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

[Roman Gushchin <roman.gushchin@linux.dev>]

Daniel Sedlak <daniel.sedlak@cdn77.com> writes:

> This patch is a result of our long-standing debug sessions, where it all
> started as "networking is slow", and TCP network throughput suddenly
> dropped from tens of Gbps to few Mbps, and we could not see anything in
> the kernel log or netstat counters.
>
> Currently, we have two memory pressure counters for TCP sockets [1],
> which we manipulate only when the memory pressure is signalled through
> the proto struct [2]. However, the memory pressure can also be signaled
> through the cgroup memory subsystem, which we do not reflect in the
> netstat counters. In the end, when the cgroup memory subsystem signals
> that it is under pressure, we silently reduce the advertised TCP window
> with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
> throughput reduction.
>
> Keep in mind that when the cgroup memory subsystem signals the socket
> memory pressure for a given cgroup, it affects all sockets used in that
> cgroup, including children cgroups.
>
> 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?

Thanks!