Re: [net] 4890b686f4: netperf.Throughput_Mbps -69.4% regression

Eric Dumazet
2022-06-27T14:08:10Z

On Mon, Jun 27, 2022 at 2:34 PM Feng Tang <feng.tang@intel.com> wrote:
>
> On Mon, Jun 27, 2022 at 10:46:21AM +0200, Eric Dumazet wrote:
> > On Mon, Jun 27, 2022 at 4:38 AM Feng Tang <feng.tang@intel.com> wrote:
> [snip]
> > > > >
> > > > > Thanks Feng. Can you check the value of memory.kmem.tcp.max_usage_in_bytes
> > > > > in /sys/fs/cgroup/memory/system.slice/lkp-bootstrap.service after making
> > > > > sure that the netperf test has already run?
> > > >
> > > > memory.kmem.tcp.max_usage_in_bytes:0
> > >
> > > Sorry, I made a mistake that in the original report from Oliver, it
> > > was 'cgroup v2' with a 'debian-11.1' rootfs.
> > >
> > > When you asked about cgroup info, I tried the job on another tbox, and
> > > the original 'job.yaml' didn't work, so I kept the 'netperf' test
> > > parameters and started a new job which somehow run with a 'debian-10.4'
> > > rootfs and acutally run with cgroup v1.
> > >
> > > And as you mentioned cgroup version does make a big difference, that
> > > with v1, the regression is reduced to 1% ~ 5% on different generations
> > > of test platforms. Eric mentioned they also got regression report,
> > > but much smaller one, maybe it's due to the cgroup version?
> >
> > This was using the current net-next tree.
> > Used recipe was something like:
> >
> > Make sure cgroup2 is mounted or mount it by mount -t cgroup2 none $MOUNT_POINT.
> > Enable memory controller by echo +memory > $MOUNT_POINT/cgroup.subtree_control.
> > Create a cgroup by mkdir $MOUNT_POINT/job.
> > Jump into that cgroup by echo $$ > $MOUNT_POINT/job/cgroup.procs.
> >
> > <Launch tests>
> >
> > The regression was smaller than 1%, so considered noise compared to
> > the benefits of the bug fix.
>
> Yes, 1% is just around noise level for a microbenchmark.
>
> I went check the original test data of Oliver's report, the tests was
> run 6 rounds and the performance data is pretty stable (0Day's report
> will show any std deviation bigger than 2%)
>
> The test platform is a 4 sockets 72C/144T machine, and I run the
> same job (nr_tasks = 25% * nr_cpus) on one CascadeLake AP (4 nodes)
> and one Icelake 2 sockets platform, and saw 75% and 53% regresson on
> them.
>
> In the first email, there is a file named 'reproduce', it shows the
> basic test process:
>
> "
> use 'performane' cpufre governor for all CPUs
>
> netserver -4 -D
> modprobe sctp
> netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> (repeat 36 times in total)
> ...
>
> "
>
> Which starts 36 (25% of nr_cpus) netperf clients. And the clients number
> also matters, I tried to increase the client number from 36 to 72(50%),
> and the regression is changed from 69.4% to 73.7%"
>

This seems like a lot of opportunities for memcg folks :)

struct page_counter has poor field placement [1], and no per-cpu cache.

[1] "atomic_long_t usage" is sharing cache line with read mostly fields.

(struct mem_cgroup also has poor field placement, mainly because of
struct page_counter)

28.69% [kernel] [k] copy_user_enhanced_fast_string
16.13% [kernel] [k] intel_idle_irq
6.46% [kernel] [k] page_counter_try_charge
6.20% [kernel] [k] __sk_mem_reduce_allocated
5.68% [kernel] [k] try_charge_memcg
5.16% [kernel] [k] page_counter_cancel

> Thanks,
> Feng
>
> > >
> > > Thanks,
> > > Feng