Re: [PATCH 3/3] sched/numa: documentation for per-cgroup numa stat

[Iurii Zaikin]

Since the documentation talks about fairly advanced concepts, every little bit
of readability improvement helps. I tried to make suggestions that I feel make
it easier to read, hopefully my nitpicking is not too annoying.
On Tue, Nov 12, 2019 at 7:46 PM çè <yun.wang@xxxxxxxxxxxxxxxxx> wrote:
> +On NUMA platforms, remote memory accessing always has a performance penalty,
> +although we have NUMA balancing working hard to maximum the local accessing
> +proportion, there are still situations it can't helps.
Nit: working hard to maximize the access locality...
can't helps -> can't help
> +
> +This could happen in modern production environment, using bunch of cgroups
> +to classify and control resources which introduced complex configuration on
> +memory policy, CPUs and NUMA node, NUMA balancing could facing the wrong
> +memory policy or exhausted local NUMA node, lead into the low local page
> +accessing proportion.
I find the below a bit easier to read.
This could happen in modern production environment. When a large
number of cgroups
are used to classify and control resources, this creates a complex
memory policy configuration
for CPUs and NUMA nodes. In such cases NUMA balancing could end up
with the wrong
memory policy or exhausted local NUMA node, which would lead to low
percentage of local page
accesses.

> +We need to perceive such cases, figure out which workloads from which cgroup
> +has introduced the issues, then we got chance to do adjustment to avoid
> +performance damages.
Nit: perceive -> detect, got-> get, damages-> degradation

> +However, there are no hardware counter for per-task local/remote accessing
> +info, we don't know how many remote page accessing has been done for a
> +particular task.
Nit: counters.
Nit: we don't know how many remote page accesses have occurred for a

> +
> +Statistics
> +----------
> +
> +Fortunately, we have NUMA Balancing which scan task's mapping and trigger PF
> +periodically, give us the opportunity to record per-task page accessing info.
Nit: scans, triggers, gives.

> +By "echo 1 > /proc/sys/kernel/cg_numa_stat" on runtime or add boot parameter
Nit: at runtime or adding boot parameter
> +To be noticed, the accounting is in a hierarchy way, which means the numa
> +statistics representing not only the workload of this group, but also the
> +workloads of all it's descendants.
Note that the accounting is hierarchical, which means the numa
statistics for a given group represents not only the workload of this
group, but also the
workloads of all it's descendants.
> +
> +For example the 'cpu.numa_stat' show:
> +  locality 39541 60962 36842 72519 118605 721778 946553
> +  exectime 1220127 1458684
> +
> +The locality is sectioned into 7 regions, closely as:
> +  0-13% 14-27% 28-42% 43-56% 57-71% 72-85% 86-100%
Nit: closely -> approximately?

> +we can draw a line for region_bad_percent, when the line close to 0 things
nit: we can plot?
> +are good, when getting close to 100% something is wrong, we can pick a proper
> +watermark to trigger warning message.

> +You may want to drop the data if the region_all is too small, which imply
Nit: implies
> +there are not much available pages for NUMA Balancing, just ignore would be
Nit: not many... ingoring
> +fine since most likely the workload is insensitive to NUMA.
> +Monitoring root group help you control the overall situation, while you may
Nit: helps
> +also want to monitoring all the leaf groups which contain the workloads, this
Nit: monitor
> +help to catch the mouse.
Nit: helps
> +become too small, for NUMA node X we have:
Nit: becomes
> +try put your workload into a memory cgroup which providing per-node memory
Nit: try to put
> +These two percentage are usually matched on each node, workload should execute
Nit: percentages
> +Depends on which part of the memory accessed mostly by the workload, locality
Depending on which part of the memory is accessed.
"mostly by the workload" - not sure what you mean here, the majority
of accesses from the
workload fall into this part of memory or that accesses from processes
other than the workload
are rare?
> +could still be good with just a little piece of memory locally.
?
> +Thus to tell if things are find or not depends on the understanding of system
are fine
> +After locate which workloads introduced the bad locality, check:
locate -> indentifying
> +
> +1). Is the workloads bind into a particular NUMA node?
bind into -> bound to
> +2). Is there any NUMA node run out of resources?
Has any .. run out of resources
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 5e27d74e2b74..220df1f0beb8 100644
> +                       lot's of per-cgroup workloads.
lots