Re: [PATCH v5] memcg: expose socket memory pressure in a cgroup

Daniel Sedlak [off-list ref] writes:

Hi Roman,

On 10/8/25 8:58 PM, Roman Gushchin wrote:

quoted

quoted

This patch exposes a new file for each cgroup in sysfs which is a
read-only single value file showing how many microseconds this cgroup
contributed to throttling the throughput of network sockets. The file is
accessible in the following path.

   /sys/fs/cgroup/**/<cgroup name>/memory.net.throttled_usec

Hi Daniel!
How this value is going to be used? In other words, do you need an
exact number or something like memory.events::net_throttled would be
enough for your case?

Just incrementing a counter each time the vmpressure() happens IMO
provides bad semantics of what is actually happening, because it can
hide important details, mainly the _time_ for how long the network
traffic was slowed down.

For example, when memory.events::net_throttled=1000, it can mean that
the network was slowed down for 1 second or 1000 seconds or something
between, and the memory.net.throttled_usec proposed by this patch
disambiguates it.

In addition, v1/v2 of this series started that way, then from v3 we
rewrote it to calculate the duration instead, which proved to be
better information for debugging, as it is easier to understand
implications.

But how are you planning to use this information? Is this just
"networking is under pressure for non-trivial amount of time ->
raise the memcg limit" or something more complicated?

I am bit concerned about making this metric the part of cgroup API
simple because it's too implementation-defined and in my opinion
lack the fundamental meaning.

Vmpressure is calculated based on scanned/reclaimed ratio (which is
also not always the best proxy for the memory pressure level), then
if it reaches some level we basically throttle networking for 1s.
So it's all very arbitrary.

I totally get it from the debugging perspective, but not sure about
usefulness of it as a permanent metric. This is why I'm asking if there
are lighter alternatives, e.g. memory.events or maybe even tracepoints.

Thanks!