> wrote:
>
> With Sub-NUMA Cluster mode enabled the scope of monitoring resources is
> per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in
> their name refer to Sub-NUMA nodes instead of L3 cache ids.
>
> Users should be aware that SNC mode also affects the amount of L3 cache
> available for allocation within each SNC node.
>
> Signed-off-by: Tony Luck <[email protected]>
>
> ---
>
> Changes since v5:
>
> Added addtional details about challenges tracking tasks when SNC
> mode is enabled.
>
> ---
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index cb05d90111b4..222c507089a5 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -345,9 +345,9 @@ When control is enabled all CTRL_MON groups will also contain:
> When monitoring is enabled all MON groups will also contain:
>
> "mon_data":
> - This contains a set of files organized by L3 domain and by
> - RDT event. E.g. on a system with two L3 domains there will
> - be subdirectories "mon_L3_00" and "mon_L3_01". Each of these
> + This contains a set of files organized by L3 domain or by NUMA
> + node (depending on whether Sub-NUMA Cluster (SNC) mode is disabled
> + or enabled respectively) and by RDT event. Each of these
> directories have one file per event (e.g. "llc_occupancy",
> "mbm_total_bytes", and "mbm_local_bytes"). In a MON group these
> files provide a read out of the current value of the event for
> @@ -452,6 +452,23 @@ and 0xA are not. On a system with a 20-bit mask each bit represents 5%
> of the capacity of the cache. You could partition the cache into four
> equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
>
> +Notes on Sub-NUMA Cluster mode
> +==============================
> +When SNC mode is enabled Linux may load balance tasks between Sub-NUMA
> +nodes much more readily than between regular NUMA nodes since the CPUs
> +on Sub-NUMA nodes share the same L3 cache and the system may report
> +the NUMA distance between Sub-NUMA nodes with a lower value than used
> +for regular NUMA nodes. Users who do not bind tasks to the CPUs of a
> +specific Sub-NUMA node must read the "llc_occupancy", "mbm_total_bytes",
> +and "mbm_local_bytes" for all Sub-NUMA nodes where the tasks may execute
> +to get the full view of traffic for which the tasks were the source.
> +
> +The cache allocation feature still provides the same number of
> +bits in a mask to control allocation into the L3 cache. But each
> +of those ways has its capacity reduced because the cache is divided
> +between the SNC nodes. The values reported in the resctrl
> +"size" files are adjusted accordingly.
> +
> Memory bandwidth Allocation and monitoring
> ==========================================
>
Sounds better, thanks.
Reviewed-by: Peter Newman <[email protected]>